Cart A of mass 0.50kg travels east at 2.0m/s on a frictionless, straight track. It collides and sticks to cart B of mass 1.0kg that is initially at rest on the same track. What is the speed of the combined carts immediately after the collision?
a) 1.0m/s
b) 0.33m/s
c) 0.67m/s
d) 2.0m/s