A 61 kg lumberjack stands on a 320 kg log that floats on a pond A 61 kg lumberjack stands on a 320 kg log that floats on a pond. Both the lumberjack and the log are initially at rest. The log points towards the shore, which we take to be the +x axis. If the lumberjack now trots along the log towards the shore with a speed v relative to the shore, the log moves away from the shore. The speed of the log relative to the shore is… A) greater than v B) less then v C) equal to v

I.e., Momentum (of the system) is conserved during a collision Collisions We are going to assume that collisions happen fast enough that any external forces can be ignored. I.e., Momentum (of the system) is conserved during a collision

A collision is “inelastic” if kinetic energy is NOT conserved Inelastic collision: A collision is “inelastic” if kinetic energy is NOT conserved

A collision is “inelastic” if kinetic energy is NOT conserved Inelastic collision: A collision is “inelastic” if kinetic energy is NOT conserved A collision is “completely inelastic” if the two objects stick together

A collision is “inelastic” if kinetic energy is NOT conserved Inelastic collision: A collision is “inelastic” if kinetic energy is NOT conserved A collision is “completely inelastic” if the two objects stick together In that case, the maximum amount of kinetic energy is lost

What is the final velocity of the pair of cars? A railroad car of mass m and speed v1,i collides with and sticks to an identical railroad car that is initially at rest. What is the final velocity of the pair of cars?

A railroad car of mass m and speed v1,i collides with and sticks to an identical railroad car that is initially at rest. After the collision, the kinetic energy of the system is _____ of its initial kinetic energy. A) 1/2 B) 1/3 C) 1/4

On a touchdown attempt, a 95 On a touchdown attempt, a 95.0 kg running back runs toward the end zone at 3.75 m/s. A 111 kg linebacker moving at 4.10 m/s meets the runner in a head-on collision. If the two players stick together, (a) What is their velocity immediately after the collision? (b) What are the initial and final kinetic energies of the system?

A collision is “elastic” if kinetic energy IS conserved Elastic collision: A collision is “elastic” if kinetic energy IS conserved Note that momentum is conserved in all collisions

A collision is “elastic” if kinetic energy IS conserved Elastic collision: A collision is “elastic” if kinetic energy IS conserved Note that momentum is conserved in all collisions (Also note that “elastic collision” is like “frictionless surface” and “massless string”)

A collision is “elastic” if kinetic energy IS conserved Elastic collision: A collision is “elastic” if kinetic energy IS conserved Note that momentum is conserved in all collisions A good example is billiard balls – a close approximation to a truly elastic collision There are also a lot of complications (friction, spin, etc.)

So we’re going to take the easy approach… No complications – no friction, no spin And we’re going to focus on a specific situation: Cart 2 is initially at rest