Energy
This page covers the core ideas now; worked examples and practice problems will keep expanding.
1. Work
Work is the bridge between force (a dynamics idea) and energy. Work is done on an object only when a force causes displacement in the direction of that force — a force with zero component along the motion does zero work, no matter how large it is.
Fapplied force (N).ddisplacement (m).θangle between the force and displacement vectors.
"If you hold a heavy box still, you're doing work on it." False — work requires displacement. Holding a box stationary, no matter how tiring, does zero physics-work on the box because d = 0.
2. Kinetic & Potential Energy
Potential energy is always defined relative to a chosen reference point — there's no such thing as an object's "absolute" potential energy, only a difference between two heights or positions.
3. Conservation of Energy
In a system with no friction or other non-conservative forces, total mechanical energy stays constant — it just trades forms between kinetic and potential.
When friction or air resistance is present, mechanical energy isn't conserved — but total energy still is, once you account for the energy converted to heat: KEi + PEi = KEf + PEf + Wfriction.
4. Power
Power measures how quickly energy is transferred, not how much. Two machines can do identical total work yet have very different power if one takes far longer to do it.
5. Simulator
6. Practice Problems
1. A 2 kg block slides from rest down a frictionless 5 m high ramp. Find its speed at the bottom.