ANSWER
Step-by-step explanation
First, let us make a sketch of the problem:
The normal force has the same magnitude as the weight of the cart.
where m = mass; g = acceleration due to gravity
Hence, the normal force is:
Applying Newton's second law of motion in the horizontal direction, the sum of forces acting on the cart is:
where Fp = force of the push
Fr = friction force
Since the waiter pushes the cart at a constant speed, it means that the acceleration of the cart (ax) is 0.
This implies that:
Solve for Fr:
That is the friction force.