Forces - 5.2 Work Done and Energy Transfer (GCSE Physics AQA)

Work Done and Energy Transfer

Forces and Work

Work Done

• Work is the transfer of energy. Work done is simply another way of saying that energy is transferred. The energy can be transferred from one store to another, such as chemical energy to kinetic energy in Fig 1.

Energy Transfers

Here are some examples of energy transfers. In each situation, one type of energy is transferred into other type(s) of energy.

Forces Doing Work

• Resultant forces do ‘work’ to move objects. When a resultant force acts on an object though a distance, two key things happen.

1. The force does work on the object. Remember, work is the transfer of energy.
2. The object will move. Due to the transfer of energy (work), the object will move.

Calculating Work Done

Formula for Work Done

We can calculate work done using the following formula:

*Distance is the distance moved along the line of action of the force

Where:

• Work done (W), is in Joules, J
• Force (F), is in newtons, N
• Distance (s), is in metres, m

Units of Work

• Work is measured in joules. As previously mentioned, the measurement unit of work is called a joule. The symbol to represent the joule is simply the capital letter ‘J’.
• 1J of work involves a force of 1N executed over 1m. There is a specific definition that you need to memorise in relation to joules:
• One joule of work is done when a force of one newton caused a displacement of one metre.
• Joules can be expressed as newton-metres. One newton-metre (Nm) is equal to one joule (J). Here’s how it works:

1. Look at the formula for work done. As seen in the previous section, the formula for work done is:

Work done (in Joules) = force (in Newtons) x distance moved (in Metres)

2. Look at the units of the formula. Ignoring the rest of the formula, only focus on the units. This will leave you with the following:

Joules = newtons x metres (J = N x m) or in other words,
Joules = newton-metres (J = Nm).

Again, we can refer to the formula for work done. This tells us that when we do 1J of work, we will have moved an object for a length of 1m, using a force of 1N. (1x 1 = 1)

Energy Transfers

• Work done needs a source of energy. Work done involves transferring energy, so we need the energy to come from somewhere before we transfer it. For example, if a person pushes a wheelbarrow, chemical energy is converted to kinetic energy to move the wheelbarrow. The chemical energy originates from the food the person eats.
• The energy transferred includes some wasted energy. Not all of the energy transferred during work is converted into useful energy. For example, if a person pushes a wheelbarrow, most of the chemical energy is converted to kinetic energy to move the wheelbarrow. But some will be converted to heart and sound energy.

Frictional Forces and Temperature Rise

• A frictional force opposes motion. When a moving object experiences friction, its motion will be opposed. As a result, it is harder to move an object over a frictional surface, since more work has to be done (Fig 2)

• Frictional forces result in more work done. When friction is involved, more work has to be done to cover the same distance, because energy is needed to overcome the frictional force. Friction makes it harder to push or pull an object.
• Friction results in thermal energy. The presence of frictional forces means that some of the energy input is transferred into thermal energy. Seeing as more work is done due to frictional forces anyway, there is a greater amount of energy transfer. This will result in a greater amount of thermal energy.
• Thermal energy leads to a rise in temperature. Thermal energy is another phrase for ‘heat energy’, so it will lead to the production of heat. The heat that is produced will lead to a rise in the temperature of the object.