Kinetic Energy
The kinetic energy of a rigid body is the energy possessed by the body motion
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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
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)
- one slug = 32.1740 pounds (as mass) - lbm
Rotational Kinetic Energy
Rotational kinetic energy can be expressed as
Er = 1/2 J ω2 (2)
where
Em = kinetic rotation energy (Joule, ft lb)
J = polar moment of inertia (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
= 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 km/h)(1000 m/km)/(3600 s/h))2
= 312,500 Joule
Note! When the speed is increased with 28% - the kinetic energy is increased 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 - which is very hard to achieve. In a modern car today it is possible to survive a crash at 70 km/h. A crash at 90 km/h is death.
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Related Topics
- Dynamics - Dynamics Motion - velocity and acceleration
- Mechanics - Kinematics, forces, vectors, motion, momentum, energy and the dynamics of objects
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Related Documents
- Angular Motion - Power and Torque - Angular velocity and acceleration - power and torque
- Flywheel Kinetic Energy - Kinetic energy and moment of inertia of a flywheel
- Formulas of Motion - Linear and angular acceleration, velocity and displacement
