What are the units of work?

Joules. One joule is one newton-metre, the work done moving one newton through one metre.

Can work done be negative?

Yes — when the force opposes the motion, as friction does, the work done by that force is negative.

PhysicsGCSE, IGCSE & A Level

Work done is the energy transferred when a force moves an object through a distance in the direction of the force.

Only the component of force along the motion does work, so a force at an angle needs a

\cos\theta

factor and a perpendicular force does none.

Power is the rate of doing work, which links work directly to time and to the power ratings of motors and engines.

W = Fd\cos\theta, \qquad P = \dfrac{W}{t}

Pushing with

50\,\text{N}

over

4\,\text{m}

transfers

200\,\text{J}

; done in

10\,\text{s}

that is

20\,\text{W}

Carrying a bag horizontally does no work against gravity — the force is vertical and the motion horizontal. Examiners test this every year.

What are the units of work?: Joules. One joule is one newton-metre, the work done moving one newton through one metre.
Can work done be negative?: Yes — when the force opposes the motion, as friction does, the work done by that force is negative.

Kinetic energy is the energy an object has because of its motion, equal to half its mass times its velocity squared.

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred between stores.

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