Physics 2 – Jan 24, 2017 P3 Challenge– Today’s Objective: Collisions A 350 g ball falling with a velocity of 2.1 m/s rebounds upward with a velocity of 1.6 m/s. If the ball was in contact with the floor for 2.5 x 10-4 s, what was the impact force of the floor on the ball? Today’s Objective: Collisions Assignment: Collisions Worksheet Agenda Conservation of Momentum Collisions Inelastic collisions Elastic collisions

Conservation of Momentum Just like energy, momentum is conserved for an isolated system. An isolated system, doesn’t have any external net forces. Similar kind of problem solving to conservation of energy problems. Draw the system, pick two points in time, asses the momentum at each point in time, and set equal. Two times are usually before and after a collision. Ask yourself “What is moving before/after the collision?”

Collisions There are three kinds of collisions. 1) Completely inelastic collision: The two colliding bodies stick together and move off together with the same velocity. 2) Elastic collision: Two objects collide and bounce off each other such that kinetic energy is conserved. 3) Inelastic collision: Two objects collide and bounce off each other such than kinetic energy is not conserved. In all momentum is conserved: 𝒎 𝟏 𝒗 𝟏𝒊 + 𝒎 𝟐 𝒗 𝟐𝒊 = 𝒎 𝟏 𝒗 𝟏𝒇 + 𝒎 𝟐 𝒗 𝟐𝒇

Inelastic collisions The general equation for inelastic collisions: 𝒎 𝟏 𝒗 𝟏𝒊 + 𝒎 𝟐 𝒗 𝟐𝒊 = 𝒎 𝟏 + 𝒎 𝟐 𝑽 𝒇 If an 800.-kg sports car slows to 13.0 m/s to check out an accident scene and a 1200.-kg pick-up truck behind him continues traveling at 25.0 m/s, with what velocity will the two move if they lock bumpers after a rear-end collision?

1D Elastic collisions In general, if you have an elastic collision you have another equation you can use: 𝟏 𝟐 𝒎 𝟏 𝒗 𝟏𝒊 𝟐 + 𝟏 𝟐 𝒎 𝟐 𝒗 𝟐𝒊 𝟐 = 𝟏 𝟐 𝒎 𝟏 𝒗 𝟏𝒇 𝟐 + 𝟏 𝟐 𝒎 𝟐 𝒗 𝟐𝒇 𝟐 Elastic collisions include items like atoms and billiard balls. Hard with no give. Algebra can get very complicated so there are equations to use that give the results of solving simultaneously. Ex: A 0.350 kg billiard cue ball moving at 6.2 m/s collides elastically a stationary ball with the same mass. Determine their velocities after the collision.

Exit Slip - Assignment Exit Slip- A 9000 kg train car moving at 32.5 m/s collides and couples with a 16000 kg train moving in the same direction at 12.5 m/s. What is the velocity of the new combined locomotion? What’s Due on Jan 26? (Pending assignments to complete.) Collision worksheet What’s Next? (How to prepare for the next day) Read 2.4 p98-108 about Momentum