Science Starter! A hockey puck has a mass of 0.115 kg and strikes the pole of the net at 25 m/s. It bounces off with the same speed but in the opposite direction at 25 m/s, as shown. • What is the impulse on the puck? • If the collision takes 0.0025 s, what is the average force on the puck?
Conservation of Momentum Total momentum of a system remains the same during a collision:
Example 1 A 600 kg car traveling east at 10 m/s makes a head-on collision with an 1800 kg truck moving west at 5 m/s. After the collision, the truck continues moving west but with a decreased speed of 0.5 m/s. With what speed and in what direction does the car move?
Example 2 Joe, 150 kg, is running at 4 m/s towards little Mike, 50 kg, who is standing still. If they stick together after the tackle, what is their combined speed?
Recoil / Explosion Objects are initially together and “explode” apart: • Total momentum before the explosion may be zero: • Example – A gun firing a bullet • Total momentum before the explosion may have a value: • Example – A boy rolling on a skateboard jumps off
Example 3 A 2.5 kg bow fires a 0.5 kg arrow so that the arrow flies off at 5 m/s. What is the recoil velocity of the bow? Example 4 A 50 kg girl is riding her 5 kg skateboard at 2.0 m/s, when she jumps off of the back at 1.5 m/s. How fast, and in what direction, does the skateboard move?