Suppose an empty grocery cart rolls downhill in a parking lot. The cart has a maximum speed of 1.3 m/s when it hits the side of the store and comes to rest 0.30 s later. If an unbalanced force of —65 N stops the cart, what is the mass of the grocery cart?

The cart comes to rest from 1.3 m/s in a matter of 0.30 s, so it undergoes an acceleration a of

a = (0 - 1.3 m/s) / (0.30 s)

a ≈ -4.33 m/s²

This acceleration is applied by a force of -65 N, i.e. a force of 65 N that opposes the cart's motion downhill. So the cart has a mass m such that

-65 N = m (-4.33 m/s²)

m = 15 kg

c. newton's third law of motion