18.1 Kinetic energy

Kinetic energy

Kinetic Energy

Kinetic energy is the energy an object has due to its motion.

Kinetic energy is the energy an object has because of its motion. This means that any moving object has kinetic energy. The faster it moves, the more kinetic energy it has. Kinetic energy ( $E_K$ ) is therefore dependent on the velocity of the object. The mass of the object also plays a role. A truck of $\mathrm{2\; 000}\mathrm{kg}$ , moving at $100\mathrm{km}·\mathrm{hr}{}^{-1}$ , will have more kinetic energy than a car of $500\mathrm{kg}$ , also moving at $100\mathrm{km}·\mathrm{hr}{}^{-1}$ . Kinetic energy is defined as:

Consider the $1\mathrm{kg}$ suitcase on the cupboard that was discussed earlier. When the suitcase falls, it will gain velocity (fall faster), until it reaches the ground with a maximum velocity. The suitcase will not have any kinetic energy when it is on top of the cupboard because it is not moving. Once it starts to fall it will gain kinetic energy, because it gains velocity. Its kinetic energy will increase until it is a maximum when the suitcase reaches the ground.

Checking units

According to the equation for kinetic energy, the unit should be $\mathrm{kg}·\mathrm{m}{}^2·\mathrm{s}{}^{-2}$ . We can prove that this unit is equal to the joule, the unit for energy.

We can do the same to prove that the unit for potential energy is equal to the joule:

Kinetic energy

1. Describe the relationship between an object's kinetic energy and its:
   1. mass and
   2. velocity
2. A stone with a mass of $100\mathrm{g}$ is thrown up into the air. It has an initial velocity of $3\mathrm{m}·\mathrm{s}{}^{-1}$ . Calculate its kinetic energy
   1. as it leaves the thrower's hand.
   2. when it reaches its turning point.
3. A car with a mass of $700\mathrm{kg}$ is travelling at a constant velocity of $100\mathrm{km}·\mathrm{hr}{}^{-1}$ . Calculate the kinetic energy of the car.