Explain physics question and answer.

Question

Ball A of mass 5 kilograms moving at 20 meters per second collides with ball B of unknown mass moving at 10 meters per second in the same direction.

 

A car traveling at 20 m/s has 1,000 J of Kinetic Energy. What would the car’s kinetic energy be if the speed was 10 m/s?

 

The answer is 250 J.

 

But why?? Explain… Show work...

 

Answer 1

E1 = kinetic energy at v1 = 1000 J; v1 = 20 m/s; E2 = Kinetic energy at v2 = ?; v2 = 10 m/s; E1 = 1/2 m v1**2; E2 = 1/2 m v2**2; Use the given information to get m from E1 and V1, them plug the result into the expression for E2, using v2: m = 2 E1 / v1**2 = 2 (1000) / (20)**2 kg = 5 kg; Therefore, E2 = 1/2 m v2**2 = 1/2 (5) (10)**2 J = 250 J