Types of Friction
Jean Brainard, Ph.D.
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Printed: October 24, 2016
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Types of Friction
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Types of Friction
• Identify four types of friction.
• Compare and contrast static, sliding, and rolling friction.
• Define fluid and describe fluid friction.
Climbing a vertical rock wall means pitting your strength and stamina against the force of gravity, which pulls you
down toward the ground. Another force helps you to climb the vertical rock wall by keeping your hands and feet
from slipping. That force is friction.
Four Types of Friction
Friction is the force that opposes motion between any surfaces that are in contact. There are four types of friction:
static, sliding, rolling, and fluid friction. Static, sliding, and rolling friction occur between solid surfaces. Fluid
friction occurs in liquids and gases. All four types of friction are described below.
Static Friction
Static friction acts on objects when they are resting on a surface. For example, if you are hiking in the woods, there is
static friction between your shoes and the trail each time you put down your foot (see Figure 1.1). Without this static
friction, your feet would slip out from under you, making it difficult to walk. In fact, that’s exactly what happens if
you try to walk on ice. That’s because ice is very slippery and offers very little friction.
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FIGURE 1.1
Q: Can you think of other examples of static friction?
A: One example is the friction that helps the people climb the rock wall in the opening picture above. Static friction
keeps their hands and feet from slipping.
Sliding Friction
Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than
static friction. That’s why it’s easier to slide a piece of furniture over the floor after you start it moving than it is to
get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write
with a pencil. The pencil “lead” slides easily over the paper, but there’s just enough friction between the pencil and
paper to leave a mark.
Q: How does sliding friction help you ride a bike?
A: There is sliding friction between the brake pads and bike rims each time you use your bike’s brakes. This friction
slows the rolling wheels so you can stop.
Rolling Friction
Rolling friction is friction that acts on objects when they are rolling over a surface. Rolling friction is much weaker
than sliding friction or static friction. This explains why most forms of ground transportation use wheels, including
bicycles, cars, 4-wheelers, roller skates, scooters, and skateboards. Ball bearings are another use of rolling friction.
You can see what they look like in the Figure 1.2. They let parts of a wheel or other machine roll rather than slide
over on another.
FIGURE 1.2
The ball bearings in this wheel reduce friction between the inner and outer
cylinders when they turn.
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Chapter 1. Types of Friction
Fluid Friction
Fluid friction is friction that acts on objects that are moving through a fluid. A fluid is a substance that can flow and
take the shape of its container. Fluids include liquids and gases. If you’ve ever tried to push your open hand through
the water in a tub or pool, then you’ve experienced fluid friction. You can feel the resistance of the water against
your hand. Look at the skydiver in the Figure 1.3. He’s falling toward Earth with a parachute. Resistance of the
air against the parachute slows his descent. The faster or larger a moving object is, the greater is the fluid friction
resisting its motion. That’s why there is greater air resistance against the parachute than the skydiver’s body.
FIGURE 1.3
Summary
• Friction is the force that opposes motion between any surfaces that are in contact. There are four types of
friction: static, sliding, rolling, and fluid friction.
• Static, sliding, and rolling friction occur between solid surfaces. Static friction is strongest, followed by sliding
friction, and then rolling friction, which is weakest.
• Fluid friction occurs in fluids, which are liquids or gases.
Review
1. List four types of friction.
2. You can move heavy boxes by sliding them over the ground. Or you can put them on a dolly, like the one in
the Figure 1.4, and then roll them over the ground. Explain which way makes it easier to move the boxes.
4. What is a fluid? Give an original example of fluid friction.
Resources
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/186727
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FIGURE 1.4
3.
References
1. Ray Sawhill. Friction helps this person to walk . CC BY 2.0
2. Image copyright Ramona Heim, 2013. Ball bearings reduce friction . Used under license from Shutterstock.com
3. Courtesy of Donna Dixon and the US Army. Parachutes work using fluid friction . Public Domain
4. Image copyright AVAVA, 2013. Practice problem illustrating how reducing friction can save energy . Used
under license from Shutterstock.com
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