# Force required to prevent something from moving on a slant

1. Jan 27, 2008

### chemboy

A box with a mass of 22 kg is at rest on a ramp inclined at 45 (degrees) to the horizontal. The coefficients of friction between the box and the ramp are: (mu(s): o.78 and mu(k): o.65)

Determine the magnitude of the smallest force that can be applied onto the top of the box, perpendicular to the ramp if the box was to remain at rest.

Fn = F
Fs = mgSinTheta
Fsmax = ukF
F >= Fsmax / uk

(22X9.8SIN45) / .65

= 234.54
= 235

is this correct??

Thanks

2. Jan 27, 2008

### Staff: Mentor

How does the applied force F change (1) the normal force between box and ramp, and (2) the friction force?

What forces act on the box?

3. Jan 27, 2008

### chemboy

The forces that are acting on the box are gravity (Fnormal) and Friction (uK)

As for your first question, Im not sure what you are looking for, but Ill try:
The applied force would have to overcome the objects ability to move down the ramp (Fa) and I would assume that increasing applied force would cause greater friction.

I imagine through your questions Im suppose to be able to find the correct method of answering this question, but have failed in doing so.

Any more help would be greatly appreciated

4. Jan 27, 2008

### Staff: Mentor

The forces on the box are: The applied force (F), the normal force, the friction force, and gravity. Draw yourself a diagram that illustrates these forces acting on the box. What direction do they act?

How does the maximum value of static friction depend on the normal force? If there were no additional force applied to the box, what would the normal force be? How does the normal force change when the force F is added?