Question

A small 1.5 kg lab cart is moving down the table at 0.25 m/s [forward]. A student drops a 1.0 kg brick vertically on it so that the brick and cart continue moving down the table. What is the final velocity of the brick-cart combo?

Answer 1

First I calculate the momentum of the moving cart.
1.5kg*0.25m/s=0.375kgm/s
Since the initial momentum of the cart is 0.375kgm/s, and the 1.0kg mass object is dropped on the moving cart, not affecting the velocity to the horizontal. Therefore, according to the conservation of momentum, the initial momentum of 0.375km/s will be conserved after the total mass has increased. Therefore increase in mass by 1.0kg is added to 1.5kg, making it 2.5kg.

Since the mass now became 2.5kg , and velocity is unknown

(2.5kg)(v)=0.375kgm/s

Therefore, v=0.15m/s

The velocity resulting from increase in mass by 1.0kg is 0.15m/s. According to conservation of momentum increase in weight only serves to slow down the velocity, but initial momentum to the horizontal is well conserved.