I kind of confused about this physics question. Assume for example a 1 kg cart is moving at a constant speed of 5 m/s on a frictionless service. Then, a 1 kg weight is dropped onto the cart. Would its initial momentum be conserved? What would be it's final velocity? At first, the cart has a momentum of 5 kg m /s. If momentum were conserved, its final velocity would be 2.5 m/s. However, I do not really understand this. According to Newton's First Law of Motion, an object will remain in equilibrium unless acted on by an external force. The cart is apparently instantaneously decelerated to 2.5 m/s. How is this possible? What is the external force applied on the cart? If the cart remained the same velocity (5 m/s before and after the weight was added), then conservation of momentum wouldn't hold. Any help and insight would be greatly appreciated. It might just be that I don't really understand the laws of motion and momentum. Thanks in advance.