# Static forces

1. Mar 15, 2005

### stealth2k4

A backpack full of books weighing 47.0 N rests on a table in a physics laboratory classroom. A spring with a force constant of 150 N/m is attached to the backpack and pulled horizontally. If the spring stretches by 2.20 cm before the backpack begins to slip, what is the coefficient of static friction between the backpack and the table?

i tried to use Fsmax = Mus of s times the normal force what should i use

Last edited: Mar 15, 2005
2. Mar 16, 2005

### learningphysics

You have the equation. What is Fsmax? What is the normal force?

Use a free-body diagram to see all the forces.

3. Mar 16, 2005

### xanthym

{Normal Force of Books on Table} = FN = (47.0 N)
{Coefficient of Static Friction} = K
{Max Frictional Force on Books} = K*FN = (47)*K
{Spring Force Constant} = c = (150 N/m)
{Spring Displacement} = d = (2.20 cm) = (2.2e(-2) m)
{Spring Force Applied to Books} = c*d = (150 N/m)*(2.2e(-2) m) = (3.3 N)

Since books began to move with the above spring displacement, we have:
{Spring Force Applied to Books} = {Max Frictional Force on Books}
::: ⇒ (3.3) = (47)*K
::: ⇒ K = (0.0702)

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4. Mar 16, 2005

### learningphysics

Please don't solve the entire problems. They won't learn from that.