Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?

The speed of the car B after the collision is same as the speed of the car A before collision.

Explanation:

lets take the mass of the car A = m kg

Initial speed of the car A = u m/s

The mass of the car B = m kg

Initial speed of the car A = 0 m/s

The final speed of the car A = 0 m/s

The final speed of the car B = v

There is no any external force on the masses that is why the linear momentum will be conserve.

Initial linear momentum = Final linear momentum

P = mass x velocity

m u + m x 0 = m x 0 + m v

m u + 0 = 0 + m v

m u = m v

v = u

Therefore we can say that, the speed of the car B after the collision is same as the speed of the car A before collision.