Work If a builder is lifting up a stone, we say that he is doing work. Similarly, you would be doing work when you carry your school bag up a flight of steps to your classroom. Work is done when a force moves against an opposing force. The opposing force is often gravity (as in the above example), or friction. Of course, the builder and you can only do work if you have some energy. The work done depends on the size of the force and the distance. In the above example, the builder does more work when he lifts a larger stone, since a larger force is needed: also more work would be done if the stone is lifted vertically through a longer distance. Work done = force distance The distance moved in the formula must be the distance moved in the direction of the force. The SI unit of work is the joule (J). 1 joule is the work done when a force of 1 Newton moves through a distance of 1 metre. In this example: W = F d W = 1 000 2.0 = 2 000J or 2kJ F = W = 600 = 60N d 10 1
d = W = 60 000 = 400m F 150 Energy Anything which is able to do work is said to posses energy. Energy is the capacity for doing work The amount of work done during an activity is equal to the amount of energy used during that same activity. This means that in each of the above example, the energy required for that particular activity is equal to the work done in each case. 2
Kinetic Energy (KE) This is the energy a body has because of its motion. For example, a moving train, a moving ship and a moving lorry all have K.E. The KE of a moving body can be calculated using the formula: Where m = mass of the object in kg and v = velocity of the moving object. Worked example: A truck, having a mass of 2 tonnes, travels at a velocity of 15m/s. Calculate the K.E. of the truck. KE = 1 / 2 mv 2 = 1 / 2 2 000 15 2 = 1 / 2 2 000 15 15 = 225 000 or 225kJ Potential Energy (PE) This is the energy a body has either due to its position or condition. Gravitational PE This type of potential energy depends on the position of the object in question. The further away the body is from the surface of the Earth, the larger would its PE be. For example, water in the tank on the roof of you home, has a lot of PE. The gravitational PE can be calculated using the formula: Where m = mass in kg g = gravity = 10N/kg h = height in metres KE = 1 / 2 mv 2 PE = mgh 3
Worked example: A small plane (mass = 800kg) glides at an altitude of 400m. Calculate the P.E. of the plane. PE = mgh = 800 10 400 = 3 200 000J or 3.2 10 6 J Law of Conservation of Energy This states that energy can neither be created nor destroyed but it changes from one form into another. The diagram shows how one form of energy is changed into another. If a mass m falls freely towards the ground, it will lose its PE and gains KE since its velocity v increases. 4
Energy Changes Power If two cars having the same weight climb up a hill, then they would be doing the same amount of work. However, if car A arrives before car B, then we would say that car A is more powerful than car B. Power is the rate of doing work. Power is measured in J/s or Watts (W) Power = work done Time taken 1J/s = 1W and 1 000J/s or 1 000W = 1kJ/s or 1kW Also 1 megawatt = 1MW = 1 000 000W 5
Worked example: A fork lifter lifts boxes of weight 1 000N through a height of 3m. If its lifts 4 boxes in 1 minute, calculate the power developed by the lifter. Work done = force distance = 1 000 3 = 3 000J Work done in lifting 4 boxes = 4 3 000 = 12 000J Power = work done Time taken = 12 000 (1 60) = 12 000 60 = 200W 6
Energy Resources Fossil Fuels Fossil fuels are mainly used in the production of electricity, heating and cooking in the home, for transport and also in the chemical and pharmaceutical industry. Fossil fuels will now be available to us forever. These are estimates of how long our fossil fuels will last. Fossil Fuel Estimated date it is expected to run out Gas 2045 Oil 2055 Coal 2500 The fact that fossil fuels will soon no longer be available to us as well as being enormous sources of pollution, have made scientists constantly look for other sources of energy. It is of greater advantage if these alternative sources are renewable as well as cleaner. Fossil fuels are non-renewable since they are not being replaced at the same rate as they are being used up, therefore they must be used carefully. Alternative Sources of Energy Nuclear Energy Uranium is the fuel used in nuclear power stations. It was formed in the Earth s crust billions of years ago. One tonne of uranium can give the same amount of energy as one million tonnes of coal. In nuclear power stations Nuclear fission is used. This involves the splitting of uranium which causes a chain reaction resulting in the release of energy. 7
The main problem associated with a nuclear power station is that it produces highly radioactive waste materials. These wastes are difficult to store and cannot be disposed of very easily. Also, leaks of radioactive material have also occurred at various sites. Nuclear fusion is another way to get energy. It is similar to the way energy is produced in the Sun. This process has not yet been developed sufficiently for use in power stations, however, the ITER (international thermo-nuclear experimental reactor) project is seeking to provide such energy by the year 2050. Hydroelectricity This is electricity generated from the energy of falling water in mountainous regions. It is a very cheap source of energy once you have built the power station, the energy is free. A disadvantage of HEP is that it often requires valleys to be flooded and communities to be moved. Geothermal energy Water is pumped into hot rocks in the Earth s crust far below ground level. The internal heat of the rocks changes the water to steam, which is used to drive turbines, thus generating electricity. This is a natural non-polluting source of energy. Wave power In this method, the energy of moving waves is used to generate electricity. A disadvantage is that a great number of generators would be required. However, this is a non-polluting source of energy. 8
Tide power The ebb and flow of the tides drives turbines built into dams or barrages across an estuary where the difference between high and low tides is large. This source of energy would cause environmental disadvantages as it world threaten the wildlife around the estuary. Wind power This method uses wind as a means of turning aerogenerators in order to produce electricity. The disadvantages are that large amounts of land are required for these wind farms. They also produce a lot of noise. Also, if there is no wind, there will not be an energy. Solar energy Two possible methods exist. 1. Photovoltaic cells or photocells use the light from the Sun to produce electricity. Disadvantages include the initial cost of cells as well as the fact that the Sun does not shine all the time. To solve this problem solar cells are usually linked to storage batteries. 2. Black-painted collector plates are used to heat water and home by using the heat from the Sun. This method is relatively cheap. Biomass When any biological material, whether animal or plant, can be changed into energy. This energy is called biomass energy. It can be taken from animal or plant material in different ways: *by burning it, for example wood *by pressing out oils that can be burned *by fermenting it to produce fuels such as alcohol or methane. 9
Some countries have already experimented with alcohol as fuel for their cars, in fact, the Brazilian government has cut down its petrol imports by up to 60% through using this alcohol/petrol mixture. Methane generated by the digestion of animal waste is called biogas. This is used for cooking, heating and lighting. Its by-product is also an excellent fertiliser. Methane can also be collected in reservoirs from engineered landfills and used to produce electricity in power stations. Renewable and non-renewable energy sources Non-renewable energy sources include fossil fuels and nuclear fuels. Once they are used up they cannot be replaced. Hydroelectricity, geothermal energy, wave energy, tide energy, wind energy, solar energy and biomass are all renewable sources of energy. These cannot be exhausted and are non-polluting. A A.G.C.. 10
WORKSHEET On the Move 1. How much work is done when a mass of 3kg is lifted vertically through 6m? 2. A hiker climbs a hill 300m high. If she weighs 50kg, calculate the work she does in lifting her body to the top of the hill. 3. In loading a lorry, a man lifts boxes each of weight 100N through a height of 1.5m. a. How much work does he do in lifting 1 box? b. How much energy is transferred when 1 box is lifted? c. If he lifts 4 boxes per minute, at what power is he working? 4. Calculate the KE. a. a 1kg trolley travelling at 2m/s, b. a 2g bullet travelling at 400m/s, c. a 500kg car travelling at 72km/hr. 5. What is the velocity of an object of mass 1kg which has a KE of 200J? 6. A 100g steel ball falls from a height of 1.8m onto a metal plate and rebounds to a height of 1.25m. Find a. the PE of the ball before the fall (g = 10m/s 2 ) b. its KE as it hits the plate. c. Its velocity on hitting the plate, d. Its KE as it leaves the plate on rebound, e. Its velocity of rebound. 7. Robin Hood exerts an average force of 100N in pulling back his bow by 0.5m. He fires the arrow (mass = 0.2kg) vertically upwards. a. How much work does he do in pulling back the bow? b. How much energy is stored in his bow? c. How high does the arrow go? 11
8. Galileo drops a stone from the leaning tower of Pisa, which is 45m high. At what speed does the stone hit the ground? 9. A body of mass 5kg falls from rest and has a KE of 1 000J just before touching the ground. Assuming there is no friction and using a value of 10m/s 2 for g, calculate; a. i. The loss in PE during the fall ii. The height from which the body has fallen b. Name an important principle which applies in this situation. 10. A rock of mass 4kg rolls over a cliff and reaches the beach below with a velocity of 20m/s. a. What is the KE of the rock just before it lands? b. What is its PE on the cliff? c. How high is the cliff? 11. At what height above the ground must a mass of 5kg be to have a PE equal in value to the KE possessed by the same mass moving at a velocity of 10m/s? 12. A boy whose weight is 600N runs up a flight of stairs 10m high in 12s. What is his average power? 13. When the energy input to a gas-fired power station is 1 000MJ, the electrical energy output is 300MJ. a. What is the efficiency of the power station in changing the energy in gas into electrical energy? b. What form does the 700MJ of lost energy take? c. What happens to this lost energy? 14. It is estimated that 7 10 6 kg of water pours over the Niagara Falls every second. If the Falls are 50m high, and if all the energy of the falling water could be harnessed, what power would be available? 15. In 1 second, a light bulb transfers 3J into light energy and 57J to heat. a. What is the energy input in 1s? b. What is the efficiency? 16. A man weighing 1 000N runs up some stairs rising a vertical height of 5m in 10s. a. What power does he develop? b. What are the energy transfers here? 17. An electric lamp is marked 100W. How many joules of energy are transformed into heat and light a. During each second b. During an hour? 12 A A.G.C..
13 A A.G.C..