1. See slide notes for more information
Centre of Gravity
See slide notes for more information
Slides 2-5 show three different starter ideas for a lesson on centre of gravity.
After that there is a game: ‘Spot the Centre of Gravity’. This needs resources to be prepared: see slide 6 notes.
Finally there are some suggestions for further activities.
This is not intended to be a self-contained lesson or a complete guide to this topic. You may want students to make notes or answer some simple questions as well.

2. A Challenge! - part 1
Place a chair facing the wall, so that when you sit in the chair you can just reach the wall.
Then stand up with your back to the wall, facing the chair.
Stand against the wall so your knees are straight and your feet and backside are up against the wall.
Keeping your knees straight and your feet and backside against the wall, try to lean forward and pick up the chair.
Starter option 1. It should not be possible to pick up the chair without breaking one of the rules. The chair should be of normal height – not a high chair as often found in science labs.
Normally, to pick up the chair you would move some of your weight backwards, behind your feet, to balance the extra weight of the chair in front. This would let you keep the combined centre of mass of you and the chair above your feet. With the wall behind you, you can't move any of your mass behind your feet, so when you lean forward and grab the chair, the combined centre of mass of you and the chair goes beyond the tips of your toes. If you could have lifted the chair off the ground, you would have instantly started to fall forwards until the chair hit the ground again.
Photos from

3. A Challenge! - part 2 Stand with the wall to your right.
Move up against the wall, so your right foot is against the wall and your right cheek is against the wall. (You may need to turn your upper body so your right arm is behind you)
Slowly, try to lift your left foot.
It should not be possible to lift your foot.
If you were to lift your left foot, your centre of gravity could no longer be above your base (which would now consist of your right foot only) and it would produce a turning effect causing you to topple sideways. (You instinctively stop yourself from doing this.)
Photos from

4. Starter option 2. Show this picture, or better, do the demo (or one of any number of fairly well-known similar ones, e.g. forks in cork). Why does this balance? The weight of the hammer is acting at a point directly below where the ruler overlaps the table.

5. Chasing the Centre of Gravity
You will need:
A 30 cm ruler (wood or stiff plastic)
1p coins / 2p coins / small masses
Place the ruler on your finger as shown and find the place where your finger needs to be so that the ruler is perfectly balanced.
Ask a partner to place one coin at a time somewhere on the ruler, and see if you can get it balanced again.
Starter option 3. You have probably balanced a ruler on your finger before. The slightest movement causes the ruler to tilt and fall off. When you find the exact spot where your finger needs to be to balance the ruler, you have found the ruler’s centre of gravity. The centre of gravity is the exact spot on the ruler where there is the same amount of weight on the one side of the spot as there is on the opposite side. Once you change the weight anywhere on the ruler, the centre of gravity changes too. (Image and idea taken from ezscience website:

6. SPOT THE Centre of Gravity
NOTE: before this activity, introduce the idea of centre of gravity/mass in whatever way you like: students try balancing objects (e.g. rulers with plasticine blobs stuck on, cardboard shapes) on their fingertips to find out Title slide. Assuming this man is in stationary balance, his centre of gravity must be above his base (his hands). What if he straightened his legs? That would move his centre of gravity to the right, and he would unbalance, tipping clockwise.

7. Online Tool to Find the Meeting Centre of Gravity for ZIP codes
Use this tool to find the best place to hold a meeting for its attendees by inputting their US ZIP code. The tool will then work out the centre of gravity for the meeting. This will be represented by a marker on the map.
“This is the centre of gravity for terrorism in the country.”
Major General Athar Abbas (speaking of South Waziristan in Pakistan, during heavy fighting there between the army and the Taliban)
The phrase ‘centre of gravity’ is widely used outside science. Here are some examples.
“Comedy's new centre of gravity Seth Rogen is box office gold thanks to two summer hits: Knocked Up and Superbad.”

8. New Zealand immigration website: The 'centre of gravity' of your family is in New Zealand if: The principal applicant has no dependent children and the number of their adult children lawfully and permanently in New Zealand is equal to or greater than those lawfully and permanently in any other single country…
Some more examples.

9. Some examples of how the location of a person’s centre of gravity can change. The picture on the right shows that it is possible for an object’s centre of gravity to be outside the object.