Forces and gravity: Is gravity real or does the Earth just suck?
1. Demonstration - to catch their interest. (Show Powerpoint Slide 1)
- To excite the children’s interest and to get them to contribute their ideas about forces. It is important to find out how much they already know on the topic.
|Fix a large brightly coloured helium balloon with ‘blu tac’ conveniently near where you will be speaking. Walk in with large cardboard box (apparently very heavy) and hold it on the edge of convenient table, ensuring that the box can be seen to project a long way over the edge of the table.|
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Ask a child to attach the balloon to the end of the box away from the table. Leave go of the box which stays in place because there is a brick inside it, which is above the table.
Ask whether the box will fall if the balloon is removed. Once they have discussed this, ask a child to remove the balloon and show that the box is still balanced. Finally after discussion you might show them the brick.
Safety Note: Bricks are heavy and have sharp edges – ensure that the brick does not fall out of or through the bottom of the box onto toes.
|Once the balloon is attached, ask whether or not the box will fall if you leave go of the box.|
When you ask whether the box will fall if the balloon is removed, there could be a show of hands to vote. Ask them for reasons. They will explain in terms of what they already know.
Full understanding requires them to have some ideas about turning forces. There is no need to go into details but they have the opportunity to talk about sizes of forces and their directions.
2. Pushes and pulls
- To ensure that they have a clear understanding of what a force is.
|Have two toys, one that needs to be pulled (e.g. toy duck on wheels with string attached) and one that needs to be pushed. Invite two children to make the toys move.|
To the child with the pushing toy give the word card with push on it, and to the other give the pull card.
Stand each child at each side of the room and tell the children that when they see a push they have to point to the ‘push’ card and when they see a pull they point to the ‘pull’ card. Then bring out more objects that need to be pushed or pulled and for each one ask a child to make it work . Large objects are best, including toys and springs.
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Demonstrate the Airzooker - pushes air
Have some more complex ones with twists and turns too.
Safety Note: Toys and items brought in for children to handle to show forces should be safe – check that there are no sharp parts, that springs are secured and there are no places where fingers can be trapped.
|Ideas about pushes and pulls are introduced at an early age.|
Ask the other children to explain what each child did to make each toy move.
Ask the question what name do we give pushes and pulls? Hopefully get the answer – forces.
Alternatively the examples of forces can be handed round to groups of children and after a few minutes each group can report back. (Remember to collect all the articles by the end of the discussion so that the children are not distracted.)
3. (Show Powerpoint Slide 2)
- To relate movement to forces. Forces start something moving, change its speed or change its direction.
- Forces will also change the shape of an object.
|Show Powerpoint Slide 2||Encourage them to say what changes happen when forces act. Then show Slide 2|
4. Big and little forces
- There are large forces and small forces
- These can be measured using a forcemeter (newton meter)
|Show Powerpoint Slide 3|
View Apparatus List
Use a forcemeter to push and pull and show sizes of forces. (Maybe relate to length of a spring).
|Discuss the push of a feather resting on a hand. Then the push needed to bend the tree.|
These measure forces by the change in shape they produce.
- Gravity is a force
|Drop a soft object.||Ask what made it fall - it moved so there must have been a force on it - was it a push or a pull?|
What do we call the force? Gravity
6. Story about Sir Isaac Newton and a fruit dropping on his head (Show Powerpoint Slides 4, 5 and 6)
- Idea that people had seen apples drop for thousands of years but it took Newton to think and understand about gravity. Many of the most important scientific ideas have come from thinking about something that others take as usual or even as a mistake.
- Weight is a force.
|What fruit? - Satsuma, apple, melon|
(Show Powerpoint Slide 7)
|Ask them which fruit is most likely to have dropped on Newton ’s head. Ask for reasons using the opportunity to talk about different weights.|
Ask whether the man in the picture (slide 6) could be Newton- wrong clothes - emphasize that Newton lived 300 years ago. ( 1643-1727).
7. Hazards and Safety Notes
- Stopping things moving can be dangerous.
- Road safety – cars.
|Could be part of discussion or be omitted.|
Hazard – stopping something moving can injure.
Safety Note: Small for slow, light objects. High for fast, heavy ones.
|Small toy car moving slowly - safe to stop it.|
Toy car moving quickly could be dangerous.
Full size car moving - VERY dangerous if a person is in the way it will not stop.
8. The unit of force is a newton
The unit of force is a newton - named after Isaac Newton.
9. Hand round some small objects with the weight of about 1 newton.
- Any thing with a mass just over 100g weighs about 1 newton on earth.
- This helps them to have a memory of the size of a newton.
|View Apparatus List|
At end of this show Show Powerpoint Slide 8
|Discuss examples of objects weighing about 1 newton.|
Bring out the idea that 1 newton is approximately the weight of a small eating apple.
Talk about estimating the forces.
10. Mystery BOX (Show Powerpoint Slide 9)
- Reinforces a memory of the size of a newton.
- Introduces the idea of estimating.
|View Apparatus List|
First ask volunteers to pull a string and estimate the size of the force in newtons.
Put the results in a table. (Fill in the table on slide 10, if a white board is used. Otherwise take some sheets of paper with tables printed on for groups to fill in the values in case no board is available.)
Safety Note: Mystery box could cause children to hurt their fingers on the string ensure that the maximum weight is not large.
Then ask more children to use a suitable forcemeter to check values.
|Discuss their answers.|
11. (Show Powerpoint Slide 11, if helpful)
|Safety Note: High range forcemeters may be unsafe for small children – avoid forces greater than 10 N.|
12. Game (Show Powerpoint Slide 12)
- An upwards pull is needed against gravity.
- Links in with the title of the session.
- When the ball does not fall the upward force caused by sucking = the force of gravity on it.
|Ask for volunteers and pass a ping-pong ball around using straws through which the children need to suck to overcome the effects of gravity; try with heavier objects.|
(Show Powerpoint Slides 13 and 14)
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Safety Note: Straws present a hygiene hazard - do not reuse or allow them to be passed round.
|They find this works best if the straw points straight downward because then it is in the direction of the force of gravity acting on the ball. Discuss the directions of the arrows on the slides.|
The game with the ping pong balls helps them to understand that to pull against gravity they must have the straw vertical.
13. Drawing forces (Show Powerpoint Slides 15,16,17)
- Forces can be represented by arrows which show the direction and the size.
|(Ignoring air resistance,) falling objects have only one force acting on them.|
Object on string or held by a straw has two equal forces, one up and one down.
|Discuss how to show that the “sucking force” is equal in size to the force of gravity on the ball when the ball is held still.|
14. Newton’s brainwave - GRAVITY
- Gravity is a force which acts between ANY two objects.
15. In Australia people are attracted to the Earth as we are – they do not fall off!
|Every object pulls every other object towards it - Gravity|
For small objects, the force is very small.
The Earth is very large and has a lot of ‘stuff’ in it and therefore the pull is large enough to feel and so objects are pulled towards the Earth when free to fall.
Show Powerpoint Slide 18 (could be omitted if does not arise in the discussion).
|They may ask ‘Shouldn’t the people in Australia be upside down?'|
16. Air resistance
- Explain that in Slide 19 the light flashes regularly so the positions of the objects are shown at the same moment.
|Drop a phone book and a ping pong ball- both hit the ground at the same time - use other demos if available .(Slide 19)|
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|Powerpoint Slide 19 shows a steel ball gaining slightly on the ping pong ball.|
17. Dropping object on the Moon
- No atmosphere on Moon so no air resistance. If they are released at the same instant, both the heavy object and the very light object reach the ground at the same time.
|Video (Show Powerpoint Slide 20)|
Video of two objects falling on the Moon – hammer and feather.
|Discuss how the feather would fall on Earth.|
Compare the effect of dropping a feather on the Moon and on the Earth.
18. Air Resistance
- Air resistance acts to slow down moving objects.
- Large surfaces cause larger air resistance than small surfaces.
|(The following could be omitted).|
Talk about the time taken for falling objects to reach the floor.
Then use a sheet of paper and the ping pong ball.
Experiments with parachutes to show there is air resistance—a force caused by the air that acts in the opposite direction to the downward movement.
The larger the load the sooner the parachute reaches the ground.
The larger the area the longer it takes for the parachute to reach the ground.
Show arrows for forces- weight same size throughout.
When getting faster (accelerating) the weight is larger than the air resistance force.
Note: They do not need the word acceleration.
This could lead to a parachute investigation by the teacher at a later date.
An example of a simple paper parachute is in the extra list.
Safety Note: Parachute investigation (extra): Any activity which might cause objects to be thrown can present hazards. The Safety Notes are small if the paper parachutes are dropped carefully. Children must not be allowed to stand on chairs, desks or above stairwells without adequate supervision to drop the parachutes.
|Tell them a story about Hippo, who jumped out of a plane. Ask them how they could save Hippo - a parachute.|
Ask whether the parachute in slide 23 would save Hippo. - No the air would spill out.
Show slide 24, ask if Hippo is safe. – Still falling very quickly.
Show slide 25 and talk about the force of the air resistance being equal in size to force of gravity on Hippo- steady speed.
Slide 26 Ask what change could be made so that Hippo slows down - larger parachute
19. Satellites (Show Powerpoint Slide 27)
- The Moon orbits the Earth. The Moon is moving in a path around the Earth.
|Satellites have to move very quickly to orbit the Earth.||Discuss what happens when they throw a ball- it drops to the Earth.|
If an object is projected at VERY high speed then it goes further round the Earth.
An object must be moving at high speed to orbit the Earth.
Talk about satellites and the fact that the Moon is a satellite of the Earth.
20. The Moon
- The Moon is many times smaller than the Moon so gravity on Moon is very much smaller than on the Earth.
|Moon smaller than Earth (Show Powerpoint Slides 28 and 29)|
Get them to reason out that the force of gravity on the Moon is much smaller than the force of gravity on the Earth.
When a person walks they are used to having to push away from the Earth so on the Moon they would push high off the surface with the strong muscles they have. (Powerpoint Slide 30)
Therefore heavy boots are worn to keep them from bouncing high above the surface. (Powerpoint Slide 31)
|The more material (stuff) in an object the larger the force of gravity between it and other objects.|
They will think that there is no gravity on the Moon. Reinforce that there is a force of gravity between ANY two objects.
21. Zero Gravity
- Understanding gravity has made space travel possible.
|When going down in a lift you feel an odd sensation. This is because the lift floor is moving downwards and is not pushing up on your feet as hard as usual.|
When in free fall there is a sensation that there is no gravity (Show Powerpoint Slide 32)
|Use this to invite discussion about space travel and other applications.|
(Powerpoint Slide 33 could be used to show that a falling object is apparently weightless.)
To be inserted where it fits – depending on the group.
|"Every object pulls every other object toward it."|
"The more matter (stuff) an object contains the harder it draws."
"The nearer an object is to another the harder it draws."
"The harder an object draws other objects, the heavier it is said to be."
"The earth is millions of times heavier than any object near to or upon its surface; so it draws every such object toward it.”