Mass, weight and work

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

Key stage:  KS 4

Curriculum topic:  Physics: Energy

Curriculum subtopic:  Energy Changes

Difficulty level:

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 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 somethings mass, we are talking about how many atoms it is made of – so if something has a mass of 15kg, and we know what types of atoms are in that object we could work out how many atoms there are in that object. 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 0, 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 Newton (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 move an object that weighs 500N 2m, how much work has she done?

1 – highlight all of the numbers.

Sarah move an object that weighs 500N 2m, how much work has she done?

2 – write down what they mean:

Weight (force) = 500N

Distance = 3m

3 – put them into the equation

Work done = 500 x 3

4 – do the sum in your calculator and write down the answer:

Work done = 15000J.

What 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 of a force.

An object moves in the direction of a force.

The effect of work is not obvious usually.

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

It will become 0

It will not change

What is the work done when a ball of weight 200N is kicked 30m? 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 dons't effect 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 60,000N, it does work. How much work is needed if the crane is to lift it 5m?

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

Without work the bicycle will eventually move downwards.

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 down hill.

5 J

200 J

2000 J

• Question 1

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

work
EDDIE SAYS
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
As a result of work, an object moves in the direction of a force.
• Question 3

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

It will not change
EDDIE SAYS
You mass never changes unless you add or remove atoms. Your weight will change depending on the gravitational field you are in.
• Question 4

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

6000 J
6000 joules
EDDIE SAYS
work done = force x distance. work done = 200 x 30 wor done = 6000J
• 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 - the gravity one is this here. If it can't be that answer that leaves only two left. 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
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 60,000N, it does work. How much work is needed if the crane is to lift it 5m?

300,000
EDDIE SAYS
Work done = force x distance word done = 60,000 x 5 work done = 300,000J
• Question 8

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

Without work the bicycle will eventually move downwards.
EDDIE SAYS
When cycling up a hill work needs to be done, so that upward movement continues to reach the top of the hill. Without work the bicycle will eventually move downwards.
• 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
The cyclist would not make it to the top of the hill without work because of the gravitational pull of the Earth.
• 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 down hill.