Energy transfers for SQA Higher Physics
This page covers the following topics:
2. Conservation of energy
Work is said to be done whenever a force causes an object to move through a distance. Work done is calculated by multiplying the force exerted by the distance moved by the object. Since work done describes energy transfer, its unit is the Joule (J).
The principle of conservation of energy states that in a closed system, the total energy before an energy change is equal to the total energy after. In other words, the total energy in a closed system is constant. This means that energy cannot be destroyed nor created.
Serkan is pushing a box horizontally for 25 m across rough ground. Given that the frictional force exerted by the ground is 50 N, calculate the work done against friction by Serkan.
W = Fd
W = 25 m × 50 N = 1250 J
A mechanical lift is being used to lift up an object of 4.5 kg. Given that the work done by the lift is 600 J and that there are no frictional forces, calculate the height through which the box is lifted.
By the principle of conservation of energy, the total energy remains constant.
GPE = W
mgh = Fd
4.5 kg × h × 9.8 = 600
h = 13.6 m (to 3 sf)
A vertical lift is lifting up a box of 700 g along a distance of 3 m. Calculate the work done by the lift to life the box up.
The force needed to lift the box will be equal to the weight of the box.
weight = mg
weight = 9.8 N/kg × 0.7 kg = 6.86 N
w = Fd
w = 6.86 N × 3 m = 20.58 J
Describe, using the principle of conservation of energy, the energy changes that occur when a ball is rolled down a hill.
At the top of the hill, the ball will possess gravitational potential energy. As the ball starts moving this will be transferred into kinetic energy. Some of the energy will also be transferred into heat energy, since the temperature of the ball will rise, as work is done against the frictional forces. By the principle of conservation of energy, the sum of the kinetic energy and the heat energy will be equal to the gravitational potential energy.
gravitational potential energy = kinetic energy + heat energy
Layla is driving her car through a horizontal distance of 120 m. Given that the work done by the car is 480000 J, calculate the thrust exerted by the car.
W = Fd
F = W ÷ d
F = 480000 J ÷ 120 m = 4000 N
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