Conservation of Momentum Collisions & Explosions!!
Conservation of Momentum Any object in motion has momentum. When objects collide or explode apart, the Total Momentum is conserved before and after the event Total initial momentum = Total final momentum
Momentum will be conserved when 2 objects push away from each other (explosion) • Or when they combine (collision)
Types of collisions • Inelastic = stick together after the collision and travel as 1 mass • Hint: they combine, not bounce off • Ex: football player being tackled • Also for when two objects split apart (explosion) • Ex: person jumping off a skateboard • Elastic = bounce off each other after the collision. • Hint: think of bouncy balls made of elastic – they don’t stick together • Example: Billiards balls • Remember – in any collision momentum is conserved during/after collisions
Perfectly inelastic collisions • After the collision, 2 objects travel together as one mass • Before the explosion, 2 objects moving together, then split • Kinetic energy will change • Converted into sound & internal energy during the collision
Elastic collisions • After 2 objects collide they travel separately • Separate before & after the collision • KE is conserved in elastic collisions
Pee-Wee Herman (54 kg) is coasting along a roadway on his bike (19 kg) at a steady speed of 3.9 m/s when he runs into a woman with a mass of 45 kg. If the woman ends up in the basket of Pee- Wee’sbike, what will be the speed of the bike immediately after the collision? • M1 = (54 kg + 19 kg) V1, i = 3.9 m/s • M2 = 45 kg V2, I = 0 m/s • (73 kg)(3.9 m/s) + (45 kg)(0 m/s) = (73 kg + 45 kg)vf • 284.7 kg m/s + 0 = (118 kg) vf • Vf = = 2.4 m/s
Suppose a kangaroo is sitting on a log that is floating in a lake, (vf = 0.0 m/s). She gets spooked and jumps off with a velocity of 15 m/s toward the bank. The log moves with a velocity of 3.8 m/s away from the bank. If the mass of the log is 250 kg, what is the mass of the kangaroo? • = 63.3 kg • What type of collision would this be? (elastic or inelastic) Just takin’ it easy…enjoyin’ the physics…