Question

A box weighing 43.2 N is pulled horizontally until it slides uniformly lat a constant

speed) over a level floor. The applied force is 6.30 N. Draw a force diagram (FBD)
showing all forces acting on the box. What is the force of friction, in Newtons,
between the box and the floor?

Answer 1

Your diagram should include four forces:

• the box's weight, pointing down (magnitude w = 43.2 N)

• the normal force, pointing up (mag. n)

• the applied force, pointing the direction in which the box is sliding (mag. p = 6.30 N, with p for "pull")

• the frictional force, pointing oppoiste the applied force (mag. f )

The box is moving at a constant speed, so it is inequilibrium and the net forces in both the vertical and horizontal directions sum to 0. By Newton's second law, we have

n + (-w) = 0

and

p + (-f ) = 0

So then the forces have magnitudes

w = 43.2 N

n = w = 43.2 N

p = 6.30 N

f = p = 6.30 N