Practice Test 1 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 B C D E
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 keep it moving. 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. 2
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 (b) (c) 3
4. Five identical crates are initially at rest, side-by-side. One is acted on by a single force of 45 N, as shown. 45 N Which of the other crates, shown below, would move side-by-side with the crate described above? (Choose as many as you think appropriate.) a) b) 25 N 10 N 50 N 25 N 10 N 40 N 20 N c) 75 N 30 N d) 5 N 60 N Briefly explain your choice(s). 5. Block A, shown below, is moving to the right. At the moment shown, three different forces are acting on the block. The strengths and directions of the three forces are shown in the force diagram. a) Will block A speed up, slow down, or move at constant speed? Briefly explain how you know. b) Suppose there was another block (B) that was identical in size and mass to block A. At the same moment shown above, block B is alongside block A and is moving at the same speed as block A. What single force should be applied to block B, so it continues to move side-by-side with block A? 4
Draw this force (strength and direction) on the block below, and briefly explain how you know. 5
6. A hockey player uses his stick to maintain a continuous constant strength push on the puck as he moves it across the smooth ice. Assuming that the effects of friction are negligible, which of the following choices best describes the motion of the puck while this constant strength push is acting on it? a) The speed of the puck will continuously decrease. b) The puck will move at a constant speed. c) The speed of the puck will continuously increase. d) Something else you describe it. Briefly explain the reasoning behind your choice (remember to include a description of the motion if you chose option d). 6
7. The child in the previous question calls for help and his mother comes and helps him push the box, in the same direction. Which one of the following statements best describes why the box now begins to move. a) The combined push of the mother and child is equal in strength to the force resisting the movement of the box. b) The mother weighs more than the box does. c) The combined push of the mother and child is greater in strength than the force resisting the movement of the box. d) The strength of the mother s push alone is greater in strength than the force resisting the movement of the box. Please explain your reasoning 8. After starting the box moving the mother and child in the previous question want to make the box move across the floor at a certain constant speed. Which one of the following statements best describes what they should do to achieve this? a) They should keep pushing with the same strength force they used to start the box moving. b) They should keep increasing the strength of their push until the box is moving at the speed they want, then keep it at that strength. c) When the box reaches the speed they want they should reduce the strength of their push to exactly match the strength of the resisting force. d) When the box reaches the speed they want they should stop pushing. Please explain your reasoning 7
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9. A ball moves in a curved path inside of a curved tube. The picture to the right shows the ball inside the tube at three different times (times 1, 2 and 3). The arrows represent the speed of the ball at those three times. At time 2 is there an unbalanced force acting on the ball? If yes, add an arrow to the picture to show the approximate direction of the unbalanced force, and briefly justify your answer. If no, explain why not. 10. A soccer ball is rolling across the ground. A child briefly taps the ball with her foot at right angles to its direction of motion. A force diagram for the interval of time that the child s foot is in contact with the ball is show below to the left (top view). During Tap After Tap In the picture to the right, draw a motion arrow showing how the ball will be moving after the tap. Pay attention to both the length and direction of the motion arrow. Briefly explain why you drew it this way. 9
11. A child s toy car moves at a constant speed round a flat circular track. (A top view is shown to the right.) A section of the track is missing, as shown, and the car leaves the track and runs across the floor. Which line in the diagram best represents the path the car will follow when it leaves the track? Briefly explain the reasoning behind your choice. D A C B 10
12. Imagine you had two different fan units. Fan #1 had more batteries than fan #2. Fan #1 can push the cart with a force of 20 N. Fan #2 can push with a force of 5 N. Suppose you start an experiment by mounting Fan #1on a cart and release the cart from rest on a very smooth track. You can ignore the effects of friction throughout this problem. Below is a speed time graph for the motion of fan #1 pushing the cart. a) Suppose you replaced fan #1 with fan #2 (the weaker one) and mounted it by itself on the same cart. When the cart is released from rest, you observe that it speeds up. Sketch what the speed-time graph for Fan #2 might look like on the graph above. Label it Fan #2. Below, briefly explain why you drew the graph the way that you did. 11
b) Now suppose you mount both fans on the same cart, but with the two fans trying to push the cart in opposite directions. When you release this cart from rest you observe that it speeds up. Sketch what the speedtime graph for the combined fans might look like on the same graph on the previous page. Label it Fans #1 & #2. Below, briefly explain why you drew the graph the way that you did. c) Below, draw a force diagram for the cart with the two fan units on it while the cart is speeding up. (Remember to include all relevant forces, label them appropriately, and also include a speed arrow if appropriate.) 12