# Understand Mass, Weight and the Work Equation

In this worksheet, students will learn the difference between mass and weight, as well as using the work done equation.

### QUESTION 1 of 10

Have you ever managed to move something with just the power of your mind? Well, that’s because it takes work to move something, and while your mind is super powerful, it cannot do physical work to move an object. You know what it can do though? Tell your muscles what to do so you can do work on an object.

Before we get into work though, I just want to mention mass and weight. They are not the same thing! Did you know that? When we talk about something's mass, we are talking about how many atoms it is made of – so if something has a mass of 15 kg, and we know what types of atoms are in that object, we could work out how many atoms there are in it. Weight is how much gravity is pulling that object down. Things that have more mass tend to have a bigger weight because gravity can do work on more atoms.

If you go into space, then your weight will drop to zero, but the number of atoms you are made of will stay the same, so your mass will not change. Remember the difference!

In order for energy to change (including lifting an object) from one store to another, work needs to be done. Work is done on an object when a force acts to move an object in the direction of the force, even if movement does not take place.

Work is essentially the amount of energy that has changed form. This can be calculated using the following formula:

work  =  force  ×  distance travelled

The force is measured in Newtons (N) and the distance in metres (m). The unit for work is Joules (J).

This equation tells us that if you increase the force or the distance an object travels, then you will increase the work as well.

Let's have a go at an example:

Sarah moves an object that weighs 500 N a distance of 3 m. How much work has she done?

1   Highlight all of the numbers.

Sarah moves an object that weighs 500 N a distance of 3 m. How much work has she done?

2   Write down what they mean:

Weight (force) = 500 N

Distance = 3 m

3   Put them into the equation

Work done = 500 x 3

4   Do the sum on your calculator and write down the answer:

Work done = 15000 J.

Job done! Let's have a go at some questions now to test out this equation.

What one-word answer is needed for energy to change from one form to another?

What is a usual outcome of work?

An object moves in the opposite direction to a force

An object moves in the direction of a force

The effect of work is not obvious usually

What will happen to your mass if you go into space?

It will become zero

It will not change

What work is done when a ball of weight 200 N is kicked 30 m? Include the unit in your answer.

Is there work done on the ball after it has been kicked?

Yes, heat is transferred into kinetic energy

Yes, air resistance works against the ball

No, gravity doesn't affect the ball

Why does the ball lose its kinetic energy?

Because it needs to do work against gravity

Because it needs to do work against the force of the kick

Because it needs to do work against friction and air resistance

When a crane lifts a vehicle with a weight of 60,000 N, it is doing work. How much work is needed if the crane lifts it 5 m?

12,000 J

300,000 J

55,000 J

Why does work need to be done when one cycles up a hill?

Without work the bicycle will eventually roll down the hill

Work is needed so the bicycle speeds up

Without work the cyclist will fall off the bicycle

Why would the cyclist not make it to the top of the hill without work?

Because of upthrust

Because of air resistance

Because of the gravitational pull of the Earth

A total force of 20 N is acting against the movement of a skier travelling down a slope for 100 m.

Calculate the work needed to keep the skier moving downhill.

5 J

200 J

2000 J

• Question 1

What one-word answer is needed for energy to change from one form to another?

work
EDDIE SAYS
If you found this tricky, then just think about what it is that you are doing now! Work needs to be done for energy to change from one form to another.
• Question 2

What is a usual outcome of work?

An object moves in the direction of a force
EDDIE SAYS
Now don't let this confuse you because we use the word 'work' in a number of different ways. For the exam, you will need to know that 'work done' is the same thing as energy transferred. The correct option here was the second one - as a result of work, an object moves in the direction of a force.
• Question 3

What will happen to your mass if you go into space?

It will not change
EDDIE SAYS
Did this one trip you up or did you manage to keep your wits about you? Your mass never changes unless you add or remove atoms. Your weight will change depending on the gravitational field you are in. So, although your weight will become zero in space, your mass will not change.
• Question 4

What work is done when a ball of weight 200 N is kicked 30 m? Include the unit in your answer.

6000 J
6000 joules
EDDIE SAYS
Now we have an equation to solve. Can you remember the formula? It is: work done = force x distance. The force = 200 N The distance = 30 m So, the work done = 200 x 30 = 6000 J That wasn't too painful was it?
• Question 5

Is there work done on the ball after it has been kicked?

Yes, air resistance works against the ball
EDDIE SAYS
When presented with a question like this, you need to look at the one answer that looks silly - it's the gravity one here. If it can't be that answer, that leaves only two to choose between. What seems more obvious, air resistance or heat? Air resistance is the correct answer here.
• Question 6

Why does the ball lose its kinetic energy?

Because it needs to do work against friction and air resistance
EDDIE SAYS
If you find some of these questions difficult, try to imagine the situation described and see if that helps you to understand about the forces working in our everyday lives. For example, think about a time when you've kicked a ball and how it behaves after it has been kicked. A ball that has been kicked and is moving, loses kinetic energy because it needs to do work against friction and air resistance.
• Question 7

When a crane lifts a vehicle with a weight of 60,000 N, it is doing work. How much work is needed if the crane lifts it 5 m?

300,000 J
EDDIE SAYS
This is another equation question but which formula do you need? Here it is: work done = force x distance The force = 60,000 The distance = 5 So, the work done is 60,000 x 5 = 300,000 J
• Question 8

Why does work need to be done when one cycles up a hill?

Without work the bicycle will eventually roll down the hill
EDDIE SAYS
It's very hard work cycling up a hill, isn't it? What would happen if you stopped all that hard work and no longer pedalled the bike? That's right - it's option one! When cycling up a hill, work needs to be done so that upward movement continues until you reach the top of the hill. Without work, the bicycle will eventually roll back down the hill.
• Question 9

Why would the cyclist not make it to the top of the hill without work?

Because of the gravitational pull of the Earth
EDDIE SAYS
Think back to the bicycle ride in the previous question. You know what would happen if you stopped pedalling, but why would you roll back down the hill? It's pretty straightforward if you think about it! That's right - it's gravity! The cyclist would not make it to the top of the hill without work because of the gravitational pull of the Earth, drawing him back downwards.
• Question 10

A total force of 20 N is acting against the movement of a skier travelling down a slope for 100 m.

Calculate the work needed to keep the skier moving downhill.

2000 J
EDDIE SAYS
The last question and another chance to use that equation. Can you remember it now? Work done = force x distance travelled Work = 20 x 100 = 2000 J Fantastic calculating skills! If you can remember equations like this one, and how to use them, you are in a great position for gaining lots of easy marks in the exam.
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