Instructor: Dr. Tatiana Erukhimova Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lectures 28-30

A block of mass m is moving along x axis with a velocity of V0 A block of mass m is moving along x axis with a velocity of V0. 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 = m0, find the velocity of each ball after a perfectly elastic collision.

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 v0. 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 2v0. Find as many equations as you need to find the velocity of the heavy piece.

Quiz 3 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.

Problem 2 A block of mass M is at rest on a frictionless table. You can hit it with either a sticky block which will stick to it or a very hard block for which the collision will be perfectly elastic. Both blocks have mass m and can be thrown with velocity of magnitude V0 . Which way will you get the block M moving fastest?

A small car weighing m1 is traveling due north when it collides with a pick-up truck weighting m2 which was traveling due east. After the collision the two vehicles move off together at an angle θ north of east. The driver of the car claimed that the truck driver was at fault because he was exceeding the speed limit, going with a velocity v1. If this were true, what was the car’s initial velocity?

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 370 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

Impulse Changes in a particle’s momentum are due to impulse, which depends on the time over which the net force acts.

Impulse Suppose you throw a ball with a mass of 0.4 kg against a brick wall. It hits the wall moving horizontally to the left at 30 m/s and rebounds horizontally to the right at 20 m/s. a) Find the impulse of the net force on the ball during its collision with the wall. b) If the ball is in contact with the wall for 0.010 s, find the average horizontal force that the wall exerts on the ball during the impact.

Polar coordinates

A ball of mass m is swung around a circle at the end of a string of length L. The string will break if the tension in it exceeds a critical value, Tc. What is the largest constant angular velocity the ball can have without breaking the string?

A ball of mass m is swung around a circle at the end of a string of length L. What is the minimum  the ball can have and still travel in a circle without string becoming slack?