Friction
A force along a surface that resists sliding. Its largest value is the coefficient of friction times the normal force.
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Analogy
Sandpaper grips more than ice.
Example
N = mg = 196 Nf max = 0.4 x 196f max = 78.4 N
In practice
A 20 kg box with a friction coefficient of 0.4 needs about 78 N to start sliding.
Practice
Flashcards
- What is the formula for maximum static friction?f max = (coefficient of friction) x (normal force).
Interview questions
- A 10 kg block slides down a 30 degree ramp with friction coefficient 0.2. Find its acceleration.
- Draw a free-body diagram
- Weight component along slope is mg sin theta
- Friction is coefficient times mg cos theta
- Net force over mass gives acceleration
Questions people ask
What is the difference between static and kinetic friction?
Static friction holds an object that is not sliding. It can be any size up to a maximum. Kinetic friction acts once sliding starts and is usually a bit smaller. This is why it takes more effort to start moving a heavy sofa than to keep it moving.
Does friction depend on contact area?
In the simple model used in first-year courses, no. Friction depends on the coefficient and the normal force. A brick on its wide face or narrow face has about the same friction. Real surfaces are more complex, but this model works well for most problems.