Resources, Tools and Basic Information for Engineering and Design of Technical Applications!
mobile - visible on handheld devices - alternatively go to www.engineeringtoolbox.com!
Engineering ToolBox > Kinetic Energy
Kinetic energy is the energy of motion. There are several forms of kinetic energy
Energy has the same units as work and work is force times distance. One joule is one Newton of force acting through one meter - N m or Joule in SI-units. The Imperial units are foot-pound.
Translational kinetic energy can be expressed as
Et = 1/2 mv2 (1)
where
Et = kinetic translation energy (Joule, ft lb)
m = mass (kg, slugs)
v = velocity (m/s, ft/s)
Rotational kinetic energy can be expressed as
Er = 1/2 Ioω2 (2)
where
Em = kinetic rotation energy (Joule, ft lb)
Io = moment of inertia (kg m2, slug ft2)
ω = angular velocity (rad/s)
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 1000/3600 (m/s))2
= 189,043 Joule
The kinetic energy of the same car at speed 90 km/h can be expressed as
Et = 1/2 1000 (kg) (90 1000/3600 (m/s))2
= 312,500 Joule
Note! When the speed is increased with 28% the kinetic energy increases with 65%. This huge rise in kinetic energy must be absorbed by the safety construction of the car to provide the same protection in a crash. This is hard to achieve. In general it is today possible to survive a car crash in a modern car at 70 km/h. A car crash at 90 km/h is death.