Momentum and Collisions Test Review

Momentum is a vector. Its direction is the direction of the velocity. Momentum is mass x velocity and its units are Kgm/s

Momentum is always conserved if there is no net force (like friction or gravity). If I shoot a rifle, the bullet has some momentum forward, and the rifle recoils backwards with an equal and opposite momentum. Momentum is equal, but kinetic energy is not, as KE has the velocity squared

Change in momentum, Dp, is the final p – the initial p. If I kick a 1 kg ball against a wall at 10 m/s, and it bounces back at 7 m/s, what is the change in momentum? 7 kgm/s – (- 10 kgm/s) = 17 kgm/s

FDt = mDv If I drop my calculator off a 10 story building onto the sidewalk, it will break. If I drop it onto a mattress, it might survive. Why? FDt = mDv FDt = mDv FDt = mDv

Types of collisions perfectly inelastic two things stick together conserved: momentum elastic two things hit and bounce off conserved: momentum and KE

A ball is going 5 m/s. A second identical ball is going 15 m/s. How is the second ball’s momentum different? How is its KE different? p = mDv KE = ½mv2

An 8 g. steel ball rolls across the floor at 5 m/s. It comes to rest after 23.5 s. What was the force of friction? mass = .008 kg vi = 5 m/s Dt = 23.5 s F = ? FDt = mDv F = mDv/Dt F = (.008 kg)(5 m/s)/23.5 s F = 1.7 x 10-3 N

Think of a pool table, if the white ball hits the black ball, describe the motion of each

I am standing on a skateboard and throw a .7 kg ball forward at 20 m/s. If I have a mass of 90 kg, what would be my velocity and direction? m1v1 = m2v2 v2 = m1v1/m2 v2 = (.7 kg)(20 m/s)/90 kg v2 = .16 m/s backwards

I throw a 2 kg lump of clay at a student at 24 m/s left. The student simultaneously throws a 2.4 kg lump at me. The lumps hit and stick to each other, moving at 4 m/s right. What was the velocity of the students lump? m1v1 + m2v2 = (m1 + m2)vf m1 = 2 kg m2 = 2.4 kg v1 = -24 m/s vf = 4 m/s v2 = ? v2 = [(m1 + m2)vf - m1v1]/m2 v2 = [(2 kg + 2.4 kg) 4 m/s – (2 kg)(-24 m/s)]/ 2.4 kg v2 = 27.3 m/s

0 m/s 24 cm 1.75 kg 3.15 kg The yellow ball smashes into the white ball, sticks, and both travel up the ramp to a vertical height of 24 cm. What was the initial velocity of the yellow ball? mgh = ½mvf2 m1 = 3.15 kg m2 = 1.75 kg v2 = 0 m/s h = .24 m v1 = ? vf = 2.17 m/s m1v1 + m2v2 = (m1 + m2)vf v1 = (m1 + m2)vf /m1 v1 = (3.15 kg + 1.75 kg)(2.17 m/s) / 3.15 kg = 3.38 m/s