Question

Find a small plastic food container and slide it on a kitchen table by giving it a gentle tap.
a) Draw a free body diagram for that container. Identify all the forces acting on the container as well as acceleration it experiences. Which forces on the diagram compensate each other? Write down the expression for the net force.
b) The container weighs 16.4g, use this to calculate the friction force for the problem, assuming a coefficient of kinetic friction between polyethylene and table surface is 0.2.

Answer 1

a)

Fn is the normal force, Fg is the gravity force, or weight, and Ffr is the frictional force. The normal and the weight of the container compensate each other, but the frictional force is unbalanced, therefore the net force is in the negative x direction:

b) By definition, the friction force can be written as:

Where mu_k is the coefficient of kinetic friction. On the other hand, from the balance in the y direction:

Replacing backwards we get:

c) When you give the same sized tap the container covers a longer distance before coming to a halt. This is because now that the surface is lubricated the kinetic friction is less: