4. A 3kg cart (A) is moving with a speed of 1.5 m/s on a horizontal airtrack. It makes a collision with another 1 kg cart (B) moving in the same direction with a speed of 0.5 m/s. After the collision, cart A moves in the original direction with a speed of 1.2 m/s.
 

a.    (15 points)
What is the velocity of cart B after the collision? Please specify direction.


 
 

b.    (10 points)
If the two carts make contact for 0.01s during the collision, what is the average force exerted on cart B by cart A? Please specify direction.