Question

an open cart is moving along a straight frictionless horizontal track. When rain starts falling vertically into the cart, what happens to the speed of the cart?

Answer 1

The speed of the cart decreases

Step-by-step explanation:

The track is frictionless, so we can consider the cart as isolated system (no external forces acting on it): therefore, the momentum of the cart is conserved.

When it starts raining, some water falls into the cart: this means that the mass of the cart, m, increases. But we said that the momentum, p, must remain constant: in order for that to happen, this means that the speed of the cart, v, must decrease (because the relationship is an inverse relationship, so if m increases, v must decrease).

Answer 2

The speed of the cart decreases

Step-by-step explanation:

The track is frictionless, so we can consider the cart as isolated system (no external forces acting on it): therefore, the momentum of the cart is conserved

`p=mv=const.`

where m is the mass and v the speed of the cart.

When it starts raining, some water falls into the cart: this means that the mass of the cart, m, increases. But we said that the momentum, p, must remain constant: in order for that to happen, this means that the speed of the cart, v, must decrease (because the relationship
`mv=const.` is an inverse relationship, so if m increases, v must decrease).