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Energy loss to friction
Intermediate
Mechanical energy lost equals friction work, converted to heat.
ΔE = −f dFriction converts mechanical energy into heat: ΔE_mech = −f·d. A sliding block's final speed follows from W_fric = f d in the work–energy theorem.
A 2 kg block at 6 m/s stops after 6 m: friction = 6 N.
Friction depends on path length, not displacement.
In more detail
Friction converts mechanical energy to thermal energy, so the total energy is conserved but the useful mechanical energy decreases. Energy lost to friction equals the friction force times the distance slid.
Real-world uses
- Brakes convert the kinetic energy of a car into heat in the discs.
- Skidding on a road heats the tyres and the surface.
- A sliding box on a rough floor loses mechanical energy to friction, which warms the box and the floor.
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Frequently asked questions
Is energy really lost?
It is converted to thermal energy, which spreads into the surroundings and is hard to use again.
Does friction always lose energy?
For sliding friction, yes. Static friction at a rolling contact does no net work.
Where does the lost energy go?
Mostly into thermal energy of the surfaces and the surrounding air, which is why friction warms things up.
Related topics in Work, Energy & Power
Energy on a pendulum · Energy in spring-mass systems · Roller coaster and loops · General energy conservation · Power · Instantaneous power