Pushcarts - Centre of Gravity

How has it been done before?

Engineering Facts

This photo shows a racing car with driver being tested to check that the centre of gravity is low enough for it not to tip over on a sharp bend or steep hillside.

Your family car design would have been tested like this as well.

Driving around corners creates forces that tip the car. It the car is poorly designed it can roll and an accident can occur.

The centre of gravity is a point from which you can suspend or support a shape so that it balances.

A spinning top spins well because the centre of gravity is on a line down the middle. The top itself is symmetrical in every direction from the centre line. To balance and spin well the centre of gravity must also be below the middle of the shape so that any slight wobble will correct itself. The spinning top falls over quickly if the centre of gravity is above its middle (like in the second diagram).

A low centre of gravity makes a shape more stable.

Suggestions and Choices

• front to back as the pusher pushes down on the pushbar. Make sure your centre of gravity is not behind the back wheel!
• side to side as you turn corners causing the pushcart to roll

The centre of gravity of cart and rider can be changed by the way it is pushed. Your pusher will need to be careful not to tip your cart. Can you design to make sure it does not happen?

When you are designing your pushcart your load will be your driver and the force downward that the pusher makes. You will need to make sure that all the downward forces act between the wheels so that the pushcart is stable. So where will you attach your seat? Where is the best place for the push bar?

Try This

Find the Centre of Gravity of a shape

1. Cut out a shape in cardboard, any shape will do.
2. Make three small holes near the edges at some distance apart. 
3. Make a plumbline (use something tied to the end of a piece of string or thread).
4. Hang the plumb-line on a pin then pin the shape through one of the holes so it is free to move
5. Draw the line made by the plumb-line onto the shape.
6. Repeat steps 4 and 5 using another hole.
7. Where the lines you have drawing intersect should be the Centre of Gravity of the shape.
8. Check it using the third hole.
9. If you pin your shape through its Centre of Gravity it should be stable and stay where you put it. It will be balanced.

Try to guess the centre of gravity of the other shapes opposite.

Once you have made your decisions:

• copy the shapes onto cardboard
• cut them out carefully
• mark the centre of gravity you predicted
• use the method outlined above to find the true centre of gravity.

How close were your predictions?

Look again at the faceless person you cut out. If this person stands on two feet it will balance, just like you do. That is because it is supported directly below its centre of gravity. Can you make this person balance on a tight rope on one foot (at the notch)? Try adding some props you have seen circus performers use like umbrellas and long sticks. Can you make the person balance on one hand or on its head?

A shape hanging from any point will hang so that its centre of gravity is along a vertical line from the hanging point.

Now draw a side-view silhouette of your pushcart design with a person sitting in it. Find its Centre of Gravity. Is it well balanced? Should you change the position of the seat to create better balance?

For Teachers

This photo shows a racing car with driver being tested to check that the centre of gravity is low enough for it not to tip over on a sharp bend or steep hillside.

A spinning top spins well because the centre of gravity is on a line down the middle.

The centre of gravity of cart and rider can be changed by the way it is pushed.

Try to guess the centre of gravity of these shapes.

What not to have happen!