Starting from Zero: Static Friction

Two coefficients of friction correspond to two different physical processes. The first, called the coefficient of static friction, applies when you start pushing something at rest to get it moving. When you already have something moving and need to keep applying a force to keep it in motion, that’s called the coefficient of kinetic friction.

The coefficient of static friction,

is usually larger than the coefficient of kinetic friction,

and both are always greater than 0.0 and almost always less than 1.0.

Sample question

1. Suppose that you need to move a 100-kg desk. If the coefficient of static friction between the floor and the desk is 0.2, how much force do you have to apply to get the desk to start to move?

   The correct answer is 200 N.

   1. The force of friction here is

      

   2. Plug in the numbers: (0.2)(100)(9.8) = 196 N. From significant figures, this answer rounds to 200 N.

Practice questions

1. You’re standing at the top of a ski slope and need 15 N of force to get yourself moving. If your mass is 60 kg, what is the coefficient of static friction?

   

2. You’ve started to pull a garbage can out to the curb. If the can has a mass of 20 kg and you need to apply 70 N to get the can moving, what is the coefficient of static friction?

   

Following are answers to the practice questions:

1. 0.03

   1. Solve for

      

      The force due to friction is

      

   2. The force due to friction, F_f, is 15 N, and the normal force, F_n, is 60g = 590 N.

   3. Use the equation

      

      to get the coefficient of static friction: 15/590 = 0.03.

2. 0.36

   1. Solve for

      

      The force due to friction is

      

   2. The force due to friction, F_f, is 70 N, and the normal force, F_n, is 20g = 196 N.

   3. Use the equation

      

      to get the coefficient of static friction: 70/196 = 0.36. From significant figures, this answer rounds to 0.4.