8.1 Energy

In the example of a falling mass the potential energy is known as gravitational potential energy as it is the gravitational force exerted by the earth which causes the mass to accelerate towards the ground. The gravitational field of the earth is what does the work in this case.

Another example is a rubber-band. In order to stretch a rubber-band we have to do work on it. This means we transfer energy to the rubber-band andit gains potential energy. This potential energy is called elastic potential energy . Once released, the rubber-band begins to move and elastic potential energy is transferred into kinetic energy.

Other forms of potential energy

1. elastic potential energy - potential energy is stored in a compressed or extended spring or rubber band. This potential energy is calculated by:
   $$\frac{1}{2}k{x}^2$$
   where $k$ is a constant that is a measure of the stiffness of the spring or rubber band and $x$ is the extension of the spring or rubber band.
2. Chemical potential energy is related to the making and breaking of chemical bonds. For example, a battery converts chemical energy into electrical energy.
3. The electrical potential energy of an electrically charged object is defined as the work that must be done to move it from an infinite distance away to its present location, in the absence of any non-electrical forces on the object. This energy is non-zero if there is another electrically charged object nearby otherwise it is given by:
   $$k\frac{q_1{q}_2}{d}$$
   where $k$ is Coulomb's constant. For example, an electric motor lifting an elevator converts electrical energy into gravitational potential energy.
4. Nuclear energy is the energy released when the nucleus of an atom is split or fused. A nuclear reactor converts nuclear energy into heat.

Some of these forms of energy will be studied in later chapters.

Investigation : energy resources

Energy can be taken from almost anywhere. Power plants use many different types of energy sources, including oil, coal, nuclear, biomass (organic gases), wind, solar, geothermal (the heat from the earth's rocks is very hot underground and is used to turn water to steam), tidal and hydroelectric (waterfalls). Most power stations work by using steam to turn turbines which then drive generators and create an electric current.

Most of these sources are dependant upon the sun's energy, because without it we would not have weather for wind and tides. The sunis also responsible for growing plants which decompose into fossil fuels like oil and coal. All these sources can be put under 2headings, renewable and non-renewable. Renewable sources are sources which will not run out, like solar energy and wind power.Non-renewable sources are ones which will run out eventually, like oil and coal.

It is important that we learn to appreciate conservation in situations like this. The planet has a number of linked systemsand if we don't appreciate the long-term consequences of our actions we run the risk of doing damage now that we will onlysuffer from in many years time.

Investigate two types of renewable and two types of non-renewable energy resources, listing advantages and disadvantages of each type. Write up the results as a short report.

Energy

1. Fill in the table with the missing information using the positions of the ball in the diagram below combined with the work-energy theorem.

   

   position	$KE$	$PE$	$v$
   A		50 J
   B		30 J
   C
   D		10 J
   E
   F
   G

2. A falling ball hits the ground at 10 m $·$ s ${}^{-1}$ in a vacuum. Would the speed of the ball be increased or decreased if air resistance were taken into account. Discuss using the work-energy theorem.
3. A pendulum with mass 300g is attached to the ceiling. It is pulled up to point A which is a height h = 30 cm from the equilibrium position.

   

   Calculate the speed of the pendulum when it reaches point B (the equilibrium point). Assume that there are no external forces acting on the pendulum.