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
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?