Friction is the force that opposes the motion of two surfaces that rub together. Friction acts in
a direction opposite to the direction of the moving object. Friction will eventually cause a moving object
to come to a stop.
What Causes Friction?
A surface that seems very smooth, like polished metal, is actually is rough when you look at it close
enough. If you examine the surface of the metal under a powerful microscope, you will see tiny bumps.
When two surfaces are pushed together, sticking happens where the bumps touch one another. These
places are called microwelds. Micro means “very small”. Friction is caused by the microwelds that form
when two surfaces are touching. To move one surface over another, a net force must be applied to
break the microwelds.
The amount of friction between two surfaces depends on two things: (i) the types of surfaces touching
each other and (ii) the amount of force pushing the two surfaces together. The rougher the surface, the
more microwelds there are and the more “sticky” the surface is. The greater the force pushing the two
surfaces together, the stronger the microwelds since more of the bumps are touching each other.
There are four types of friction: static friction, sliding friction, rolling friction and fluid friction.
Static friction
Acts on objects that are not moving
A table resting on the floor
Sliding friction
Occurs when 2 solid surfaces slide across one another
Moving a box across the floor
Rolling friction
When an object rolls across a surface
A tire rolling on a road
Fluid friction
When an object moves through a fluid (liquid or gas)
A boat moving through water
Air Resistance
Air resistance is a special type of fluid friction that occurs when an object falls through the
air. When you try to walk against a very strong wind, you can feel air resistance acting on you. It also
occurs on skydivers falling through the air.
Air resistance is not the same for all objects. The greater the surface area of an object, the greater the
air resistance. Parachutes work because their very large surface area maximizes air resistance to slow
down a skydiver. Air resistance also increases with velocity. As the velocity of a falling object increases,
air resistance increases until it is equal with the force of gravity. When the two forces are balanced, the
falling object stops accelerating and the velocity remains the same. This greatest velocity that can be
achieved by a falling object is called terminal velocity.
1. What is the definition of friction?
2. Friction acts in a direction _______________ to the object’s direction of motion.
3. The strength of the force of friction depends on what two factors?
4. Describe how microwelds explains how there can be friction between two seemingly smooth surfaces.
Match each type of friction with its definition by writing the letter of the correct definition
in the right column in the space beside the term in the left column.
Type of Friction
Fluid friction
A. The kind of friction that results when a wheel turns on a surface.
Static friction
B. The kind of friction that results when you rub sandpaper against wood.
Rolling friction
C. The kind of friction that exists between oil and a door hinge, for example.
Sliding friction
D. The kind of friction that slows a falling object as it moves through the air.
Air resistance
E. The kind of friction that occurs even when two objects are not moving.
5. What two factors can increase the force of air resistance for an object?
6. What is terminal velocity? What can be said about the forces on the object at terminal velocity?