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Energy, work and power
This topic connects the force you apply to an object with the energy it gains and how quickly that energy is transferred. We'll explore how pushing a box (doing work) gives it speed (kinetic energy) or height (potential energy), and how doing this quickly requires more power.
Need to know
What you need to know
- The SI unit for work is the Joule (J), which is equivalent to a Newton-metre (Nm).
- Work is a scalar quantity, not a vector.
- Work done is positive if the force helps the motion ($0^\circ \le \theta < 90^\circ$).
- Work done is negative if the force opposes the motion ($90^\circ < \theta \le 180^\circ$). For example, work done by friction.
- Work done is zero if the force is perpendicular to the motion ($\theta = 90^\circ$). For example, the normal contact force on an object moving on a horizontal surface.
Explanation
From Pushing to Power
- Work is done when a force causes displacement. It's calculated as the force component in the direction of motion multiplied by the distance moved: $W = Fs \cos \theta$.
- Energy comes in two main forms for mechanics: kinetic energy ($KE = \frac{1}{2}mv^2$) from motion, and gravitational potential energy ($GPE = mgh$) from height.
- Power is the rate at which work is done or energy is transferred. It can be found by dividing work by time ($P = W/t$) or by multiplying force by velocity ($P = Fv$).
- The principle of conservation of energy states that in a system with no external non-conservative forces like friction, the total mechanical energy (KE + GPE) remains constant.