Focal distance of a concave lens is always: O A. smaller than the image distance.
O B. positive.
O C. negative.
O D. larger than the object distance.

An open cart is moving along a straight frictionless horizontal track. when rain starts falling vertically into the cart, what happens to the speed of the cart?

Consider a system to be two train cars traveling toward each other. what is the total momentum of the system before the train cars collide?   kg • what must the total momentum of the system be after the train cars collide?   kg •  

What is friction? how does it affect the motion of an object?