Exam 2 Review
Lab Analysis Question
Two students are dropping marbles of various mass through the air and using a photogate to measure the marble’s terminal velocity. The data they have recorded is shown here. Analyze this data and draw a conclusion about the force of air resistance on the marbles as they fall.
Paragraphs
1. Suppose you are holding a heavy board vertically and it begins to slip from your hand. You try to keep from dropping the board by gripping tighter. Use physics to explain why this lets you hold on to the board.
2. You will hopefully never be in a head-on automobile collision. If you were, however, use Newton’s Laws to explain why the airbag would protect you from serious injury.
Problem Solving
3. A 4-kg crate and 6-kg box are attached to each other by a rope and sitting on a frozen lake. You pick up a chain and pull the 6-kg box across the ice with a force of 30 N. Find the acceleration of the box and the tension in the rope (a) ignoring friction and (b) assuming a coefficient of friction of 0.2.
4. A 90-kg astronaut floating out in space is carrying a 1-kg TV camera and a 10-kg battery pack. She is drifting toward her ship, but in order to get back faster, she hurls the camera out into space at 15 m/s and then throws the battery at 10 m/s in the same direction. What is the resulting increase in her speed after each throw?
5. A skydiver of mass 100 kg opens his parachute when he is going at 25 m/s. The parachute experiences 1200 N of air resistance. How fast will the skydiver be falling 6 seconds after opening the chute?
Multiple Choice
6. A 4.0 kg rock and a 1.0 kg stone fall freely from rest from a height of 100 meters. After they have fallen for 2.0 seconds, the ratio of the rock’s speed to the stone’s speed is:
a) 1:1
b) 2:1
c) 1:2
d) 4:1
7. Compared to the inertia of a 0.10 kg steel ball, the inertia of a 0.20 kg Styrofoam ball is:
a) ½ as great
b) the same
c) 2x as great
d) 4x as great
8. A 60 kg astronaut weighs 96 newtons on the surface of the Moon. The acceleration due to gravity on the Moon is
a) 0.0 m/s/s
b) 4.9 m/s/s
c) 1.6 m/s/s
d) 9.8 m/s/s
9. From how high (in meters) must a car fall if it is to have the same momentum it would have driving at 30 m/s?
a) 3 m
b) 10 m
c) 30 m
d) 45 m
e) 60 m
10. In order to catch a ball, a baseball player moves his or her hand backward in the direction of the ball’s motion. Doing this reduces the force of impact on the player’s hand principally because
a) the velocity of impact is reduced.
b) the momentum of impact is reduced.
c) the time of impact is increased.
d) the time of impact is decreased.
e) none of the above.
Additional Problems
11. James Bond uses his secret ejector seat button to bail out of his Aston-Martin convertible as it is moving at 25 m/s toward the solid cliffs of Montenegro. The 1200 kg sports car is stopped by the cliffs in 0.12 seconds. What was the force that the cliffs put on the expensive custom-built sports car before it became a twisted pile of metal?
12. Blofeld, whose mass is 75 kg, jumps from his retreat house’s balcony (2 meters high) just as Bond manages to blow the house up. Foolishly, the evil "genius" holds his body perfectly stiff so that his body strikes the ground like a spear. He jolts to a stop in only 0.016 seconds. The force bringing Blofeld to a stop is split between each leg. A tibia can withstand a shock of 13200 N without breaking. Can the criminal mastermind run away on healthy legs, or can he do nothing but fall to the ground and wait for Felix Lighter to come arrest him?
13. Imagine the typical wedge-shaped door stopper. Use physics to explain why the wedge shape works to stop the door while a block would not.
14. A 300 kg motorboat is turned off as it approaches a dock and it coasts in toward the dock at 0.5 m/s. Isaac, whose mass is 62.0 kg, jumps off the front of the boat with a speed of 3.0 m/s relative to the water. What is the velocity of the boat after Isaac jumps?
15. A 620 kg zombie moose stands in the middle of the railroad tracks, frozen by the lights of an oncoming 10000 kg locomotive that is traveling at 10 m/s. The train hits the zombie moose and the moose rides down the track sitting on the cowcatcher. What is the new combined velocity of the train and zombie moose?
16. Gilligan has climbed a 30 m coconut tree to harvest its fruit. The tree is 20 m from the far side of a deep ravine. Gilligan wants to throw the coconuts over the ravine so he can pick them up on the other side.
(a) With what horizontal velocity does Gilligan need to throw the coconuts in order to clear the ravine?
(b) Gilligan’s arm can apply a force of 40 N to the 2 kg coconuts for a total time of 0.5 seconds. Can Gilligan give the coconuts the required velocity?
17. A bulldozer slides a large mound of dirt (3500 kg) across a concrete driveway. The coefficient of friction between the dirt and the concrete is 0.3. If the bulldozer pushes with 15000 N of force, what will be the acceleration of the dirt?
18. A 20000 kg rocketship at rest in deep space turns on its thrusters for 10 seconds, then shuts them off. At the end of the 10 seconds, the ship is moving at 25 m/s.
a) What was the ship’s acceleration while the rocket was on?
b) How much force did the thrusters apply to the ship?
c) How far did the ship move during the time its thrusters were on?
d) How far will the ship move during the next 10 seconds?
19. A 6-kg box sled is coasting across frictionless ice at a speed of 9 m/s when a 12 kg package is dropped on it from above. What is the new speed of the sled?
20. The coefficient of friction between a 150 kg chair and the carpet is 0.9. (a) How much would you have to push on the chair to move it across the carpet at a constant speed? (b) How much would you have to push on the chair in order to accelerate it at 0.5 m/s/s?
21. A loaf of bread weighs 5.3 N on Earth.
(a) What is its mass?
(b) What would it weigh on the moon, where g = 1.6 m/s/s?
(c) What is its mass on the moon?
(d) What would it weigh on Jupiter, where g = 25.2 m/s/s?
(e) What is its mass on Jupiter?
22. Mr. Otis does his physics experiments in an elevator. While the elevator is at rest, he hangs a brick from a spring scale. The scale reads 25 N.
(a) What is the mass of the brick?
(b) The elevator accelerates upward at 0.5 m/s/s. What is the reading on the scale?
(c) The elevator now moves upward at a constant speed of 3 m/s. What is the reading on the scale?
(d) The elevator now slows to a stop with a negative acceleration of –1 m/s/s. What is the reading on the scale?
23. In the figure shown here, find the acceleration of the (200 kg) cart that is required to prevent the (5 kg) block from falling. The coefficient of static friction between the block and cart is 0.4.
Answers
3. (a) 3 m/s/s & 12 N; (b) 1 m/s/s & 12 N
4. 0.15 m/s; 1.1 m/s
5. 13 m/s
6. A
7. C
8. C
9. D
10. C
11. 2.5 x 105 N
12. no; 14800 N on each leg
14. –0.016 m/s
15. 9.4 m/s
16. (a) 8.2 m/s; (b) yes: 10 m/s
17. 1.3 m/s/s
18. (a) 2.5 m/s/s; (b) 50000 N; (c) 125 m; (d) 250 m
19. 3 m/s
20. (a) 1350 N; (b) 1425 N
21. (a) 0.53 kg; (b) 0.85 N; (c) 0.53 kg; (d) 13.4 N; (e) 0.53 kg
22. (a) 2.5 kg; (b) 26.25 N; (c) 25 N; (d) 22.5 N
23. 25 m/s/s