7K Forces and their Effects

Transcription

1 7K Forces and their Effects 0

2 Forces A force is a pull or a push. It is measured in Newton s. Complete the following sentences: 1. Push and pull are both words used to describe a _. When things touch you get a of forces. When the legs of a stool touch the floor there is a force on the floor from the. There is a force on the legs from the. 2. The forces on the diagram are shown with. The size of the force is shown by the of the arrow. The direction of the force is shown by the direction of the arrow. The size of a force is measured in. 1

3 Force-meters measure force A force-meter measures the size of a force in Newton s. Look at the drawings of the blocks on the force-meters. 1. Complete this table: Block A B C D E F Pull of block on force-meter 2. Which force-meter can measure the largest force? 3. Which force-meter would be the best one to measure a force of just under 1N as accurately as possible? Give a reason for your answer. 2

4 All about forces A force is a push or a pull measured in Newton s. Forces occur in many places, for example between magnets. A force-meter measures the size of a force. A force-meter has a spring inside it. Write the answers to the clues in the correct places on the grid. Clues down 1 It stretches inside a force-meter 4 Two things do this when there is a pair of forces pulling them together 5 The unit we use to measure the size of a force Clues across 2 A metal that is attracted to a magnet 3 Something that will attract steel but not brass 6 The north poles of two magnets do this to each other 7 The name for a push or a pull 3

5 Marking forces A force can be shown by an arrow that points in the direction in which the force acts. 1. Complete the diagrams by adding the missing forces 2. For each force you have marked, write a sentence stating what effect you think the force 4

6 Combining Forces A force can be shown by an arrow that points in the direction in which the force acts. You can represent a force by an arrow. The direction of the arrow shows the direction of the force. The length of the arrow represents the size of the force. If you draw arrows to scale, you can use them to work out how forces combine. The combined effect of two forces is called their resultant. Look at these examples. Use the following grid to work out the resultant effect of these forces. a) 3 N pointing left and 4 N pointing left b) 4 N pointing left and 3 N pointing right c) 7 N pointing up and 4 N pointing up d) 6 N pointing up and 4 N pointing down e) 6 N pointing left and 2 N pointing left f) 3 N pointing right and 3 N pointing left g) 4 N pointing up and 5 N pointing down h) 4 N pointing up and 4 N pointing down 5

7 Weight and mass Weight is a force, mass is a measure of how much material there is in something. 1. Write the words force and Newton s on the second map where you think they fit best. 2. The table gives some information about some everyday objects. Fill in the missing spaces. The first one has been done for you. Use this key idea: 6

8 Comparing weight and mass 1. The table below gives some information about some everyday objects. Fill in the missing spaces in the table. Use these key ideas. 1 Kg has a weight of 10 N 100 gm has a weight of 1 N Object Mass Weight Bag of flour 30 N Small tin of beans Medium sized apple 250 gm 100 gm Bottle of juice 10 N Full bottle of milk Full pencil case Supermarket shopping bag full of shopping Cast iron frying pan Mug of tea Notebook computer 800 gm 18 Kg 1.3 Kg 1.8 Kg 3 N 5 N 2. For each of the statements in the table below, write True or False in the last column. Statement True or False A bag with a mass of 200 gm has a weight of 2 N A bag with a weight of 1 N has a mass of 100 gm A bag with a weight of 3 N has a mass of 400 gm To make a mass of 500 gm you could add a 200 gm bag to a 300 gm bag When you multiply by 100 you get the mass in grams 7

9 Weight thought-map Use words from the map to complete the sentences below. Weight is a. Weight is measured in. Weight is caused by gravity. Gravity is a pull force that happens because things that have a mass attract other things with. On the Earth the force of gravity pulls a 1 Kg mass down with a force of about. Gravity is a small force that attracts all towards each other. We only notice it with big masses like the. The weight of a mass varies from place to place. It is large near large masses like the planets. It is a lot smaller near smaller masses like our. 8

10 Newton s and grams Follow these instructions Step 1 Check the mass hanger on a top pan balance Step 2 Measure its weight as accurately as you can, using a force-meter Step 3 Record the readings in the table Step 4 Add a slotted mass to the hanger Step 5 Repeat the mass check and weight measurement. Write the readings in the table Step 6 Repeat the instruction steps until the table is complete Mass (Hanger) = 50 gm = mass of 1 slotted mass Mass in grams Weight in Newton s Mass hanger alone Mass hanger plus a slotted mass Mass hanger plus 2 slotted masses Mass hanger plus 3 slotted masses Mass hanger plus 4 slotted masses Mass hanger plus 5 slotted masses Step 6 Plot a graph of weight in Newton s (Y-axis) against mass in grams (X-axis) Step 8 Draw a line of best fit that goes through the origin 1. Describe the relationship between the mass in grams and the weight in Newton s as fully as you can 2. Why might some of your measurements give points that are not in the line of best fit? 3. Do you think that the statement The mass in grams is 100 times the weight in Newton s is true? 9

11 Springs and forces The length of a spring can be used to measure a force. 1. Write down the length of the spring in each diagram. 2. Fill in the gaps in the sentences: When you put a weight on the end of a spring it gets. Every time 1 N is added to the spring it gets If 12 N were added to the spring it would be cm longer. cm longer. 10

12 Springs and forces 1. What is the length of the spring: a) When it has 1 N on it? b) When it has 2 N on it? 2. How much longer does the spring get: a) When the weight changes form 2 N to 3 N? b) Each time 1 N is added? 3. How long would you expect the spring to be if it had 6 N on it? Explain how you worked out your answer. 4. How long would you expect the spring to be if it had 4.5 N on it? Explain how you worked out your answer. 5. What weight do you think would be on the spring if the spring was 14 cm long? Explain how you worked out your answer. 11

13 All about upthrust The name for the force on a floating object is upthrust Write words that answer the clues into the grid. Clues across 1 You measure the size of an upthrust in this unit 2 What objects do if their weights are equal to their upthrust 3 What happens to an object if the weight is bigger than the upthrust Clues down 4 The upthrust and the weight are when an object floats 5 The force due to gravity that pulls down on something 6 the force upwards on something because it is in a liquid 12

14 Upthrust and floating An object floats when the upthrust equals its weight. 1. What happens to the reading on force-meter when the block of metal is moved into the water? 2. Why do you think the change in the reading happens? 3. What is the reading on the force-meter for the block of wood when it is floating? 4. What is the weight of the wooden block? 5. What provides the force that keeps the wooden block up in the air when it is hanging from the force-meter? 6. What provides the force that cancels out the weight of the wood when it is floating? 7. What is the size of the upthrust from the water on the wooden block when it is floating? 8. What would happen to the size of the upthrust on the block if you dissolved a lot of sugar in the water? Give a reason for your answer. 9. How would the reading on the force-meter change if sugar was dissolved in the water? 13

15 Density and floating An object floats if it is less dense than the liquid. Densities of some substances are shown in the table. Substance Ice (solid) 0.92 Pine (wood used for making furniture, solid) 0.5 Lignum vitae (wood used for making bowls, solid) 1.3 Iron (solid) 7.0 Cork (solid) 0.2 Beeswax (solid) 0.95 Water (liquid) 1.0 Mercury (liquid) 13.6 Gold (pure, solid) 19.3 Gold (22 carat, solid) 17.5 Gold (9 carat, solid) 11.3 Olive oil (liquid) Which of the solids in the table will float in water? 2. Which of the solids will float in water but sink in olive oil? 3. Why will an iron nail float on mercury? Density in grams per cubic centimetre 4. 9 carat gold has a lower density than 22 carat gold. Rings made from 9 carat gold are cheaper to buy than rings made from 22 carat gold. Suggest a reason why. Archimedes was a scientist who lived in Greece form 287 BC to about 220 BC. He was given the problem of finding out if the king s crown was made of pure gold. He used the method shown to find the volume of the crown. He found the mass of the crown by weighing it. 5. How could he use his two measurements to check if the crown was pure gold? 14

16 What is friction? Friction is a force that opposes motion. For each statement, write in its box whether it is True or False. Statement True or False 1. Friction is needed between a match and the box so it will light. 2. If there were no friction between your shoes and the floor, you could not walk. 3. You need friction between water and a boat for it to float. 4. The friction force on a parachute is called drag. 5. The drag on a car can be made less by streamlining. 6. If you increase the drag on a car, it will go faster. 7. If there were zero friction on your shoelaces, you could not keep them tied in a knot. 8. If there were no friction between wood and steel, nails and screws would just drop out and the things they were holding together would fall apart. 9. If there were no friction between the fibres that make your clothes, then they would all fall apart. 15

17 All about friction Friction is a force that opposes motion 1. What is produced when the person pushes back on the ground with their foot? 2. In which direction, forwards or backwards, does the friction force act on the person? 3. What does this forward force do to the person and the buggy? 4. What is the direction of the friction force on the person when they go downhill? 5. Why is this friction force useful? 6. What do you think might happen if the ground were covered in ice? Give a reason for your answer. 7. Complete the following sentences: Friction is a force that happens when two surfaces friction force is in the the friction between two surfaces is to use through water or air the friction force is called. across each other. The direction to the movement. One way of reducing. When something moves 16

18 Coefficient of friction Friction is a force that opposes motion. Scientists use a number called the coefficient of friction to measure how big the friction force is between surfaces. The smaller the coefficient, the smaller the friction force. The table gives some examples: Materials Conditions Coefficient of friction Wood on wood Clean and dry Wood on wood Wet 0.2 Steel on steel Clean 0.58 Steel on steel Oiled 0.2 Patterned rubber tyre on road Dry 0.7 Patterned rubber tyre on road Wet 0.4 Smooth rubber tyre on road Dry 0.9 Smooth rubber tyre on road Wet 0.1 Waxed ski on dry snow Teflon on steel Clean and dry What do you notice about the way different types of tyre grip the road in different conditions? 2. Suggest why Formula One racing cars use smooth tyres with no tread if the conditions are dry? 3. Smooth tyres with no tread give a better grip than patterned tyres on a dry road. Why do you think such tyres are illegal on normal cars driving on the roads? 4. What is the effect of oiling the moving parts in an engine made from steel? 5. Why do you think Teflon is referred to as a non-stick substance? 6. Why do think the coefficient of friction varies so much for dry wood on dry wood? 7. What do you notice about the coefficient of friction for waxed skis compared to the coefficient of friction for Teflon? Why is such a low coefficient a good thing when skiing? 17

19 Investigating friction Friction is a force that opposes motion. You are going to investigate the friction between a box and a wooden surface. Method: Step 1 Set up the equipment as shown in the diagram Box letter Length of side Area of base A 1 cm 1 cm 2 B 2cm 4 cm 2 C 3cm 9 cm 2 D 4cm 16 cm 2 E 5cm 25 cm 2 Step 2 For each of the boxes provided, find the angle of slope where it just slides down the board Step 3 Repeat the measurement five times for each box and take an average for the angle of the slope. Step 4 Draw a results table and record your findings in it 1. What is the name of the force that stops the box sliding? 2. What is the connection between the size of the friction force and the angle of the slope at which the box slides? 3. Do your results show that the area of the box has an effect on the size of the friction force? What is the connection? 4. Why is it important that all the boxes have the same mass? 5. How could you use the same equipment to see if the mass in a box has an effect on the friction force? 18

20 Investigating drag Objects fall because gravity pulls them down. A force called drag from the air slows down how quickly an object falls. You can change the drag of the air on a falling bun case by spreading the case out. Method: Step 1 Set up the equipment as shown in the picture Step 2 Measure the diameter of the bun case Step 3 Record the diameter in the results table below Step 4 Drop the bun case from 1.5 m and time its fall Step 5 Do this for three drops and record your results in the table Step 6 Work out an average time Step 7 Spread the bun case a little bit to give a bigger diameter Step 8 Repeat steps 2 to 6 Step 9 Spread the bun case as much as you can without totally flattening it Step 10 Repeat steps 2 to 6 Diameter of bun case Time to fall 1.5 m Average 1. What is the name of the force which pulls the bun case downwards? 2. What is the name of the force from the air which makes the bun case take longer to fall? 3. What was the diameter of the bun case when it took the longest time to reach the ground? 4. Is the drag on the bun case bigger or smaller when it is spread out? 19

21 Stopping distances Use sentences from the bottom of the sheet that best fit in the correct columns. Things that might increase your stopping distance Things that might make your stopping distance as good as it can be Bald tyres Icy road Driving in the morning after a good night s sleep Driving when tired Using a mobile phone when driving New tyres Concentrating on the road ahead Driving under the influence of drink Going downhill Going uphill 20

22 What affects stopping distance? The stopping distance for a car depends on several different things. Eric and Sonja are arguing over what affects how fast a car can stop. Eric thinks it all depends on how good the driver is. Sonja thinks that there are lots of possible things that might affect it. She has started a list. 1. Say whether you agree with each thing on Sonja s list and explain why. 2. Write down any things you think might be important that are not on Sonja s list. Explain why you would add them. 3. What do you think about Eric saying It all depends on how good the driver is? 21

23 Reading distance/ time graphs You can use a graph to show how distance changes on a journey. The graph shows a journey on a bike. Below are some descriptions of part of the journey. Match each description to a section of the line and write the correct letter on the graph. A B C D E The rider starts his journey The rider cycles quickly home again The rider continues on his journey The rider realises he is late, stops and turns the bike around The rider stops for 2 minutes to talk to a friend 22