0.1 Gravity and mechanical energy  (Page 3/5)

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.
   

Mechanical energy

Mechanical energy, $E_M$ , is simply the sum of gravitational potential energy ( $E_P$ ) and the kinetic energy ( $E_K$ ). Mechanical energy is defined as:

Conservation of mechanical energy

The Law of Conservation of Energy states:

Energy cannot be created or destroyed, but is merely changed from one form into another.

Conservation of Energy

The Law of Conservation of Energy: Energy cannot be created or destroyed, but is merely changed from one form into another.

So far we have looked at two types of energy: gravitational potential energy and kinetic energy. The sum of the gravitational potential energy and kinetic energy is called the mechanical energy. In a closed system, one where there are no external forces acting, the mechanical energy will remain constant. In other words, it will not change (become more or less). This is called the Law of Conservation of Mechanical Energy and it states:

The total amount of mechanical energy in a closed system remains constant.

Conservation of Mechanical Energy

Law of Conservation of Mechanical Energy: The total amount of mechanical energy in a closed system remains constant.

This means that potential energy can become kinetic energy, or vice versa, but energy cannot 'disappear'. The mechanical energy of an object moving in the Earth's gravitational field (or accelerating as a result of gravity) is constant or conserved, unless external forces, like air resistance, acts on the object.