Kinetic Energy

The kinetic energy of a rigid body is the energy possessed by the body motion

Kinetic energy is the energy of motion. There are several forms of kinetic energy

  • vibration - the energy due to vibration motion
  • rotational - the energy due to rotational motion
  • translational - the energy due to motion from one location to another

Energy has the same units as work and work is force times distance. One Joule is one Newton of force acting through one meter - Nm or Joule in SI-units. The Imperial units are foot-pound.

  • 1 ft lb = 1.356 N m (Joule)

Translational Kinetic Energy

car average velocity speed

Translational kinetic energy can be expressed as

Et = 1/2 m v2         (1)


Et = kinetic translation energy (Joule, ft lb)

m = mass (kg, slugs)

v = velocity (m/s, ft/s)

  • one slug = 32.1740 pounds (as mass) - lbm

Rotational Kinetic Energy

Rotational kinetic energy can be expressed as

Er = 1/2 I ω2         (2)


Em = kinetic rotation energy (Joule, ft lb)

I = moment of inertia, an object's resistance to changes in rotation direction (kg m2, slug ft2)

ω = angular velocity (rad/s)

Example - Car and Kinetic Energy

The kinetic energy of a car with mass of 1000 kg at speed 70 km/h can be expressed as

Et = 1/2 (1000 kg) ((70 km/h) (1000 m/km) / (3600 s/h))2

= 189043 (Joule)

The kinetic energy of the same car at speed 90 km/h can be expressed as

Et = 1/2 (1000 kg) ((90 km/h) (1000 m/km) / (3600 s/h))2

= 312500 (Joule)

Note! When the speed is increased with 28% (from 70 to 90 km/h) - the kinetic energy is increased with 65% (from 189043 to 312500 J). Be aware that this huge rise in kinetic energy must be absorbed by the safety construction of the car to provide the same protection in a crash - which is very hard to achieve. In a modern car it is possible to survive a crash at 70 km/h. A crash at 90 km/h is more likely death.

Related Topics

  • Dynamics - Dynamics Motion - velocity and acceleration
  • Mechanics - Kinematics, forces, vectors, motion, momentum, energy and the dynamics of objects

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