What is the threshold velocity vthreshold(water) (i. e., the minimum velocity) for creating Cherenkov light from a charged particle as it travels through water (which has an index of refraction of n=1.33)?

On a frictionless air track, a blue glider with mass 0.200 kg is moving to the right with a speed of 8.00 m/s. it strikes a red glider that has mass 0.600 kg and that is initially at rest. after the collision, the blue glider is moving to the left with a speed of 3.00 m/s.(a) what are the magnitude and direction of the velocity of the red glider after the collision? (b) is this collision elastic?