Work, Energy and Power
Work–energy theorem, conservative forces, potential energy and collisions.
What is Work, Energy and Power?
Scalar product of force and displacement, W = F s cos(θ), in joules; zero when force is perpendicular to displacement
Key formula / rule: Work and theorem
Key points
- Compute work for constant and variable forces and apply the work-energy theorem and power relations
Common exam trap
Dropping the cos(θ) factor in work
Definitions
- Name
Work
- Definition
Scalar product of force and displacement, W = F s cos(θ), in joules; zero when force is perpendicular to displacement
- Name
Key idea
- Definition
Net work equals the change in kinetic energy; with only conservative forces, K + U stays constant
- Name
Exam insight
- Definition
Centripetal force does zero work in uniform circular motion because it is always perpendicular to the displacement
- Name
Worked example
- Definition
50 N pulling a body 4 m at 60 degrees: W = 50 x 4 x cos60 = 100 J, so kinetic energy rises by 100 J
Learning objectives
Compute work for constant and variable forces and apply the work-energy theorem and power relations
Formulae
- Name
Work and theorem
- Expression
W = F s cos(θ); W = integral F dx; Wnet = DeltaK
- Name
Power
- Expression
P = W/t = F . v
Common mistakes
Dropping the cos(θ) factor in work
Treating work done against friction as recoverable
Keywords
work
energy
power
work energy theorem
kinetic energy
potential energy
joule
watt