Roughly speaking, frictional forces are caused by the roughness of the materials in contact, deformations in the materials, and molecular attraction between materials. You needn’t worry too much over the causes of friction, though: SAT II Physics isn’t going to test you on them. The most important thing to remember about frictional forces is that they are always parallel to the plane of contact between two surfaces, and opposite to the direction that the object is being pushed or pulled.
There are two main types of friction: static friction and kinetic friction. Kinetic friction is the force between two surfaces moving relative to one another, whereas static friction is the force between two surfaces that are not moving relative to one another.
Static Friction
Imagine, once more, that you are pushing a box along a floor. When the box is at rest, it takes some effort to get it to start moving at all. That’s because the force of static friction is resisting your push and holding the box in place.
In the diagram above, the weight and the normal force are represented as
W and
N respectively, and the force applied to the box is denoted by
. The force of static friction is represented by
, where
. The net force on the box is zero, and so the box does not move. This is what happens when you are pushing on the box, but not hard enough to make it budge.
Static friction is only at work when the net force on an object is zero, and hence when
. If there is a net force on the object, then that object will be in motion, and kinetic rather than static friction will oppose its motion.
Kinetic Friction
The force of static friction will only oppose a push up to a point. Once you exert a strong enough force, the box will begin to move. However, you still have to keep pushing with a strong, steady force to keep it moving along, and the box will quickly slide to a stop if you quit pushing. That’s because the force of kinetic friction is pushing in the opposite direction of the motion of the box, trying to bring it to rest.
Though the force of kinetic friction will always act in the opposite direction of the force of the push, it need not be equal in magnitude to the force of the push. In the diagram above, the magnitude of
is less than the magnitude of
. That means that the box has a net force in the direction of the push, and the box accelerates forward. The box is moving at velocity
v in the diagram, and will speed up if the same force is steadily applied to it. If
were equal to
, the net force acting on the box would be zero, and the box would move at a steady velocity of
v, since Newton’s First Law tells us that an object in motion will remain in motion if there is no net force acting on it. If the magnitude of
were less than the magnitude of
, the net force would be acting against the motion, and the box would slow down until it came to a rest.
