An object rests on an inclined surface thanks to the presence of static friction forces between the object and the surface.

Question

The magnitude of the friction force that prevents the object from sliding is found to be 30 N. Now, the angle that the surface makes with respect to the horizontal is increased, and the object is found to be still at rest.The magnitude of the friction force that prevents the object from sliding is now ...

a) zero
b) larger than zero less than 30N
c) exactly equal to 30N
d) greater than 30N

I think it is greater than 30N

Answer 1

It could be larger, if someone has stuck a nail through the object. But barring that,

the normalforce is mg*CosTheta. What does CosTheta do as the angle gets larger?

Answer 2

but isn't costheta periodic?

Answer 3

Only if you got 180 degrees, but here in Texas, we only angle to ramp from zero to 89 degrees. At that point, all things not nailed fall off.
What does CosTheta do between zero and 90 deg?

Answer 4

costheta decreases so the answer is B? right

Answer 5

yes.