# Forces - Kinetic Energy of Objects

This GCSE Physics quiz on forces will challenge you on the kinetic energy of objects. All objects in motion have kinetic energy. The amount of kinetic energy depends on the speed they are moving and their mass and is measured in joules. When you compare two or more objects moving at the same speed, the heaviest will be the one that has the greater kinetic energy. Objects with more kinetic energy can do more work, which is why high speed collisions or collisions involving larger vehicles are more destructive.

When an object speeds up or slows down, its kinetic energy increases or decreases. When energy is transferred, it means that work has been done - the forces which cause a change in speed therefore do so by doing work. If friction and air resistance are ignored, the kinetic energy changes by an amount that is equal to the work done by the applied force.

Friction and air resistance reduce the amount of kinetic energy by transferring it into heat energy. The law of conservation of energy means that the work done is therefore equal to amount of heat transferred plus the kinetic energy of the object. Kinetic energy can be stored by using a heavy wheel called a flywheel. This spins at high speed on low-friction bearings but most of the time, kinetic energy is transferred from other forms of energy only when it is needed.

For the GCSE, you should be able to discuss the transfer of kinetic energy in particular situations. Examples might include the space shuttle re-entry or meteorites burning up in the atmosphere. It could also include braking systems as they are designed to transfer kinetic energy into heat energy to slow or stop a vehicle. Most vehicles currently use non-renewable fossil fuels as the source of energy that is transferred into kinetic energy.

As humans are becoming more aware of the damage caused to the Earth by large scale burning of fossil fuels, car manufacturers are designing electric cars. There are even cars that produce their electricity from solar cells, some is used to charge the batteries and the rest is transferred directly into kinetic energy without first being stored. These do not emit (give out) gases into the atmosphere but the electrical energy needs to come from somewhere. Most electricity is generated from the burning of fossil fuels so they do cause pollution at the power station. The parts all need to be manufactured, which uses up the Earth's resources and creates more pollution at the factories - there is no such thing as an environmentally-friendly vehicle.

You may also be required to make calculations of kinetic energy or required to work out factors such as speed and mass from knowing the kinetic energy of an object, so make sure that you learn the equation used to work out kinetic energy. The equation includes the term v2 so when the speed of an object is doubled, the kinetic energy increases fourfold.

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1. What is the formula for kinetic energy?
Kinetic energy is determined by the speed and mass of an object
2. An athlete has a mass of 70 kg and is running at a speed of 10 m/s. What is their kinetic energy?
From the kinetic energy equation, it is half of 70 multiplied by 100
3. What is the mass of a ball which has 2.5 J of energy and is travelling at 5 m/s?
This needs you to rearrange the equation to isolate m. Once that is done, you can substitute in the figures from the question
4. Which formula correctly shows the relationship between speed and acceleration?
Kinetic energy questions can sometimes seem a bit tricky as you often need to use more than one equation to arrive at the answer. The equation for acceleration is sometimes required, usually you will need to rearrange it to find the final velocity
5. What is the acceleration of a car as it increases in speed from 3 m/s to 10 m/s in a period of 5 seconds?
Straightforward substitution of the figures from the question, no need to rearrange the equation
6. What is the mass of a person who accelerates from 0 m/s to 10 m/s in 30 seconds? When travelling at 10 m/s, the person has a kinetic energy of 5000 J.
The first piece of information here is not actually needed to answer the question. Keep an eye out for this in your exams - cruel examiners sometimes try to catch you out!!! You can use the relationship between kinetic energy and speed to work out the mass
7. Which of the following can be converted into kinetic energy?
You don't need to remember loads of examples of energy changes, you should be able to work them out from your knowledge and understanding of energy transfers
8. A ball is dropped from a height of 10 m and is pulled towards the Earth due to the gravitational field of the Earth. The strength of the Earth's gravitational field is 10 N/kg. What is the mass of the ball, if the ball has 100 J of GPE?
Even though the question is about a moving object, in this case, the equation for gravitational potential energy is sufficient to arrive at the answer
9. What is the weight of the ball in the question above?
Weight is calculated by multiplying the mass of an object by the strength of the gravitational field in which it lies. Although you know the more accurate figure for the acceleration due to gravity is 9.81 N/kg, the question refers to the previous one and so you must use the figure given in that
10. What is the speed of a bird of mass 3 kg which has kinetic energy of 400 J?
That's one quick bird!