Practice Midterm Exam 1 Physics 14

Transcription

1 Booklet Number Practice Midterm Exam 1 Physics 14 Last Name First Name To get a full credit show the all calculations steps in the spaces provided. All work must be shown in order to receive FULL credit. 1. Shown below is a speed-time graph for a cart moving in front of the motion sensor. For convenience it has been divided into five sections (A,B,C,D,E). B C D A E During each of the five separate periods shown on the graph (A, B, C, D, E), was the combination of forces acting on the cart balanced or unbalanced? If unbalanced, did the strongest force act in the same direction as the cart s motion, or against it. Briefly explain how you can tell this from the graph? A The cart is at rest (constant speed of zero). The forces acting on it must be balanced for it to remain at rest. B The speed of the cart is increasing. For this to happen the forces must be unbalanced, with the strongest force in the same direction as the motion. C The speed of the cart is decreasing. For this to happen the forces must be unbalanced, with the strongest force in the opposite direction to the motion. D The speed of the cart is still decreasing. For this to happen the forces must be unbalanced, with the strongest force in the opposite direction to the motion.

2 E The cart has stopped and is at rest (constant speed of zero). The forces acting on it must be balanced for it to remain at rest 2. A child playing with a toy car gives it a quick shove on a smooth level floor. (The car does not have any type of motor inside it.) After his push, the car very gradually slows down and stops. Four students are discussing why the car very gradually slows down and stops, after the shove. The car slows down because the force pushing it forward is getting weaker and weaker. It slows because, after the shove, there is no more force to Kristen It slows down because the forces acting on it are balanced, and balanced forces make a moving object come to rest. Daryl The car slows down because there is a force acting on it in the opposite direction to its motion. Samantha Victor Which student do you agree with (if any)? Please explain your reasoning.

3 I agree with Victor. He has an idea that is consistent with those we developed in class; that objects slow and stop because a force (or unbalanced combination of forces) acts on them in a direction opposite to their motion. Kristen s idea is not consistent with our class ideas. She seems to be saying that there is still a force pushing the car forward after the initial shove, but our idea was that the force of the shove is gone as soon as the hand loses contact. Her idea that the car slows down because the forward force gets weaker is also inconsistent with our class ideas. We said objects slow down because stop because a force (or unbalanced combination of forces) on them in a direction opposite to their motion. Daryl s idea might be OK, but without more information it is impossible to tell. He implies that for the car to keep moving a force is needed in the direction of motion. If he is accounting for the effects of the friction force that slows the car down, he is correct, but he doesn t say that. On the other hand his thinking could be something like Kristen s, in that he thinks that for any motion to continue a continuous forward force is needed, even in the absence of friction, and without such a forward force it is just natural for the car to stop. This is not consistent with our class ideas. Samantha thinks that if balanced forces act on a moving object, then it will slow and stop. However, in class we found out that when balanced forces act on an object its speed will remain constant. So if balanced forces acted on the car after the initial shove then it would not slow down, but continue at a constant speed. 3. In the left column below there are three situations where a cart is acting on by more than one force. The direction and strength of each force is shown. In the second column state whether the forces acting on that cart are balanced or unbalanced. If balanced, leave the third column blank. If unbalanced, in the third column draw the net force on the cart (the single equivalent force) and make sure you include its direction and strength in N. (a) Object with forces acting on it Are forces balanced or unbalanced? If unbalanced, draw the net force 7 N Unbalanced

4 (b) Balanced (c) Unbalanced 8 N 4. A man wants to move a large box across a level floor by pulling on a rope attached to the box. The floor is not perfectly smooth so friction acts between the floor and the box. Below are force diagrams representing four possible arrangements of forces that could be acting on the block after it has already started moving to the right. The situations are also described briefly in words. A. The man s pull to the right is stronger than the force of friction opposing the motion. B. The man does not pull at all. Friction opposes the motion. C. The man pulls to the right with a force strength that is exactly the same as that of the frictional force. D. The man s pull to the right is weaker than the force of friction opposing the motion.

5 a) Indicate all the situations shown above (if any) that you think would result in the block moving to the right at a constant speed (after it has already been started moving.) Briefly explain the reasoning behind your choices. The diagrams all show the block moving to the right. Since it is already in motion any unbalanced combination of forces would result in a change in speed. Only a balanced combination of forces would result in motion at a constant speed. So ONLY situation C would work. b) For those situations that you did not select in part a), what do you think the motion of the cart would be like? Again, explain your reasoning. A would increase in speed because the forces are unbalanced with the stronger force in the same direction as the motion B and D would both slow down (and eventually stop) because the forces are unbalanced, with the strongest (or only) force in a direction opposite to the motion. 5. A small child wants to move his box of toys across the room. The floor is carpeted, so there is obviously friction. Create an explanation for why the box begins to move as the child pushes it. Draw the Force Diagram: Force exerted on box by child Frictional force exerted on box by floor Write the explanation: For an object at rest to start moving an unbalanced combination of forces must act on it. Therefore, if the box in question does start to move this must mean that the strength of the pushing force must be stronger than that of the force opposing its motion. So the box starts to move because the strength of the child s push is stronger than that of the frictional force exerted on it by the floor. 6. The famous chef, Antonio, is making pizza. He shapes the dough for the base into a large round, flat shape, and then tosses it into the air. It rises about three feet before falling back down, where he catches it.

6 Which of the following diagrams best represents the forces acting on the pizza after it has reached its highest point and has begun falling back down. Circle your choice. A. B. C. D. Explain your reasoning. Diagram B is correct. The gravitational force of the Earth pulls downward on the pizza and all the diagrams show that. However, after the pizza has left Antonio s hands the force of his hands no longer acts, so both A and C can be eliminated. Like friction, the force of air resistance always opposes the motion of objects and since the pizza is moving downward, this force must be pushing upward on the pizza. Thus gravity acts downward and air resistance acts upward, so the correct diagram is B. 7. Use the parallelogram rule to carefully construct the resultants of these pairs of vectors.

7 Answers: The heavy ball is supported by two strands of rope. The tension for each tension strand is shown the vectors. Use a parallelogram rule to find a resultant. Answer: 8. A soccer goalie is practicing by punting a ball straight up into the air and then catching it again when it falls back down. Consider a moment just after the ball has been kicked, but is still moving upward (as shown in the picture). Which of the following forces do you think are acting on the soccer ball at this moment? (Choose all those that you think are present.)

8 a) A force pushing upward due to the motion of the ball. b) A force of gravity pulling downward. c) A force from the kick pushing upward. d) A force of gravity pushing upward. e) Some other force (describe what you think it is below) Briefly explain the reasoning behind your choice(s). As soon as the foot loses contact with the ball the force of the foot is gone. As the ball moves upward, the force of gravity is pulling downward on it, that s why it slows down as it is rising. (The force of air resistance is also acting on the ball, opposing its upward motion.) 9. A small child attempts to push a box full of toys across the floor in his playroom. However, despite pushing as hard as he can, the box does not move. Which one of the following statements best describes the reason the box does not move while he is pushing it. c) The force resisting moving the box is greater in strength than the child s push. d) The child weighs less than the box does. e) The force resisting moving the box is equal in strength to the child s push. f) The strength of the child s push is greater than the strength of the force resisting moving the box, but not great enough to move it. Please explain your reasoning If an object is at rest and remains at rest, then the forces acting on it must be balanced. This means the force of the boy and the force resisting him must be equal. (Note: If a) were true the box would start to move in the opposite direction to the child s push!)

9 10. Momentum and momentum conservation