1. An object is moving to the right with speed vi when a force of magnitude F is exerted on it. In which of the following situations is the object’s direction of motion changing and kinetic energy decreasing at the instant shown?
2. A ball is suspended by a lightweight string, as shown in the figure above. The ball is displaced to position 1 and released. The four labeled positions are evenly spaced along the arc of the ball’s motion. Between which adjacent pairs of positions is the change in kinetic energy of the ball greatest?
(A) 1 and 2
(B) 2 and 3
(C) 3 and 4
(D) The change is the same for all adjacent pairs.
3. A newly discovered planet is found to have density 2/3ρE and radius 2RE , where ρE and RE are the density and radius of Earth, respectively. The surface gravitational field of the planet is most nearly
(A) 1.7 N kg
(B) 3.3 N kg
(C) 6.7 N kg
(D) 13 N kg
4. A bus is initially traveling north at a constant speed, as shown in the figure above. As the bus starts to make a left turn without changing speed, a passenger notices that a box on the floor starts sliding toward the right side of the bus. Which of the following top views of the box, when correctly labeled, would best represent all of the horizontal forces exerted on the box as it starts sliding?
Questions 5-6 refer to the following material.
Three identical blocks each take a different path from a height h to the ground. Block A is released from rest and falls vertically. Block B is released from rest and slides down a frictionless incline. Block C is projected horizontally with an initial speed v.
5. Which block takes the longest time to reach the ground?
(D) The blocks take the same time to reach the ground.
6. Which block has the greatest speed just before hitting the ground?
(D) The blocks reach the ground with the same speed.
1. (7 points, suggested time about 13 minutes)
The figure above represents two carts, with magnets attached, that make up a system. The mass of one cart and magnet is 1 kg and the mass of the other is 5 kg. The carts are initially at rest on a frictionless track. They are released from rest and exert a repulsive force on each other. The track is not quite horizontal, with the right side slightly lower than the left side.
The speeds of the carts are measured over a 10 s interval. The graph below shows the momentum of the two carts as a function of time for this interval as they move along the x-axis.
(a) Based on the graph, were the measurements started at the instant the carts were released?
(b) Calculate the magnitude of the external force exerted on the system.
(c) Suppose the experiment is repeated with different carts, so that the masses of cart plus magnet are 2 kg and 4 kg. Would your answer to part (b) be different with the new masses?