# EdPlace's Key Stage 3 Home Learning Science Lesson: Gravity

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**Get them started on the lesson below **and then jump **into our teacher-created activities** to practice what they've learnt. We've recommended five to ensure they feel secure in their knowledge - 5-a-day helps keeps the learning loss at bay (or so we think!).

Are they keen to start practising straight away?** Head to the bottom of the page to find the activities. **

**Now...onto the lesson!**

**Key Stage 3 Statutory Requirements for Science**

**Year 8 Students**

*should be taught about gravity force. They also need to be taught the equation to calculate weight, which is:*

weight = mass x gravitational field strength

weight = mass x gravitational field strength

## Gravity

We are all aware of **gravity** in some form or another. It is an invisible pulling force which acts all around us and it is what keeps us on the ground instead of floating off into the atmosphere. However, as we can only see the effects of gravity (like dropping a book) but not actually **gravity** itself, it can be a tricky topic to explain to children. A key part in learning about **gravity** is understanding the difference between **mass** and **weight**. In everyday language these two words are used interchangeably and are often used to mean the same thing. In Science, however, the meanings of the two words are different.

Understanding the equation to weight will help with understanding gravity. This equation is:

**weight** = **mass** x **gravitational field strength**

Students also need to learn that weight is different on other planets and stars.

We're confident that if you follow the step-by-step approach below your child will be able to:

1) **Understand** the difference between mass and weight.

2) **Apply** this understanding to **calculate** weight using the equation weight = mass x gravitational field strength.

3)** Explain **why the weight of an object changes on different planets.

## Step 1: Key Terminology

First of all, it is a good idea to make sure your child understands the key terminology associated with this topic:

A **force** is defined as a push or a pull. There are many different types of forces (e.g. magnetic, air resistance, friction etc.) but these are categorised into two groups: contact forces and non-contact forces. A **non-contact force** is one that acts between two objects that are not physically touching each other. **Gravity** is probably the most familiar example of a **non-contact force** and it is a force that pulls objects together. **Gravity** acts between all objects and the greater the **mass** of an object, the stronger the **force** of **gravity** will be. However, we only notice the effects of **gravity** when objects fall towards the ground as they are attracted towards the centre of the Earth (as planet Earth has such a huge mass). A book and a pencil for example, are attracted to each other because of **gravity** but because their **masses** are so small it is not noticeable.

## Step 2: Key Terminology

Now let us focus on what the new keywords mean:

We’ve just read that the **force** of **gravity** depends on the **mass** of an object. The word **mass**** **is used to describe the amount of **matter** an object is made up of and it is usually measured in **kilograms** (kg). Imagine you are stepping on the weighing scales and make a note of what you see. You are not actually measuring your **weight** but your **mass**. Say a doctor asks for your **weight**. To be correct he or she should really be asking you for your **mass**.

So, what does** weight **mean in scientific terms? The weight of an object is the size of the **force** of **gravity** between it and the Earth. **Weight** is a **force** and all **forces** are measured in **newtons** (N).

The **weight** of an object will depend on its **mass** (remember the greater the **mass** of on object, the stronger the **force** of **gravity** will be) and it will also depend on the **gravitational field strength** of the planet. When an object is placed in the **gravitational field** of a planet, it will be attracted to the centre of that planet. The **gravitational field strength** of Earth is 10 N/kg (10 **newtons per kilogram**). This means an object with a **mass** of 1 kg would be attracted towards Earth with a force of 10 N. Your child (through lots of practice) will become familiar with this equation to calculate the **weight** of an object:

**weight** (in **newtons**) = **mass** (in **kilograms**) x **gravitational field strength** (in **newtons per kilogram**).

## Step 3: The Weight Equation

Questions on **mass** and **weight** in textbooks and assessments usually focus on asking about these keywords in different contexts (like comparing the **mass** and **weight** of an object on Earth and then on different planets) to check that the student really understands the difference between the two.

The important thing to remember is that the **mass** of an object will never change no matter where you are in the solar system. A 1 kg bag of flour will measure the same on Earth, Mars, the moon or in outer space. However, the **weight** of an object can change depending on its location.

Let’s think about the **weight** of a 1 kg bag of flour on Earth.

**weight** (N) = **mass** (kg) x **gravitational field strength** (N/kg)

**weight** (N) = 1 kg x 10 N/kg (remember the Earth’s **gravitational field strength** is 10 N/kg)

**weight** = 10 N

This means the bag of flour is being pulled towards the Earth with a force of 10 N.

Now imagine an astronaut has transported this bag of flour to the moon. The mass of the bag of flour will still be 1 kg (this can’t change) but the **gravitational field strength** here is only 1.6 N/kg as the moon is a lot smaller than the Earth. What would the weight of the bag of flour be on the moon? We use the same equation making sure to change the value for gravitational field strength.

**weight** (N) = **mass** (kg) x **gravitational field strength** (N/kg)

**weight** (N) = 1 kg x 1.6 N/kg

**weight** = 1.6 N

The **force of gravity** (or **weight**) is less between the bag of flour and the moon compared to the bag of flour and the Earth because the moon is smaller. If the bag of flour then went to a larger planet than Earth, its weight would be more. If the bag of flour happened to get lost in outer space the **weight** would become zero (or weightless) as there would be no planets nearby to exert their **gravitational field** on the object.

## Step 4: Practice makes perfect

We will now go through some practice questions to check understanding:

1. What unit of measurement is used for **mass**? (**Newtons** or **kilograms**)

2. What unit of measurement is used for **weight**? (**Newtons** or **kilograms**)

3. **Mass** is a measure of how much ________ there is in an object.

4. **Weight** is the measure of the force of ________ on an object.

5. What measurement would be different on the Earth compared to the moon. **Mass** or **weight**?

6. John has a 440 kg bicycle (remember the **gravitational field strength** of Earth = 10 N/kg and the **gravitational field strength** of the moon = 1.6 N/kg).

(A) What is the **mass **of the bicycle (i) on Earth and (ii) on the moon

(B) What is the **weight **of the bicycle (i) on Earth and (ii) on the moon

(C) The **gravitational field strength **of the planet Venus is 8.8 N/kg. (i) What is the **mass** of the bicycle on Venus? (ii) What is the **weight** of the bicycle on Venus? (iii) What does your answer to 6c(ii) tell you about the **mass** of planet Venus compared to the **mass** of planet Earth?

## Step 5 - Let's put your grouping knowledge to the test while it's still fresh in your mind...

Now, you’ve had some practice, why not assign your child the following five activities in this order to test their understanding.

All activities are **created by teachers and automatically marked.** Plus, with an EdPlace subscription, we can **automatically progress your child** at a level tailored to their needs. Sending you progress reports along the way so you can **track and measure progress, together** - brilliant!

Activity 3 - Satellites in Orbit

Why not have a go at this GCSE worksheet to stretch themselves further:

Activity 4 - Calculate Mass, Weight and Gravity

Activity 5 - Understand Newton's First Law

**Answers**

1. Kilograms (kg)

2. Newtons (N)

3. Matter

4. Gravity

5. Weight

6. (a) (i) 440 kg (ii) 440 kg

(b) (i) 4400 N (ii) 704 N

(c) (i) 440 kg (ii) 3872 N (iii) Venus has a smaller mass compared to Earth

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