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## Homework Statement

A cart of mass m moving at 12 m/s to the right collides elastically with a cart of mass 4.0 kg that is originally

at rest. After the collision, the cart of mass m moves to the left with a velocity of 6.0 m/s. Assuming an elastic

collision in one dimension only, what is the velocity of the center of mass (vcm) of the two carts before the collision?

(A) v_cm = 2.0 m/s

(B) v_cm = 3.0 m/s

(C) v_cm = 6.0 m/s

(D) v_cm = 9.0 m/s

(E) v_cm = 18 m/s

## Homework Equations

## The Attempt at a Solution

I just gave an arbitrary distance of 60 meters initially between the carts with the moving cart being at x=0 at t=0. During t=0 the center of mass is (60-0)/2=30 and at t=1 the center of mass is (60+12)/2=36, implying that C is correct however this is B, what am I doing wrong?