How forces do work and energy changes form
In physics, work is done when a force moves an object in the direction of the force: W = Fs cos θ, measured in joules. Carrying a bag across a level floor does no work against gravity, because the force and movement are at right angles. Energy is the capacity to do work. Kinetic energy, the energy of motion, is ½mv², and gravitational potential energy near Earth is mgh. A 2 kg book lifted 3 m gains 2 × 9.8 × 3 ≈ 59 J of potential energy.
Energy is never created or destroyed, only changed from one form to another; a falling ball turns potential energy into kinetic energy. Power is the rate of doing work, P = W/t, measured in watts. In Class 11, the work-energy theorem states that the net work done on a body equals its change in kinetic energy. Work done by a variable force is the area under a force–displacement graph, and a spring stores energy ½kx². Collisions conserve momentum; elastic collisions also conserve kinetic energy.
Why it matters
Hydroelectric dams convert the potential energy of stored water into electricity, and car engines are rated by power. Understanding energy conservation helps compare appliances, design roller coasters and analyse sports such as pole vaulting. These ideas connect mechanics with heat, electricity and every later topic that involves energy.
Key terms
- Work
- The product of a force and the displacement in the direction of the force.
- Kinetic energy
- The energy an object has because of its motion.
- Power
- The rate at which work is done or energy is transferred, measured in watts.
- Work-energy theorem
- The net work done on a body equals the change in its kinetic energy.