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From the largest structures in the universe to the microscopic world of the atom, forces play a part in every interaction. Sometimes the forces are very obvious and you can feel their effects, for example gravity or the wind blowing. Other times these forces are less obvious like the forces that hold chemical elements together to form compounds or the reaction force that a table exerts on a book that is placed on it.

When a force is applied to an object and causes it to move, energy is transferred to the object and work is done. In everyday language, work means a lot of different things but in physics it means only one thing - a force has made something move.

It is calculated by multiplying the force needed to move an object by the distance it moved. The unit of work is the joule (J) and one joule is the amount of work done when a force of one newton (N) moves through a distance of one meter (m). Joules are also the units of energy so the work done must therefore be equal to the energy transferred to or from the object that has moved.

The greater a force that is applied to an object, the more energy the object gains. An example of this is a tennis player making different shots. When they make a smash, they use as much force as they can so the ball gains a lot of energy. When the ball hits the court, it will bounce a long way. Another kind of tennis shot is the drop shot. The player uses much less force so that when the ball lands, it has a lot less energy and the idea is that the ball bounces as little as possible.

As a force causes an object to move, it takes a certain amount of time. Power is a measure of how quickly that work is done and therefore is also a measure of the rate at which energy is transferred. Whenever you come across anything in physics that is described as a 'rate' you know immediately that it is something that is divided by time. In the case of power, it must be the work done (or energy transferred) divided by the time taken. So, if a question asks you to calculate power, it will either talk in terms of work done or energy transferred. The SI units of power are watts (W), one watt of power is equal to one joule of energy being transferred in one second.

1.

What is gravitational potential energy?

The energy an object has due to its position within a gravitational field

The strength of the gravitational field at the center

The speed of an object due to the force of gravity

The energy an object radiates to other objects within a gravitational field

The amount of gravitational potential energy depends on the position of the object and its mass in a given strength of gravitational field

2.

What is work done?

The amount of energy transferred to an object that is being moved

The amount of energy transferred to an object at rest

The force needed to move an object

The distance an object can be moved

The definition of work in physics is very specific

3.

What is the gravitational potential energy of a ball of mass 100 g which is at a height of 1 m above the Earth? (Note: The gravitational strength of the Earth is 9.81 N/Kg.)

1000 J

1 J

0.1 J

1 N

Remember mass is measured in kg and so you need to convert the 100 g into 0.1 kg before you multiply by g and h

4.

What are the units used for power?

Joules

Watts

Volts

Amperes

The watt is names after James Watt, a Scottish inventor who invented an efficient type of steam engine

5.

What is the formula for gravitational potential energy?

The gravitational potential energy of an object is related to its mass, the strength of the gravitational field and the object's distance from the center of the gravitational field. In practical terms, on Earth, *h* is measured from the ground and often at senior high school level, this equation is used to calculate energy changes due to changes in height

6.

What is transferred when work is done?

Charge

Voltage

Energy

Electrons

The joule denotes a unit of energy and is used to describe various types of energy-related values including work done

7.

What is the formula for power?

Since power is the rate of transfer of energy, you can eliminate two of the options immediately

8.

What is the time taken to move an object if the total power used is 100 W and the energy transferred by a force to the object is 1,000 J?

10 seconds

100 seconds

1,000 seconds

0.1 seconds

Make sure that you are able to rearrange equations

9.

What is the formula for work done?

If a force does not move, then no work is done

10.

What are the units used for work done?

Joules

Watts

Coulombs

Amperes

When the force is measured in newtons and the distance in meters, the work done will be calculated in joules