Physics Week 9

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?

Decreases (The addition of the coal increases the mass of the system. To conserve momentum, the speed of the system must decrease.)

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

False

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

False

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.

Statement I only (Momentum is conserved, not necessarily kinetic energy.)

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?

The momentum of the ball changes and the momentum of Earth also changes by the same amount. (The momentum of the ball and Earth change by the same amount by virtue of the conservation of momentum.)

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

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

True

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

is half as much as before. (In an inelastic collision, the total momentum of the system is conserved, but not KE.)

Two different masses have equal, non-zero kinetic energies. The momentum of the smaller mass is

smaller than the momentum of the larger mass. (Try to use the mathematical definitions of KE and momentum to convince yourself of the answer.)