Chpt. 6 HW
A hungry fish is about to have lunch at the speeds shown. Assume the hungry fish has a mass 5 times that of the small fish. A) v = 4 m/s v = 0 B) v = 4 m/s v = 1 m/s C) v = 5 m/s v = 2 m/s Rank from greatest to least. To rank items as equivalent, overlap them.
C) A) B)
How is Dr. Hewitt able to break a piece of wood in his demonstration? a) He moves his hand very quickly. b) He moves his hand very slowly. c) He uses a heavy object to break the piece of wood.
a) He moves his hand very quickly.
Now, consider the collision between two happy balls described in Part A. How much of the balls' kinetic energy is dissipated? a) None of it b) Half of it c) All of it
a) None of it
Bumper cars A and B undergo a collision during which the momentum of the combined system is conserved. Which equation(s) correctly states the principle of conservation of momentum? a) P⃗ A,i+P⃗ B,i=P⃗ A,f+P⃗ B,f b) P⃗ A,i+P⃗ A,f=P⃗ B,i+P⃗ B,f c) ΔP⃗ A=ΔP⃗ B d) ΔP⃗ A+ΔP⃗ B=0
a) P⃗ A,i+P⃗ B,i=P⃗ A,f+P⃗ B,f d) ΔP⃗ A+ΔP⃗ B=0 Two of the equations express the conservation principle in a different way. The equation P⃗ A,i+P⃗ B,i=P⃗ A,f+P⃗ B,f states that the total momentum of the system is the same before and after the collision; the equation ΔP⃗ A+ΔP⃗ B=0 states that the change in the total momentum of the system is zero.
The video shows an animated billiards experiment in which a cue ball strikes a glued-in-place eight-ball. Which of the following explains why the momentum of the eight-ball is conserved? a) The "glue force" cancels the collision force. b) The eight-ball is an isolated system. c) The cue ball rebounds with the same speed it had coming in. d) The collision is elastic.
a) The "glue force" cancels the collision force. During the collision, the force exerted by the glue (on the eight-ball) cancels the force exerted by the cue ball (on the eight-ball). Thus, the momentum of the eight-ball system is conserved because the net external force acting on it is zero.
Click "Reset." Now, predict what will happen if you change the velocity of the red truck so that it has a magnitude smaller than that of the green truck but is positive. a) The trucks will stick together after the collision and move to the right. b) The trucks will stick together after the collision and move to the left. c) The trucks will stick together after the collision and not move.
a) The trucks will stick together after the collision and move to the right.
An impulse can be increased by a) increasing the force or increasing the time interval. b) decreasing the force and decreasing the time interval. c) decreasing the force and maintaining time interval. d) maintaining the force and decreasing the time interval.
a) increasing the force or increasing the time interval.
Suppose we repeat the experiment from the video, but this time we use a rocket three times as massive as the one in the video, and in place of water we use a fluid that is twice as massive (dense) as water. If the new fluid leaves the rocket at the same speed as the water in the video, what will be the ratio of the horizontal speed of our rocket to the horizontal speed of the rocket in the video after all the fluid has left the rocket? (Ignore air resistance.) a) 1/3 b) 2/3 c) 1 d) 1/2 e) 3/2
b) 2/3 The rocket's speed is proportional to the ratio of the fluid's mass to the rocket's mass.
Imagine that you replace the block in the video with a happy or sad ball identical to the one used as a pendulum, so that the sad ball strikes a sad ball and the happy ball strikes a happy ball. The target balls are free to move, and all the balls have the same mass. In the collision between the sad balls, how much of the balls' kinetic energy is dissipated? a) All of it b) Half of it c) None of it
b) Half of it
What would Dr. Hewitt need to have done to exert an even greater force than he did in his karate demonstration? a) Decrease the change in momentum, and increase the time duration. b) Increase the change in momentum, and decrease the time duration. c) Increase the change in momentum, and increase the time duration. d) Decrease the change in momentum. and increase the time duration
b) Increase the change in momentum, and decrease the time duration. (Think about the relation between force, change in momentum, and duration. To increase the force, should you increase or decrease the change in momentum? Should you increase or decrease the duration?)
Why is a force that is applied for a short time more effective in karate? a) The average force is independent of the time. b) The average force is increased. c) The average force becomes zero for short times. d) The average force is decreased.
b) The average force is increased.
Which has a greater momentum: a heavy truck at rest or a moving skateboard? a) The heavy truck at rest. b) The moving skateboard. c) Both have zero momentum. d) Both have the same momentum
b) The moving skateboard.
Which statement must be true for the momentum of a system to be conserved? a) The net external force on the system is non-zero. b) The net external force on the system is zero. c) The internal forces sum to zero. d) There are no external forces acting on the system.
b) The net external force on the system is zero. When the net external force on a system is zero, the system has zero acceleration and constant momentum.
Predict what will happen if you reduce the magnitude of the velocity of the green truck to a magnitude smaller than that of the red truck. a) The trucks will stick together after the collision and not move. b) The trucks will stick together after the collision and move to the left. c) The trucks will stick together after the collision and move to the right.
b) The trucks will stick together after the collision and move to the left
Click "Reset." Now, predict what will happen if you change the velocity of the red truck so that it has a magnitude smaller than that of the green truck but is negative. a) The trucks will stick together after the collision and move to the left. b) The trucks will stick together after the collision and move to the right. c) The trucks will stick together after the collision and not move.
b) The trucks will stick together after the collision and move to the right.
Click "Reset." Now, predict what will happen if you increase the mass of the green truck so that it is greater than the mass of the red truck a) The trucks will stick together after the collision and move to the left. b) The trucks will stick together after the collision and move to the right. c) The trucks will stick together after the collision and not move.
b) The trucks will stick together after the collision and move to the right.
Is it correct to say that, if no net impulse is exerted on a system, then no change in the momentum of the system will occur? a) Sometimes. b) Yes, always. c) No, never. d) Almost always.
b) Yes, always.
When two vehicles collide, momentum is conserved _______. a) if the collision is inelastic. b) whether the collision is elastic or inelastic. c) only if deformation of either vehicle does not occur. d) if the collision is elastic
b) whether the collision is elastic or inelastic.
What does it mean to say that momentum is conserved? a) It means the change in momentum is equal to the impulse. b) It means the net momentum is zero. c) It means momentum does not change. d) It means momentum is zero.
c) It means momentum does not change.
In which is momentum conserved: an elastic collision or an inelastic collision? a) Momentum is conserved only in inelastic collisions. b) Momentum is conserved only in elastic collisions. c) Momentum is conserved in elastic and inelastic collisions. d) Momentum is conserved in neither elastic nor inelastic collisions.
c) Momentum is conserved in elastic and inelastic collisions.
While goofing off at the ice skating rink, a student takes off her shoes and places each of them on the ice. Her friend, a hockey player, then shoots a hockey puck at each shoe. The first puck immediately comes to rest after it collides with the left shoe. The second puck rebounds after it collides with the right shoe. If each hockey puck has the same incoming speed, which shoe has greater speed after the collision? a) Both shoes have the same speed. b) The left shoe c) The right shoe
c) The right shoe In each collision, the change in momentum of the puck has the same magnitude as the change in momentum of the shoe. The puck has a greater magnitude of change in momentum when it rebounds, so the right shoe must undergo a greater change in momentum than the left shoe!
Before you click "Play," predict what will happen if the red and green truck with equal mass and equal magnitude of velocity collide. a) The trucks will stick together after the collision and move to the left. b) The trucks will stick together after the collision and move to the right. c) The trucks will stick together after the collision and not move.
c) The trucks will stick together after the collision and not move.
How does Dr. Hewitt break the piece of wood? a) He creates a change in momentum of his hand over a long period of time. b) He causes a change in momentum over a long period of time. c) He exerts a large force to break the wood. d) He causes a change in momentum of his hand over a short period of time.
d) He causes a change in momentum of his hand over a short period of time.
When a falling firecracker explodes, the momenta of its pieces _______. a) illustrate a case where momentum is not conserved b) is conserved only if the fragments eject in one direction c) disintegrate d) vectorally add up to equal the initial momentum of the firecracker
d) vectorally add up to equal the initial momentum of the firecracker
Suppose we replace both hover pucks with pucks that are the same size as the originals but twice as massive. Otherwise, we keep the experiment the same. Compared to the pucks in the video, this pair of pucks will rotate. a) one-half as fast. b) four times as fast. c) twice as fast. d) one-fourth as fast. e) at the same rate.
e) at the same rate. By doubling the mass but keeping the velocities unchanged, we doubled the angular momentum of the two-puck system. However, we also doubled the moment of inertia. Since L=I ω, the rotation rate of the two-puck system must remain unchanged.
