Momentum transfer: Trolley colliding with spring

[coconut62]

Homework Statement

A trolley travels with constant velocity to the right on a horizontal ground and collides with a light helical spring attached to a wall fixed to the ground (earth) as shown. At the instant that the trolley comes momentarily to rest during collision, what has happened to the initial momentum of the trolley?

A) It has been transferred to the earth
B) It has been stored in the spring
C) It has been dissipated as sound and heat.
D) it has been destroyed by the friction due to the ground.

Homework Equations

None

The Attempt at a Solution

My answer was B. But the correct answer is A.
I don't understand how can the momentum be transferred to the earth. The trolley was moving horizontally, not vertically. Also, it causes the spring to contract, so the momentum or energy should be stored in the spring, and then the spring relaxes then pushes the trolley again. Why am I wrong?

 

[CWatters]

I don't understand how can the momentum be transferred to the earth. The trolley was moving horizontally, not vertically.

Momentum can be transferred to the Earth's rotation. Depending on which direction the trolley was moving the Earth's rotation may either speed up or slow down (slightly).

Energy has been stored in the spring.

 

[haruspex]

Further to CWatters' answer, the spring starts off at rest and is again at rest when at maximum compression, therefore it cannot be storing any momentum. Unlike energy, momentum always involves movement.