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  1. Glossary
  2. /Physics
  3. /Friction

What is Friction?

In short

Friction is the force that opposes the motion, or the attempt at motion, of two surfaces in contact. It acts along the surfaces, in the direction opposite to the sliding.

In simple words

Static friction acts when there is no sliding. It grows to a maximum of fs=μsNf_s = \mu_s Nfs​=μs​N, where NNN is the normal force and μs\mu_sμs​ is the coefficient of static friction. Once sliding starts, the kinetic friction is smaller: fk=μkNf_k = \mu_k Nfk​=μk​N. Friction is the reason you can walk and why brakes work, but it also wastes energy as heat.

Example

A 10 kg10\ \text{kg}10 kg box rests on a floor with μs=0.4\mu_s = 0.4μs​=0.4. The most friction available is 0.4×10 kg×9.8 m/s2=39 N0.4 \times 10\ \text{kg} \times 9.8\ \text{m/s}^2 = 39\ \text{N}0.4×10 kg×9.8 m/s2=39 N. A push of 30 N30\ \text{N}30 N does not move the box.

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Related terms

  • Newton's Second Law (F = ma)
  • Weight and Mass

← All glossary terms

Learn it in

  1. Friction and Circular MotionClass 11 to 12
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Practise

  1. A wedge of mass M = 3.0 kg rests on a smooth horizontal floor…Advanced
  2. A fixed horizontal cylinder has a rough surface…Advanced
  3. A plank B of mass 4 kg and length 0.9 m lies on a long horizontal table…Hard
  4. A block is sent up a slope at 27° with a speed of 8.0 m/s…
Hard
All problems on this topic →

Quick questions

  1. A block A of mass 4 kg rests on a rough incline that makes an angle of 37° with the…Hard
  2. A block of mass 1 kg rests on a plank of mass 3 kg, which lies on a smooth floor. The…Advanced
  3. A uniform chain of length 2.0 m and mass 4.0 kg lies partly on a rough horizontal table…Hard

More Physics terms

  1. Acceleration
  2. Acceleration Due to Gravity (g)
  3. Adiabatic Process
  4. Amplitude
  5. Angle of Incidence
  6. Angle of Reflection
All Physics terms →

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