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Overview
Kinetic energy is the energy of motion. Any moving object has kinetic energy, including a rolling ball, flowing river, moving car, flying aircraft, vibrating molecule, or orbiting satellite.
Energy of Motion
Kinetic energy describes a moving object's capacity to do work because of its motion. A moving hammer can drive a nail, moving water can turn a turbine, and a moving vehicle can deform material in a collision.
Mass and Speed
Kinetic energy increases with mass and with the square of speed. Doubling an object's speed makes its kinetic energy four times larger, assuming mass stays the same.
Kinetic Energy Is a Scalar
Kinetic energy has magnitude but no direction. Two identical objects moving at the same speed in opposite directions have the same kinetic energy even though their velocities are different.
Work-Energy Theorem
The work-energy theorem states that net work equals change in kinetic energy. Positive net work increases kinetic energy, while negative net work decreases it.
Kinetic Energy in Collisions
In elastic collisions, total kinetic energy is conserved. In inelastic collisions, some kinetic energy is transformed into heat, sound, deformation, or internal energy.
Rotational Kinetic Energy
Rotating objects also have kinetic energy. A rolling wheel has translational kinetic energy from motion of its center of mass and rotational kinetic energy from spinning.
Common Mistakes
A common mistake is treating kinetic energy as a vector because motion has direction. Another is assuming kinetic energy doubles when speed doubles, even though speed is squared.
Applications
Kinetic energy is important in transportation, sports, machinery, safety engineering, turbines, collisions, fluid flow, and thermal physics.
Why This Matters in Physics
Kinetic energy connects motion to work, power, collisions, conservation laws, safety, and real-world design.