# Physics

posted by Daniella

A 15 kg box sits on a horizontal force.

the coefficient of static fricition between the box and the surface is 0.70. if a force P is exerted on the box at an angle directed 37 degrees below the horizontal, what must be the magnitude of P to get the box moving??

1. Damon

force down = 15 * 9.8 + P sin 37
force horizontal = P cos 37
0.7 (15 + P sin 37) = P cos 37

2. drwls

How can a box sit on a force? Do you mean that it sits on a horizontal surface?

The vertical force applied to the surface by the box is
15*g + P*sin37.

The maximum friction force (which starts it moving) is
Ff = 0.70(15*g + P*sin37)
This must equal the horizontal component of P, so
Pcos37 = 15*g + P*sin37
P(cos37 - sin37) = 15 g
P = (15/0.1968)*g = 747 N

3. Daniella

i mean horizontal surface not horizontal force..sorry..

4. drwls

I believe Damon neglected to include a g term when considering the weight of the mass. Otherwise, we agree.

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