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Pushing hard against a wall all afternoon feels exhausting, but in the strict physics sense you’ve done no work at all — the wall never moved. "Work" in this course has a precise, sometimes counter-intuitive meaning, and it’s the doorway into energy and power calculations that run through the rest of the mechanics module.
What you'll be able to do
by a force is the energy transferred when that force moves its point of application through a distance, in the direction of the force. If a force acts but produces no movement at all (like pushing on an immovable wall), zero work is done, however hard you push.
Tip — When the force acts EXACTLY perpendicular to the direction of motion (θ=90°), cosθ=0 and no work is done at all — this is why the normal contact force on a level surface never does any work on a moving object.
is the energy an object possesses due to its motion; is the energy it possesses due to its position in a gravitational field, relative to some reference height.
is the rate of doing work (equivalently, the rate of energy transfer), measured in watts (W), where W J s⁻¹. For a force moving at constant velocity, power can equivalently be found from the force and the speed at which its point of application moves.
measures what fraction of the energy (or power) supplied to a system is usefully transferred, rather than wasted (often as heat) — always less than 100% for any real machine, since some energy dissipation is unavoidable.
Tip — P=Fv only applies at the instant an object moves at that particular speed v — if speed changes, so does the (instantaneous) power, even for a constant force.
Equation recap
Common mistakes to avoid
Key takeaways
Test yourself
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