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!
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!
Why not have a go at this GCSE worksheet to stretch themselves further:
1. Kilograms (kg)
2. Newtons (N)
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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