how does normal force affect frictional force?

Answer 1

Normal force presses the object and the surface this object moves upon together. The more they are pressed together the higher is the force of friction.

f=uN

f-force of friction
u-coefficient of friction
N - normal to the surface force exerted by the object

Answer 2

Friction depends on two things 1. The nature of the surface and 2. The weight. On a horizontal surface the weight is equal to the normal force. So the greater the normal force, the greater the friction.

Answer 3

Normally.

f = kN; where k is the friction coefficient, N is the normal force, and f is the force pushing a mass (M) along a surface. k is typically found experimentally because it is really determined by a lot of things that are not well defined by equations.

The normal force is usually some portion of the weight (W = Mg) of the mass (M) sitting on some sort of ramp (the surface) in physics questions. The portion of W that is N is given by N = W cos(theta); where theta is the angle of slope for the ramp. Note, where theta = 0 deg, N = W, so, in that special case, all the weight of the mass is the normal force.

Note also, if you begin to increase the slope of the ramp from the horizontal (theta = 0), when the mass just starts to slip back down the ramp, tan(THETA) = k; where THETA is the critical angle at which the mass began to slide.

This stems from noting that force along the surface of the inclined ramp is f = F sin(theta); where F = all the forces acting on the mass. But when the weight of the mass makes up all the forces acting on it (W = F), we have f = W sin(theta) = N k = W cos(theta) k. Thus, sin(theta) = cos(theta) k; and k = sin(theta)/cos(theta) = tan(theta).