# Physics test 2- chapter 9

Two objects of equal mass are traveling with identical speeds v and collide. After the collision they travel together with a speed v/2. What was the angle between the original velocities?

120°

An air track cart with mass m and speed v moving along an air track collides with a stationary cart with a mass m/3. After the collision the first cart has a speed v/2. What is the speed of the second cart?

3v/2

A ping-pong ball moving east at a speed of 4 m/s, collides with a stationary bowling ball. The Ping-Pong ball bounces back to the west, and the bowling ball moves very slowly to the east. Which object experiences the greater magnitude impulse during the collision? A) Neither; both experienced the same magnitude impulse. B) the Ping-Pong ball C) the bowling ball D) It's impossible to tell since the actual mass values are not given. E) It's impossible to tell since the velocities after the collision are unknown.

A) Neither; both experienced the same magnitude impulse.

An artillery shell explodes in midair and breaks up into many fragments. Which of the following statements are true regarding conditions immediately before and immediately after the explosion: I. The total momentum of the fragments is equal to the original momentum of the shell. II. The total kinetic energy of the fragments is equal to the original kinetic energy of the shell. A) Statement I only B) Statement II only C) Both Statement I and Statement II D) Neither statement is true.

A) Statement I only

An inelastic collision of two objects is characterized by the following. A) Total momentum of the system is conserved. B) Total kinetic energy of the system remains constant. C) Both A and B are true. D) Neither A nor B are true.

A) Total momentum of the system is conserved.

A rope is held from one end in such a way that the lower end just touches a table. The rope is then lowered onto the table with constant velocity. The force that is needed to hold the upper end of the rope is A) equal to the weight of the portion of the rope that is not on the table. B) greater than the weight of the portion of the rope that is not on the table. C) less than the weight of the portion of the rope that is not on the table but not zero N. D) zero N. E) Cannot be determined without additional information.

A) equal to the weight of the portion of the rope that is not on the table.

Two different masses have equal, non-zero kinetic energies. The momentum of the smaller mass is A) smaller than the momentum of the larger mass. B) larger than momentum of the larger mass. C) equal to the momentum of the larger mass. D) zero. E) There is not enough information to answer the question.

A) smaller than the momentum of the larger mass.

A rubber ball and a lump of putty have equal mass. They are thrown with equal speed against a wall. The ball bounces back with nearly the same speed with which it hit. The putty sticks to the wall. Which objects experiences the greater momentum change? A) the ball B) the putty C) Both experience the same non-zero momentum change. D) Both experience zero momentum change. E) Cannot be determined from the information given.

A) the ball

A red ball with a velocity of +3.0 m/s collides head-on with a yellow ball of equal mass moving with a velocity of -2.0 m/s. What is the velocity of the yellow ball after the collision? A) zero B) +3.0 m/s C) -2.0 m/s D) +2.5 m/s E) +5.0 m/s

B) +3.0 m/s

An object that is initially at rest breaks up into two pieces of unequal masses when a spring-loaded device is released. If you compare the kinetic energy of the larger mass to that of the smaller mass immediately after they separate, which of the following statements is correct? A) The kinetic energy of the larger mass is greater. B) The kinetic energy of the smaller mass is greater. C) The kinetic energies of the two masses are equal. D) The total kinetic energy is zero J

B) The kinetic energy of the smaller mass is greater.

A tennis ball undergoes an elastic collision when it hits the ground and bounces up. It is given that the speed of the ball just before it hits the ground is -v0 and immediately after rebounding it is +v0. Which of the statements below is true? A) The momentum of the ball changes but the momentum of Earth does not. B) The momentum of the ball changes and the momentum of Earth also changes by the same amount. C) The momentum of the ball does not change but the momentum of Earth changes. D) Neither the momentum of the ball nor the momentum of Earth changes. E) Additional information is needed to answer this question.

B) The momentum of the ball changes and the momentum of Earth also changes by the same amount.

A rope is lying on a table. You pick up one end and start raising it with a constant velocity. The force you have to exert on the rope is A) equal to the weight of the portion of the rope that is not on the table. B) greater than the weight of the portion of the rope that is not on the table. C) less than the weight of the portion of the rope that is not on the table but not zero N. D) zero N. E) Cannot be determined without additional information.

B) greater than the weight of the portion of the rope that is not on the table.

A railroad car of mass m and speed v collides and sticks to an identical railroad car that is initially at rest. After the collision, the kinetic energy of the system A) is the same as before. B) is half as much as before. C) is one third as much as before. D) is one fourth as much as before. E) is one quarter as much as before.

B) is half as much as before.

A photograph of the two-dimensional elastic collision of two particles shows a particle of mass m1 approaching a stationary particle of mass m2. As a result of their interaction, the incident particle moves away along a line that makes an angle of 30° with its original path, and the other particle moves away along a line that makes an angle of 30° to the other side of the original path of the incident particle. What is the ratio of the masses, m1/m2? A) 0.5 B) 1 C) 2 D) 3 E) 4

C) 2

A ping-pong ball originally at rest is hit head-on by a bowling ball moving with initial speed v0. It is given that the mass of the ping-pong ball is as good as negligible in comparison with the mass of the bowling ball. What is the speed of the ping-pong ball after this elastic collision? A) v0/2 B) v0 C) 2v0 D) 4v0 E) 8v0

C) 2v0

A puck with a mass m1 = 50.0 g moving at 1.00 m/s approaches a stationary puck with a mass m2 = 100 g on an air table and they undergo a two-dimensional elastic collision. After the collision, both pucks have identical speeds, but travel in different directions. What is the angle between the original and final paths of m1? A) 30.0° B) 60.0° C) 90.0° D) 120° E) 150°

C) 90.0°

An elastic collision of two objects is characterized by the following. A) Total momentum of the system is conserved. B) Total kinetic energy of the system remains constant. C) Both A and B are true. D) Neither A nor B are true.

C) Both A and B are true.

A freight car moves along a frictionless level railroad track at constant speed. The freight car is open on top. A large load of coal is suddenly dumped into the car. What happens to the speed of the freight car? A) It increases. B) It remains the same. C) It decreases. D) It becomes zero. E) Cannot be determined from the information given.

C) It decreases.

In a collision between two unequal masses, how does the impulse imparted to the smaller mass by the larger mass compare with the impulse imparted to the larger mass by the smaller one? A) It is larger. B) It is smaller. C) They are equal. D) The answer depends on the ratio of the masses. E) The answer depends on how fast they are moving.

C) They are equal.

Identical forces act for the same length of time on two different masses. The change in momentum of the smaller mass is A) smaller than the change in momentum of the larger mass, but not zero. B) larger than the change in momentum of the larger mass. C) equal to the change in momentum of the larger mass. D) zero. E) There is not enough information to answer the question.

C) equal to the change in momentum of the larger mass.

Jacques and George meet in the middle of a lake while paddling in their canoes. They come to a complete stop and talk for a while. When they are ready to leave, Jacques pushes George's canoe with a force to separate the two canoes. What is correct to say about the final momentum and kinetic energy of the system? A) The final momentum is in the direction of but the final kinetic energy is zero J. B) The final momentum is in the direction opposite of but the final kinetic energy is zero J. C) The final momentum is in the direction of and the final kinetic energy is positive. D) The final momentum is zero kg∙m/s and the final kinetic energy is zero J. E) The final momentum is zero kg∙m/s but the final kinetic energy is positive

E) The final momentum is zero kg∙m/s but the final kinetic energy is positive

A system that has zero momentum must also have zero kinetic energy.

FALSE

The impulse delivered to an object is equal to the change in the object's velocity.

FALSE

When objects stick together after colliding, the collision is completely elastic.

FALSE

internal forces cannot change the total kinetic energy of a system.

FALSE

State the Conservation of Linear Momentum.

If the net force acting on an object is zero, its momentum is conserved.

Define an elastic collision in terms of conservation of momentum and energy.

In an elastic collision, the momentum and kinetic energy are conserved.

Define an inelastic collision in terms of conservation of momentum and energy

In an inelastic collision, the momentum of the system is conserved, but its kinetic energy is not.

In space there is nothing for the rocket to "push against" so how does it accelerate?

In terms of forces, the rocket pushes the exhaust gases back and they in turn (action-reaction) push the rocket forward. In terms of momentum, in order to conserve momentum since the exhaust gases acquire negative momentum (moving backwards) the rocket acquires positive momentum (increasing velocity forward).

State the Conservation of Momentum for a System of Objects

Internal forces have absolutely no effect on the net momentum of a system. If the net external force acting on a system is zero; its net momentum is conserved

When a ball undergoes a one-dimensional elastic collision with a wall, the velocity of the ball is reversed, while the wall remains stationary. Explain why this does not violate conservation of momentum.

One can regard this as the collision of the ball with a very massive object. As the mass of the object is increased, its speed after the collision becomes smaller and smaller. In the limiting case of an infinite mass, that mass does not move and the ball's velocity is reversed. The momentum is still conserved, but since the mass of the wall is essentially infinite, its velocity is infinitesimal

In the two-dimensional elastic collision of a particle with a stationary particle that has the same mass, the trajectories of the two particles after the collision are at right angles to each other. Explain why this should be so.

Since the masses are equal, conservation of momentum can be stated in the form v1 + v2 = v, which states that the three vectors form the sides of a triangle, and conservation of energy becomes v12 + v22 = v2, which says that the sum of the squares of the two sides is equal to the square of the third. That makes it a right-angled triangle, with the two sides corresponding to the final velocities at right angles to each other.

A particle that has zero momentum must also have zero kinetic energy.

TRUE

In a one-dimensional elastic collision of two identical masses, the masses exchange velocities.

TRUE

Internal forces cannot change the total momentum of a system.

TRUE

The center of mass of a continuous, uniform object is located at the geometric center of the object.

TRUE

The center of mass of an object does not have to be located within the object.

TRUE

The net force acting on an object is equal to the rate of change of its momentum.

TRUE

When a gun is fired, which has the greater kinetic energy: the bullet or the recoiling gun?

The bullet

A novice marksman fires a rifle while holding the butt of the rifle a couple of centimeters away from his shoulder. Explain what happens in terms of the physical principles involved.

The initial momentum of the bullet/gun system was zero kg∙m/s, so after firing the rifle the forward momentum of the bullet is equal and opposite to the backward momentum of the rifle alone. The rifle kicks back and hits the marksman on the shoulder with sufficient speed to cause a bruise. An expert marksman would hold the rifle tight against the shoulder. The recoil then involves both the mass of the rifle and the mass of the upper part of the body of the marksman.

State Newton's second law in terms of momentum.

The rate of change of momentum of an object is equal to the net force applied to it.

An air track cart with mass m and speed v moving along an air track collides with a stationary cart with a mass m/3. After the collision the first cart has a speed v/2. Is the collision elastic?

Yes

Two objects approach each other with velocities such that the total momentum of the masses is zero kg∙m/s, and undergo an elastic collision. Is the sum of the kinetic energies of each object the same before and after the collision?

Yes