Momentum! AP Physics C Miss Espina

Warm up -

1. Turn your spring energy lab in to the top bin.

2. Take out your notebook/binder for notes on momentum.

Momentum

• Linear momentum – the product of an object’s mass times velocity.

• Law of conservation of momentum: the total momentum of an isolated system at all times equals its initial momentum

p mv=

Impulse

• Impulse is a change in momentum

J = ∆p = F∆t

F =dpdt

Natalie throws a 0.5 kg ball at a wall at 15 m/s. It is in contact with the wall for 0.10 seconds and rebounds with a speed of 12.5 m/s.

What is the impulse delivered to the ball by the wall? What is the force the wall exerts on the ball?

In a crash test, a 1500 kg car collides with a wall. The car hits the wall at v0 = -15 m/s and rebounds with v = +2.6 m/s. If the collision lasts 0.150 s, find the impulse caused by the collision and the average force exerted on the car.

Collisions in one dimension

• Elastic:

• Inelastic:

the total kinetic energy (in addition to linear momentum) of the system is the same before and after the collision

only momentum is conserved, not kinetic energy

Perfectly inelastic collision

• When two objects stick together

A 10.0-g bullet is fired into a stationary block of wood (m = 5.00 kg). The relative motion of the bullet stops inside the block. The speed of the bullet-plus-wood combination immediately after the collision is 0.600 m/s. What was the original speed of the bullet?

A 12.0-g wad of sticky clay is hurled horizontally at a 100-g wooden block initially at rest on a horizontal surface. The clay sticks to the block. After impact, the block slides 7.50 m before coming to rest. If the coefficient of friction between the block and the surface is 0.650, what was the speed of the clay immediately before impact?

Four railroad cars, each of mass 25,000 kg are coupled together and coasting along horizontal tracks at a speed vi toward the south. Richie, who is really strong is riding on the second car, uncouples the front car and gives it a strong push, increasing the speed to 4 m/s southward. The remaining three cars are now moving at 2 m/s southward. (a) Find the initial speed of all four cars (b) How much work did Richie do?

A skater is standing on a frictionless ice rink. Her friend throws a Frisbee straight at her. In which of the following cases is the most momentum transferred to the skater? (a) the skater catches the Frisbee and holds on (b) the skater catches the Frisbee momentarily but then drops it downward. (c) the skater catches the Frisbee, holds on momentarily, then throws it back.

Homework

• Chapter 9: • Problems 5, 6, 11, 17, 65, 68 • (Due the second day after break – Monday)

• Test corrections – due the day we get back • I am only adding test corrections if you do not have

any outstanding assignments.

• The first day back is the very last day to get any assignments in for a grade from the previous units.