Kinetic energy (KE) is the energy of motion. Any moving object — from a baseball to a planet — has kinetic energy proportional to its mass and the square of its velocity.
Formula
KE = ½mv², where m = mass (kg), v = velocity (m/s). Units: Joules (J).
Key Properties
- Doubling velocity quadruples kinetic energy (v² relationship)
- A car at 60 mph has 4× the KE of one at 30 mph — this is why high-speed crashes are so much more dangerous
Work-Energy Theorem
The net work done on an object equals its change in kinetic energy: W = ΔKE.
Kinetic Energy in Vehicle Safety
Because kinetic energy scales with velocity squared, small speed increases dramatically raise crash severity — going from 30 to 40 mph (a 33% speed increase) increases kinetic energy by 78%. This nonlinear relationship is a core reason speed limits in pedestrian-heavy areas are set so much lower than highway limits.
Kinetic Energy at the Molecular Level
Temperature is essentially a measure of the average kinetic energy of a substance's molecules — "hotter" literally means faster-moving particles. This connects the everyday formula KE = ½mv² directly to thermodynamics and explains why heating a gas increases its pressure (faster molecules collide with container walls harder and more often).