# Homework Help: Find friction force on box.

1. Nov 1, 2015

### MattRC

1. The problem statement, all variables and given/known data
At a construction site, a 62.0 kg bucket of concrete hangs from a light (but strong) cable that passes over a light friction-free pulley and is connected to an 80.0 kgbox on a horizontal roof (see the figure (Figure 1) ). The cable pulls horizontally on the box, and a 50.0 kg bag of gravel rests on top of the box. The coefficients of friction between the box and roof are shown. The system is not moving. μs = 0.700, μk = 0.400

https://session.masteringphysics.com/problemAsset/2039115/1/YF-07-29.jpg

The question I am trying to answer is part B:
Find the friction force on the box.

2. Relevant equations
F = μN

3. The attempt at a solution
m_box = 80.0kg
m_sand = 62.0kg

F = μN
F = μmg
F = μ(mbox + msand)g
F = 0.700(80.0kg + 62.0kg)9.81
F = 893 N

So I have already got the answer wrong on mastering physics after 5 attempts and it revealed that the answer is 608N. I can't for the life of me figure out what I did wrong or how to actually get 608N.

2. Nov 1, 2015

### CWatters

u(mbox + mgravel)g would be the maximum static friction possible. You need to calculate the actual frictional force.

Hints
What force acts on the box?
Why do they tell you its not moving?
Where is your free body diagram for the box?

3. Nov 1, 2015

### Mister T

This equation doesn't look right to me. It will give you the maximum value of static friction, but if the tension in the string is less than this the box won't move.

This is not right, either.