Forces and motion 4: Gravity and freefall

Transcription

1 Forces and motion 4: Gravity and freefall University of York

2 Gravity and freefall The questions in this set probe pupils understanding of gravity and freefall. The idea of friction is first introduced in the National Curriculum at Key Stage 2, where pupils should be taught that objects are pulled downwards because of the gravitational attraction between them and the Earth (Sc4/2b). The idea of an air resistance force on objects moving through the air is also introduced at this stage (Sc4/2c). At Key Stage 3, the distinction between weight and mass is introduced in statement SC4/2b. In the QCA Scheme of work for Key Stages 1&2 air resistance is introduced in Unit 4E, Friction, and ideas about objects falling through air are discussed in Unit 6E, Forces in action. In the QCA Scheme of work for Key Stage 3, mass and weight are discussed in Unit 7K, Forces and their effects and gravity is explored at some length in Unit 9J, Gravity and space. Motion through air, including the motion of parachutes is also included in Unit 9K, Speeding up. The only explicit mention of these ideas at Key Stage 4 is in statements Sc4/2h and Sc4/2i which specify that pupils be taught how the forces on falling objects change with velocity and why falling objects reach a terminal velocity. Questions to probe understanding of friction also bring in ideas about the relationship of forces and motion, which are probed by diagnostic questions in the set Forces and motion 2. Several questions in the set Forces and motion 1 also probe understanding of motion and rest under the influence of vertical forces including the force of gravity. Question 1 This question checks that pupils recognise that gravity acts on objects near the Earth, whether they are moving or stationary. Questions 2-7 These probe understanding of freefall, and the way in which objects that are light relative to their surface area (including parachutes) fall. The two-tier format of Q2 probes understanding of how the stone falls and then of the explanation for this. The question states explicitly that air resistance can be ignored in this situation. Q3 then looks at a situation where this is not the case, again using the two-tier format to probe understanding of what happens separately from understanding of the explanation. Q4-6 then look at the fall of two objects of different weights, released together. In Q4, the wording states that forces caused by the air can be ignored. If so, the balls should land exactly together a result which some pupils may recall as the right answer. For that reason, part (b) provides a useful check on understanding. 1 An advantage of Q5 is that it is less wordy. For objects of mass 1kg and 2kg, the correct 1 The third option is only correct if we ignore forces due to the air. If we do not ignore these, there are three forces to consider. The downward force of gravity is twice as big for the heavy ball. As the two balls have the same diameter, the upthrust due to displaced air is the same for both. Also the air resistance force is the same for both, at any given speed. So the resultant force on the heavy ball is more than double the resultant force on the lighter one. The heavy ball will therefore have a slightly larger acceleration and so should land first. 162 University of York 2003

3 answer is A, though B might also be defended as plausible. The situation in Q6 is significantly different from that in Q5, as here the weight/area ratio for the table tennis ball is very much smaller than for the golf ball. In Q7, the two falling objects have exactly the same weight. It is not easy to write concise explanations for part (b) here. Answer 3 is not correct in that the air resistance force depends on the speed, and so is not always bigger for a large flat object than for a smaller one. Answer 4 is seen as the correct one here. Questions 8-10 Questions like Q2-7 in this set are difficult to devise. This is because an explanation involves a chain of reasoning. Pupils may reason several of these steps correctly, but then make an error. Q8-9 are devised to enable you to probe pupils ability to construct a whole explanation of freefall in the case where the object only falls a short distance and air resistance can be ignored; and where the fall is much longer and air resistance plays a significant role. These questions, and Q10, are suitable for use as small-group discussion activities. One way of using these is to give each pupil a question sheet and allow them about 10 minutes to write their own individual answers. Then put pupils into groups of three, and give each triad another blank copy of the question. Encourage each group to discuss their answers and write the group s agreed best answer on the new sheet. This might take minutes. It is important that every member of the group is prepared to explain and justify this group answer. Then the groups answers can be discussed by the whole class, with the aim of trying to agree the correct answer. In this way, pupils may come to a better understanding of the ideas involved. Questions These questions probe understanding of the difference between weight and mass. In particular they probe whether pupils appreciate that the weight of an object can change when it is moved from one place to another. Some may believe that even a small change in height leads to a measurable change in weight. If so, this could be checked out by a practical measurement. On the other hand, going up a mountain (Q12) might make an observable difference (it will depend on the sensitivity of the weighing equipment). On the moon (Q13), the weight (and hence the stretch of a spring) will be about 1 / 6 that on Earth. The two questions on mass (Q14-15) ask about the acceleration of an object when pulled by the same force in different locations. Pupils who can clearly distinguish mass and weight and who recognise that mass is the property that gives objects their resistance to changes in motion may appreciate that the acceleration will be the same each time, even though the weight is not. Although these ideas are specified in the National Curriculum as Key Stage 3 material, and not mentioned again at Key Stage 4, data we have collected in piloting these questions suggests that only a minority can answer these correctly at Key Stage 4. Questions Previous research has shown that some pupils think gravity is due (at least in part) to having an atmosphere. Q16 probes ideas about gravity on the moon compared with University of York

4 that on Earth. The final statement asks about mass rather than weight, linking with Q The only true statement here is b; all the others are false. Finally Q17 explores understanding of the weightlessness observed in Earth orbit. The true statements here are d and e; all the others are false. Questions These two concept cartoons deal with some of the ideas probed by earlier questions. They could be used as posters that pupils might discuss informally while this topic is being taught. Or they could be used as individual or small-group activities, using the response sheet provided with each, to make pupils think about their response to the comments of the cartoon characters. 164 University of York 2003

5 1 The drawing shows an apple falling from a tree. Position 1 apple attached to tree Position 2 apple falling down Position 3 apple on the ground In which position (or positions) is there a force of gravity acting on the apple? Tick ONE box () In position 1 only. In position 2 only. In position 3 only. In positions 1 and 2 only. In positions 1 and 3 only. In positions 2 and 3 only. In positions 1, 2 and 3. University of York

6 2 A stone is dropped from a first floor window and falls to the ground. You can ignore any forces caused by the air as these are too small to have any effect. stone (a) Which of the following best describes how the stone falls? Tick ONE box () The stone falls at a constant speed. For a brief moment after it is released the stone stays stationary. Then it falls at a constant speed. The speed of the stone increases at first, until it reaches its maximum speed. Then it falls at this constant speed to the ground. The speed of the stone increases steadily throughout its fall. The speed of the stone increases rapidly at first, then decreases again as it gets closer to the ground. (b) Which of the following best explains the way the stone falls? Tick ONE box () The only important force on the stone is the downward pull of gravity. This is constant in size. It takes a short time after the stone is released for the force of gravity to have an effect on it. Then it pulls it down. The force of gravity on the stone gets a lot bigger as it gets closer to the ground. The force of gravity on the stone is a lot bigger when it is higher up, and gets smaller as it gets closer to the ground. 166 University of York 2003

7 3 Bilal makes a model parachute out of a piece of cloth and some string. He ties a stone to the parachute, and drops it from a first floor window. The parachute falls to the ground. parachute (a) Which of the following best describes how the parachute falls? Tick ONE box () The parachute falls at a constant speed. For a brief moment after it is released the parachute stays stationary. Then it falls at a constant speed. The speed of the parachute increases at first, until it reaches its maximum speed. Then it falls at this constant speed to the ground. The speed of the parachute increases steadily throughout its fall. The speed of the parachute increases rapidly at first, then decreases again as it gets closer to the ground. (b) Which of the following best explains the way the parachute falls? Tick ONE box () The only important force on the parachute is the downward pull of gravity. This is constant in size. It takes a short time after the parachute is released for the force of gravity to have an effect on it. Then it pulls it down. At first, gravity is the main force on the parachute. As the parachute gets faster, the air resistance force increases. The force of gravity on the parachute gets a lot bigger as it gets closer to the ground. The force of gravity on the parachute is a lot bigger when it is higher up, and gets smaller as it gets closer to the ground. University of York

8 4 Two metal balls are the same size, but one weighs twice as much as the other. twice as heavy The two balls are dropped at the same instant from the roof of a single storey building. You can ignore any forces caused by the air as these are too small to have any effect. (a) Which of the following best describes what happens? Tick ONE box () The heavy ball will reach the ground in exactly half the time of the light one. The light ball will reach the ground in exactly half the time of the heavy one. The two balls will reach the ground at exactly the same time. The heavy ball will reach the ground well before the light one, but not necessarily in half the time. The light ball will reach the ground well before the heavy one, but not necessarily in half the time. (b) Which of the following best explains the way the balls fall? Tick ONE box () The force of gravity on the heavy ball is twice as big, so it has twice as big an effect. The force of gravity is the same for both balls. So it has less effect on the heavy one. The force of gravity on the heavy ball is twice as big, but it has twice as much mass. So both balls have the same acceleration. 168 University of York 2003

9 5 Bilal wants to do an experiment to see how objects fall. He takes an aluminium ball and a steel ball of exactly the same size. The aluminium ball has a mass of 1 kg. The steel ball has a mass of 2 kg. Bilal ties the two balls together with a piece of string. 1 kg 2 kg He stands on a stool, holding them side by side like this, 2 metres above the floor. He then drops them at the same moment. (a) Which of the diagrams below shows how the balls will look just before they hit the floor? Write your answer in this box: A B C D E F (b) Explain your answer: University of York

10 6 Tim wants to do an experiment to see how objects fall. He takes a golf ball and a table-tennis ball. These are the same size but the golf ball is quite a lot heavier than the table-tennis ball. Tim ties the two balls together with a piece of string. table-tennis ball golf ball He stands on a stool, holding them side by side, 2 metres above the floor. He then drops them at the same moment. Just before they hit the floor, Tim notices that the balls look like this: table-tennis ball golf ball Look at each of the statements below about this situation. For each, put a tick () in one column to show if you think it is true or false. a b c d e f A heavy object always falls faster than a lighter one. The force of gravity on the golf ball is bigger. The force of gravity on the two balls is the same size. The air resistance force on the table-tennis ball is bigger. The air resistance force on the two balls is the same size. The air resistance force is bigger, relative to the gravity force, for the table-tennis ball. True False Don t know 170 University of York 2003

11 7 Pam takes two identical sheets of paper. She crumples one into a tight ball. Then she drops the paper ball and the sheet of paper at the same moment from the same height above the ground. paper ball sheet of paper (a) Which of the following best describes the way the paper ball and the sheet of paper fall? Tick ONE box () The paper ball will reach the ground well before the sheet of paper. The paper ball will reach the ground just a little bit before the sheet of paper. The paper ball and the sheet of paper will reach the ground at exactly the same time. The sheet of paper will reach the ground before the paper ball. (b) Which of the following best explains how the two objects fall? Tick ONE box () The force of gravity on the paper ball is bigger than the force of gravity on the sheet of paper. There is an air resistance force on the sheet of paper, but not on the paper ball. The air resistance force on a large flat object is always bigger than on a small compact object. The air resistance force on the sheet of paper becomes equal to the gravity force at a slower speed than for the paper ball. The acceleration due to gravity (g) is a constant for all objects. University of York

12 8 Dropping a stone from 1 metre above the ground Imagine that you hold a stone at arm s length and let it go, so that it falls to the ground a distance of about 1 metre. The statements in the boxes below link together to form an explanation of the way the stone falls. Some boxes contain more than one statement. In each of these boxes, pick the statement that you think is correct and fits into the whole explanation. Indicate your choice by putting a line through the other statement(s) in the box. Continue until you have chosen one statement from every box, to produce a complete explanation for the way the stone falls. 1 There is a force of gravity on the stone. 2 This acts downwards. 3a It is the same size throughout the fall. 3b It gets a lot bigger as the stone gets closer to the Earth. 3c It is biggest when the stone is high up and gets a lot smaller as it falls. 4a This force makes the stone begin to accelerate downwards. 4b This force makes the stone begin to move downwards at a steady speed. 5 Once the stone begins to move, there is also an air resistance force on it. 6a This acts downwards, in the direction the stone is going. 6b This acts upwards, in the opposite direction to the stone s motion. 172 University of York 2003

13 7a The size of the air resistance force on the stone is constant throughout the fall. 7b The air resistance force gets bigger as the stone gets faster. 8a Over a short distance like this, the air resistance force on the stone is much smaller than the force of gravity, and so it can be ignored. 8b Even over a short distance like this, the air resistance force on the stone is quite large, and has to be taken into account. 9a So the total force on the stone is equal to the force of gravity, and is constant. 9b So the total force on the stone is the sum of the gravity force and air resistance, and this gets gradually less as it falls, because the air resistance increases. 10a Therefore the stone has a uniform acceleration throughout its fall. 10b Therefore the stone accelerates to begin with, until it reaches its terminal velocity and then falls with a steady speed. 10c Therefore the stone falls at a steady speed throughout its fall. University of York

14 9 Dropping a box from a plane Imagine that a box is dropped from an aeroplane, flying at a height of 1000 metres. It falls to the ground. The statements in the boxes below link together to explain how the box falls. Some boxes contain more than one statement. In each of these boxes, pick the statement that you think is correct and fits into the whole explanation. Indicate your choice by putting a line through the other statement(s) in the box. Continue until you have chosen one statement from every box, to produce a complete explanation for the way the box falls. 1 There is a force of gravity on the box. 2 This acts downwards. 3a It is roughly the same size throughout the fall. 3b It gets a lot bigger as the box gets closer to the Earth. 3c It is biggest when the box is high up and gets a lot smaller as it falls. 4a This force makes the box begin to accelerate downwards. 4b This force makes the box begin to move downwards at a steady speed. 5 Once the box begins to move, there is also an air resistance force on it. 6a This acts downwards, in the direction the box is going. 6b This acts upwards, in the opposite direction to the box s motion. 174 University of York 2003

15 7a The size of the air resistance force on the box is constant throughout the fall. 7b The air resistance force gets bigger as the box gets faster. 8a The air resistance force on the box is much smaller than the force of gravity, and so it can be ignored. 8b The air resistance force on the box becomes quite large, and has to be taken into account. 9a So the total force on the box is equal to the force of gravity, and is constant. 9b The total force on the box is the sum of the gravity force and air resistance, and this gets gradually less as it falls, because the air resistance increases. 10a Therefore the box has a uniform acceleration throughout its fall. 10b Therefore the acceleration of the box is biggest to begin with, and gets gradually less. Once the air resistance force becomes equal to the gravity force, the acceleration is zero and the box then falls at a steady speed. 10c Therefore the box falls at a steady speed throughout its fall. University of York

16 10 Dropping a golf ball and a table-tennis ball A golf ball and a table-tennis ball are dropped from the same height. On the diagrams below, draw all the forces acting on each ball: (a) early in the fall, shortly after it has been released; (b) late in the fall, shortly before it hits the ground. Represent forces: by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) golf ball early in fall (a) table-tennis ball early in fall (b) golf ball late in fall (b) table-tennis ball late in fall 176 University of York 2003

17 11 Tim is in a lab on the ground floor of the school. He hangs a small box on a spring. It stretches the spring by 20 centimetres. 20 cm He then carries the spring and box up to a classroom on the top floor of the school, two flights up. Then he repeats the experiment. (a) How much will the spring stretch now? Tick ONE box () More than 20 centimetres. Exactly 20 centimetres again. Less than 20 centimetres. (b) How would you explain this? Tick ONE box () The box is now higher above the ground. The change in distance from the centre of the Earth is too small to have any effect. The box is now further from the centre of the Earth. The weight of an object is always the same. University of York

18 12 Pam is in a lab on the ground floor of the school. She hangs a small box on a spring. It stretches the spring by 35 centimetres. 35 cm She then takes the spring and box up to the top of a mountain, and repeats the experiment. (a) How much will the spring stretch now? Tick ONE box () More than 35 centimetres. Exactly 35 centimetres again. Less than 35 centimetres. (b) How would you explain this? Tick ONE box () The box is now higher above the ground, so gravity pulls it down harder. The box is further from the centre of the Earth, so the gravity force is less. The box is further from the centre of the Earth, so the gravity force is less, but the change is much too small to see. The weight of an object is always the same, so the spring stretches by exactly the same amount. 178 University of York 2003

19 13 An astronaut carries out an experiment. Before leaving Earth, he hangs a small box from a spring. It stretches the spring by 25 centimetres. 25 cm He then takes the spring and the box to the Moon, and repeats the experiment. (a) How much will the spring stretch on the Moon? Tick ONE box () More than 25 centimetres. Exactly 25 centimetres again. Less than 25 centimetres. Zero - no stretch at all. (b) How would you explain this? Tick ONE box () The box is now higher above the ground, so gravity pulls it down harder. The Moon is smaller than the Earth, so gravity is weaker there. There is no gravity on the moon, because it has no atmosphere. The weight of an object is always the same. University of York

20 14 An astronaut tries out an experiment on Earth before setting off on a mission. He uses a spring to pull a block along a smooth level surface. As he pulls, he keeps the spring stretched by exactly 2 centimetres all the time. The block has an acceleration of 1 unit. acceleration = 1 unit 2 cm pull (a) He then repeats this experiment on the Moon. The Moon s gravity is only one-sixth as strong as on Earth. As before, he keeps the spring stretched by exactly 2 centimetres all the time. What will the acceleration of the block be now? Tick ONE box () More than 1 unit Exactly 1 unit again Less than 1 unit (b) How would you explain this? Tick ONE box () The weight of the box is less than on Earth. The weight of the box is the same as on Earth. The mass of the box is less than on Earth. The mass of the box is the same as on Earth. It takes a bigger force to make things move on the Moon. 180 University of York 2003

21 15 An astronaut tries out an experiment on Earth before setting off on a mission. He uses a spring to pull a block along a smooth level surface. As he pulls, he keeps the spring stretched by exactly 2 centimetres all the time. The block has an acceleration of 1 unit. acceleration = 1 unit 2 cm pull (a) He then repeats this experiment in his spacecraft while it is in Earth orbit and everything is weightless. As before, he keeps the spring stretched by exactly 2 centimetres all the time. What will the acceleration of the block be now? Tick ONE box () More than 1 unit Exactly 1 unit again Less than 1 unit (b) How would you explain this? Tick ONE box () The box now has no weight. The mass of the box is the same as on Earth. In space, it takes a bigger force to make things move. University of York

22 16 An astronaut goes for a moon walk. Look at each of the sentences below. For each sentence, put a tick () in one box to show if you think it is true or false. a b c d e There is no gravity on the moon, because it has no atmosphere. So he feels weightless. He can jump up and down easily, because gravity on the moon is only one-sixth as strong as on Earth. The force of gravity on the astronaut is bigger than on Earth. If the astronaut drops a spanner he is carrying, it doesn t fall. It just floats beside him. It is much easier to start and stop a Moon buggy moving than on Earth, because its weight is less. True False Don t know 182 University of York 2003

23 17 An astronaut in a spacecraft in orbit around the Earth experiences weightlessness. She can float around inside the spacecraft, and other objects in the cabin also float around. Earth Look at each of the sentences below. For each sentence, put a tick () in one box to show if you think it is true or false. a b c d e f g She feels weightless because the spacecraft is far away from the Earth, beyond the range of the Earth s gravity. She feels weightless because the spacecraft is outside the Earth s atmosphere, where there is no gravity. The force of gravity on the astronaut is bigger than on the Earth, because she is higher up. There is a force of gravity on the astronaut, but it is smaller than on the Earth. The astronaut feels weightless because the spacecraft s orbit is equivalent to freefall, and everything inside it is falling with it. The astronaut is moving in a circle, so the centrifugal force balances gravity and makes her feel weightless. The Earth s gravity only affects the spacecraft, so things inside it feel weightless. True False Don t know University of York

24 18 Dropping two pebbles Some pupils are dropping pebbles from a bridge into a stream. They have two pebbles, one twice as heavy as the other. They are discussing how much time it will take for the pebbles to reach the water. The heavy pebble will fall in half the time of the light one. The heavy pebble will take a lot less time than the light one, but I don t think it will be exactly half. The smaller pebble will fall faster, because it cuts through the air more easily. I think both pebbles will hit the water together. 184 University of York 2003

25 Dropping two pebbles What do you think? Tick a box in each row to show if you agree or disagree with each of the pupils: Alex The heavy pebble will fall in half the time of the light one. Agree Disagree Not sure Ben Chris Dana The heavy pebble will take a lot less time than the light one, but I don t think it will be exactly half. The smaller pebble will fall faster, because it cuts through the air more easily. I think both pebbles will hit the water together. Please explain your answer in the space below: University of York

26 19 Can the weight of an object change? Sam puts a box on a top pan balance. It reads 225 g. If she took it up to the top floor of a very tall building, would it still have the same reading? What do you think? It is getting further from the Earth, so the force of gravity will be weaker. It will weigh less. I think it will change, but the difference will be too small to see. The pull of gravity on the box will get bigger because the box is higher up. So it will weigh more. It will stay the same weight, because it is still the same box. 186 University of York 2003

27 Can the weight of an object change? What do you think? Tick a box in each row to show if you agree or disagree with each of the pupils: Adrian Betty Charlene Dee It is getting further from the Earth, so the force of gravity will be weaker. It will weigh less. I think it will change, but the difference will be too small to see. The pull of gravity on the box will get bigger because the box is higher up. So it will weigh more. It will stay the same weight, because it is still the same box. Agree Disagree Not sure Please explain your answer in the space below: University of York

28 Answers and discussion Gravity and freefall In the discussion below, answer options are referred to by number (1, 2, 3 ), in the order in which they appear in the question. 1 7 (In positions 1, 2 and 3) 2 (a) 4 (b) 1 3 (a) 3 (b) 3 4 (a) 3 (b) 3 5 (a) A (b) Two reasonably heavy, compact objects fall with the same acceleration. So the two balls will stay side by side as they fall. 6 (a) false (Two compact objects of different mass fall with the same acceleration.) (b) true (c) false (d) false (The air resistance force on an object depends on the size and shape of the object and the speed it is moving at. These are the same shape and size, and the golf ball is moving slightly faster. So the air resistance force on the table-tennis ball is not larger but see part (f) below.) (e) false (This would be true of they were both moving at the same speed. Here the golf ball is moving slightly faster (it has fallen further) so the air resistance force on it is slightly bigger.) (f) true (This is more important for the motion than the absolute size of the air resistance force (part (d).) 7 (a) 1 (b) 4 Part (b) here requires some care. Option 3 may be tempting for some students, but whilst this is true if the two objects are travelling at the same speed, it is not generally true. 8 The correct statements are: 1, 2, 3a, 4a, 5, 6b, 7b, 8a, 9a, 10a 9 The correct statements are: 1, 2, 3a, 4a, 5, 6b, 7b, 8b, 9b, 10b If students are able to select the correct statements for both Q8 and 9, this indicates a good understand of the forces acting on falling objects. 188 University of York 2003

29 10 (a) force exerted by the Earth (gravity) force exerted by the Earth (gravity) (b) air resistance air resistance force exerted by the Earth (gravity) force exerted by the Earth (gravity) 11 (a) 2 (b) 2 Option 4 for (b) is too general a statement. Over the short distance involved in this question, the weight of the box is the same. But over a larger distance (see Q12), it is not true. 12 (a) 3 (b) 2 Some students might argue that options 2/3 are correct. It would depend on the height of the mountain, and the accuracy to which the length of the spring can be measured. 13 (a) 3 (b) 2 14 (a) 2 (b) 4 It is mass, not weight, that affects the acceleration. Mass does not change, wherever an object is moved to. 15 (a) 2 (b) 2 See Q (a) false (There is no atmosphere on the moon, but this does not make the gravity force zero.) (b) true (c) false (It is roughly one-sixth as big.) (d) false (It falls, with an acceleration about one-sixth of that on Earth.) University of York

30 (e) false (The force needed to start and stop things depends on their mass, not their weight.) 17 (a) false (The force exerted by the Earth on the astronaut due to gravity gets smaller as her distance from the Earth gets bigger. But it does not become zero.) (b) false (The feeling of weightlessness has nothing to do with being outside the Earth s atmosphere. The Earth s gravity continues to act outside the atmosphere.) (c) false (It is smaller, as she is further away.) (d) true (e) true (This is the best way to think of apparent weightlessness due to falling along with the container you are in. In orbit, the rate of change of speed towards the Earth is the same as if falling straight downwards.) (f) don t know (This statement is not a good way to explain the situation, but some people might argue that it is correct, provided you really know what you mean by centrifugal force. Someone inside the spacecraft, who didn t know they were moving in a circle, could explain their observations by arguing that two forces are acting on them: a gravity force towards the Earth and a second force away from the Earth, which is sometimes called the centrifugal force. Someone looking at the situation from outside, however, does not need to invent this extra imaginary force acting on the spacecraft, and can explain their observations without it. There is a steady force of gravity on the spacecraft, which means that there is a resultant force acting towards the centre of its orbit (a centripetal force), not away from it. The force of gravity keeps the spacecraft moving in a circle rather than moving off at a tangent, into space, which would happen if there were no forces acting on it. It is the effect of this constant acceleration due to gravity which makes the orbit equivalent to freefall ) (g) false 18 Dana is right about what happens here. In practice, the lighter pebble might take a little longer to reach the water. But the difference will be very small. 19 Adrian is right that the force of gravity on the box gets smaller as it is moved further from the centre of the Earth. But, as Betty says, the difference will be too small to notice. Dee may be confusing weight with mass, which doesn t change. Charlene may be mixing up ideas about potential energy (which gets bigger as an object is raised) and weight. 190 University of York 2003

31 Acknowledgements Some of the questions in this pack are based on probes used by science education researchers to explore learners understanding of key ideas about forces and motion. These have been modified, often quite substantially, to improve their clarity or to make them easier to use in class. Others are new questions, written for EPSE Project 1: Using Diagnostic Assessment to Enhance Teaching and Learning in Science. Questions which are knowingly based on previous probes are listed below, with an indication of the original source. If precursors of any questions in this pack are not acknowledged below, we would be grateful to have this pointed out, so that it can be rectified in any subsequent publication. Q1 Q2 Q4 Q10 Based on a question from the TIMSS survey IEA (1994). TIMSS Population 2 Item Pool. The Hague: IEA. Based on question 3 in the revised version of the Force Concept Inventory (FCI) (Halloun, I., Hake, R., Mosca, E. & Hestenes, D. 1995), reproduced in: Mazur, E. (1997). Peer Instruction (pp ). Upper Saddle River NJ: Prentice Hall. Based on question 1 in the Force Concept Inventory (FCI): Hestenes, D., Wells, M. & Swackhamer, G. (1992). Force Concept Inventory. The Physics Teacher, 30, The format of this question is based on an idea developed by Pam Mulhall and Brian McKittrick at Monash University. They refer to items of this format as Conceptual Understanding Procedures (CUPs). A selection can be downloaded from URL: Q18-19 These are new items, but are based on the Concept Cartoon format developed by Stuart Naylor and Brenda Keogh: Naylor, S. & Keogh, B. (2000). Concept Cartoons in Science Education. Sandbach: Millgate House Publishers. University of York

32 192 University of York 2003