Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lectures 27, 28

Momentum is a vector! Vector equation!

If

Conservation of Momentum If there is no external force on a system, then the total momentum of the system is a constant

True in X and Y directions separately!

Problem Solving For Conservation of Momentum problems: 1.BEFORE and AFTER 2.Do X and Y Separately

Before X Y

After Y X

Inelastic collision A collision in which the total kinetic energy after the collision is not equal to the kinetic energy before the collision is called an inelastic collision. A B BEFORE AFTER A B V after ?

Perfectly elastic collision A collision in which the total kinetic energy after the collision is the same than that before the collision is called an elastic collision. A B

A block of mass m is moving along x axis with a velocity of V 0. It collides with a block of mass M, initially at rest. 1) What is the change in kinetic energy of the system of two balls: a) if the collision is perfectly elastic; b) if the collision is perfectly inelastic (balls stick together after collision). 2) For m = M = m 0, find the velocity of each ball after a perfectly elastic collision. Quiz 1

Problem 2 from handout At the intersection of Texas Avenue and University Drive, a blue, sub compact car with mass 950 kg traveling east on University collides with a maroon pickup truck with mass 1900 kg that is traveling north on Texas and ran a red light. The two vehicles stick together as a result of the collision and, after the collision, the wreckage is sliding at 16.0 m/s in the direction 24.0 degrees east of north. Calculate the speed of each vehicle before the collision. The collision occurs during a heavy rainstorm; you can ignore friction forces between the vehicles and the wet road.

Quiz 2 A block of mass m is sliding on a frictionless table with velocity v 0. It explodes into two pieces, one with mass m/3. The light piece flies off horizontally, perpendicular to the original direction of motion, with velocity 2v 0. Find as many equations as you need to find the velocity of the heavy piece.

You are standing on a frictionless surface. Some idiot throws a rock at you which you catch. In terms of your mass, the rock’s mass and the rock’s velocity find your position as a function of time after you catch the rock. Quiz 3

Problem 4 p.200 In a nuclear collision an incoming proton has initial velocity of magnitude m/s. It collides with another proton, initially at rest. After the collision one proton goes off at 37 0 to the x axis. If the collision is perfectly elastic, find the velocities of the two protons after the collision.

The ballistic pendulum

“Famous Problem” from the book

Have a great day! Reading: Chapter 12 Hw: All Chapter 10 problems and exercises