Potential of a hand cart

A hand cart (the left one in the picture), which was on the top of a $1$ m high slope, has slid down the slope and stopped (the right one in the picture), pushing a box on the ground by $1$ m to the right. If the frictional force between the box and floor is constant, how large is this frictional force? (Ignore the frictional force between the hand cart and floor and assume the gravitational acceleration is $10 \text{ m/s}^2$ .)