The driver of a sports car, of mass 1000 kg, has now noticed that the trace lights ahead of her are red. She slams on the brakes. Unluckily, her car skids 50 m and collides with a stationary car, of mass 1200 kg. Both cars move together another 120 m before coming to rest. The coecient of kinetic friction before the collision is before = 0:25. After the collision (when the brakes no longer work!) the coecient of kinetic friction decreases to after = 0:10. How fast was the sports car moving when the driver noticed that the lights were red?
Hints: divide the problem into its three obvious stages: before the collision, the collision, and after the collision. What are the principles involved at all stage?