You are attempting to slide your fridge to clean the floor underneath. You apply a horizontal pulling force of 215 N, but the fridge does not move.

What must be true about the static friction force acting on the fridge?

A. It must have a magnitude greater than 215 N.
B. It must have a magnitude equal to 215 N.
C. It must have a magnitude less than 215 N.
D. It could have a magnitude either as great as 215 N, or less than 215 N.
E. It must have a magnitude equal to the weight of the fridge, but this weight is not given in the question.

The static friction force acting on the fridge when it does not move with a pulling force of 215 N is equal to 215 N because it counters the pulling force exactly, keeping the fridge stationary. Therefore, the correct answer is b .

When you apply a horizontal pulling force of 215 N to the fridge and it does not move, the static friction force acting on the fridge must have a magnitude equal to 215 N. This is because the forces are in balance and therefore, the static friction force is exactly countering your pulling force, which conforms to Newton's first law of motion. Once the fridge is in motion, the kinetic friction force would be less than the static friction force. This explains why it's easier to keep an object moving at a constant velocity once it has started moving, as the kinetic friction is then less than the static friction.