How to find the coefficient of static friction.

[mateye10]

Homework Statement

In order to hold a book of mass m stationary against a wall, you must push horizontally with a force of at least P.
a. make a force diagram and find the coefficient of static friction
b. calculate the coefficient of static friction for the wall and book when at least a 35-N horizontal push is needed to hold a 1.2-kg book steadily at rest against the wall.

Homework Equations

F=ma
F_n=mg
F_f=μF_net

The Attempt at a Solution

I find it difficult to grasp the concept that, somehow, there is still friction between the wall and the book even though there is no velocity. My best guess would be:

F_f = F_net so μ would have to equal 1. help would be greatly appreciated.

 

[Flipmeister]

If there is no velocity and no acceleration, the system is in equilibrium. Try drawing the force diagram without the static friction and then you'll see how friction plays a role.

 

[mateye10]

so is friction equal to weight?

 

[Flipmeister]

Yes, and knowing that and the basic equation of static friction (f=μ*n), you can now calculate μ.

 

[mateye10]

great, I see. thank you so much.