Ch.6.3 Elastic and Inelastic Collisions Warm Up How would you define “collision?” ●Is a tennis racket hitting a ball a collision? ●Is a baseball glove catching a baseball a collision? ●What’s different about these two situations? ● Collisions [NBC Learn Video-NHL] Collisions happen whenever two objects impact each other ● Sometimes the objects bounce off of each other Sometimes the objects stick together Elastic vs Inelastic Collisions Elastic Perfectly Inelastic Perfectly Inelastic Collisions In a perfectly inelastic collision, two objects collide and stick to each other with some deformation ● deformation m1v1,i + m2v2,i = (m1 + m2)vf Kinetic energy is NOT conserved because deformation takes away energy (sound, friction, etc.): KEi ≠ KEf ●Momentum is conserved: p = p i f ● Real World Examples Textbook Example pg.213 A 1850 kg luxury sedan stopped at a traffic light is struck from the rear by a compact car with a mass of 975 kg. The two cars become entangled as a result of the collision. If the compact car was moving at a velocity of 22.0 m/s to the north before the collision, what is the velocity of the entangled mass after the collision? D vf = ? F v1,i= 0 v2,i= 22.0 m/s A U m1 = 1850 kg m1v1,i + m2v2,i = (m1 + m )v →2vf f= m1v1,i + m2v2,i m1 + m 2 vf = (1850 kg)(0) + (975 kg)(22 m/s) 1850 kg + 975 kg m2 = 975 kg v1,i = 0 v2,i = 22.0 m/s to the north vf = ? S vf = 7.59 m/s to the north Homework Pg.214 #1-4 Elastic Collisions In an elastic collision, two objects collide and bounce off of each other ● m1v1,i + m2v2,i = m1v1,f + m2v2,f Kinetic energy is conserved because motion continues uninterrupted: KEi = KEf ●Momentum is conserved: p = p i f ● Real World Examples Typical Elastic Collisions Realistic Collisions some deformation Most collisions are somewhere between elastic and perfectly inelastic ●For our class, we will be assuming collisions are either elastic or perfectly inelastic ● Real World Examples Textbook Example Pg.219 A 0.015 kg marble moving to the right at 0.225 m/s makes an elastic head-on collision with a 0.030 kg shooter marble moving to the left at 0.180 m/s. After the collision, the smaller marble moves to the left at 0.315 m/s. What is the velocity of the 0.030 kg marble after the collision? m2 m1 m1v1,i + m2v2,i = m1v1,f + v1,i v2,i D F mv 2,f = m v + m v →2v2,f – 1 1,i 2 2,I v v1,f 2,f mv 1 1,f A U m1 = 0.015 kg m2 = 0.030 kg v1,i = 0.225 m/s → v2,i = 0.180 m/s ← v1,f = 0.315 m/s ← v2,f = ? → S m2 (0.15 kg)(0.225 m/s)+(.03 kg)(-.18 m/s) – (0.15 kg)(-.315 m/s) 0.03 kg v2,f = 0.09 m/s → Homework I’m not the hero I thought I was… Pg.219 #1-4a *only do part a’s …because I didn’t do my homework.