Question

6-7 Collisions and Explosions (1-D)

1. A flatcar with mass M, traveling at speed v, collides with and couples to a parked
boxcar. They move off together with a speed of 1/3 v. What is the mass of the boxcar?

Answer 1

We know that (momentum after) = (momentum before)

We remember that (Momentum) = (mass) x (speed)

Mass of the flatcar = M

Mass of the boxcar = m

Speed of the flatcar before the collision = v

Speed of the two cars stuck together after the collision = v/3

Total momentum before the collision = M · v

Total momentum after the collision = (M + m) · (v/3)

Momentum is conserved. (Total after) = (total before)

(M + m) · (v/3) = (M · v)

Eliminate parentheses on the left. Mv/3 + mv/3 = Mv

Divide both sides of the equation by v . M/3 + m/3 = M

Multiply both sides of the equation by 3 . M + m = 3M

Subtract M from both sides of the equation. m = 2M

The mass of the boxcar is 2 times the mass of the flatcar.