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amingad
i apply a contact force on a rough horizontal surface and it has constant magnitude . now my question is what happens to the angle between the force and the vertical is decreased when i apply frictional force between the surface and the body
Force of friction is equal to the normal force times a constant of the materials. F = N*u. If you reduce the angle with the surface, you decrease the normal, so you decrease the force. That’s logic because think that when you press with angle, the X component (along the surface) doesn’t approach more the particles between the bodies, but the Y component does, so it makes harder to move along any direction.
Unless you are pulling rather than pushing, of course, then the normal force gets smaller for a given pull as the angle above horizontal increases by the factor F sin(theta)