Friction and the Normal Force
Kinetic Friction
Push a box or table across the floor (moving). Friction opposes the motion with a constant force that depends on the surfaces (coefficient of friction) and the force of interaction between the box and floor (normal force).
This was the extent of our discussion of friction last year. We'll use the symbol fk to stand for this type of sliding or KINETIC friction. The coefficient of kinetic friction is symbolized by the greek letter (pronounced mu) and a subscript for kinetic.
Push a box or table across the floor (moving). Friction opposes the motion with a constant force that depends on the surfaces (coefficient of friction) and the force of interaction between the box and floor (normal force).
This was the extent of our discussion of friction last year. We'll use the symbol fk to stand for this type of sliding or KINETIC friction. The coefficient of kinetic friction is symbolized by the greek letter (pronounced mu) and a subscript for kinetic.
Note what is NOT a part of this formula. The speed of the object does not matter, nor does whether or not it is accelerating, or how quickly it accelerates. All that matters is the perpendicular interaction force between the surface and the object (i.e. the normal force, N) and the types of surfaces that are in contact (represented by , which has no standard unit).
Static Friction
Attempt to push a box or table across, (not actually moving). Friction opposes the expected motion. Friction will automatically adjust itself (magnitude and direction) to cancel whatever force you apply (up to a point). STATIC friction depends on the other forces present that try to move the object. But it has a maximum value that depends on the same type of quantities as we discussed for kinetic friction. There is a different value for the coefficient of friction, and in general, this value of is larger than for kinetic friction. We use a different subscript to signify static versus kinetic friction.
Attempt to push a box or table across, (not actually moving). Friction opposes the expected motion. Friction will automatically adjust itself (magnitude and direction) to cancel whatever force you apply (up to a point). STATIC friction depends on the other forces present that try to move the object. But it has a maximum value that depends on the same type of quantities as we discussed for kinetic friction. There is a different value for the coefficient of friction, and in general, this value of is larger than for kinetic friction. We use a different subscript to signify static versus kinetic friction.
Some problem-solving ideas you should know:
- If the object does not move, we know the magnitude of the (static) frictional force equals the force parallel to the surface.
- If the object is about to breakaway, we know the (static) frictional force has reached its maximum value.
- If the object slides (with either constant or changing speed), we know kinetic friction must be used as a force opposing the motion (parallel to the surface.)
Imagine a block sitting at rest on a horizontal surface.
A small force is then applied but the block does not move. Gradually the force is increased until the block moves, but once it moves the force is decreased so that the block moves at a constant speed. See the graphs below to see how friction changes from static to kinetic.
A small force is then applied but the block does not move. Gradually the force is increased until the block moves, but once it moves the force is decreased so that the block moves at a constant speed. See the graphs below to see how friction changes from static to kinetic.
Let's take a few minutes to look at some example situations that involve the normal force, FN (which is labeled N in the pictures).