Pushcart - Structures

How has it been done before?

Engineering Facts

• squash a structure or part of it (compression forces)

• stretch it (tension forces)

• bend it (bending forces)

• twist it (torsion forces)

• cut it (shearing forces)

To be able to withstand all these forces your pushcart frame needs to be light, strong and stable.

To add lightness a framework can be made rather than building a solid structure. Beams that are hollow are lighter than solid beams of the same material and dimensions. The materials you find or buy are also important here.

To add strength a framework can be made of strong materials and/or covered with a shell. The shell material needs to be as light as possible if weight is important.

There are two ways to add stability to a structure:

• make sure the centre of gravity is between the wheels of your pushcart and close to the ground.

• use triangular shapes rather than rectangles. This is why bracing is used in many frame structures. Bracing is added to form triangles in the structure. e.g. Harbour Bridge

Suggestions and Choices

• choosing materials
• how forces work
• centre of gravity

Look at the pictures on the right of this page. Can you find triangles that brace the structure?

Look back at all the pictures of pushcarts you found, or at real pushcarts. Can you find triangles? These might be frame triangles or solid shells that work as triangles. How is the frame braced in each of the examples you find?

Try This

With straws

• Fold drinking straws into a variety of shapes (e.g. square, hexagon, rectangle, triangle...)
• Squeeze the shapes gently to see which ones are hardest to distort.

With arms

• Hold one arm out horizontally. 
• How long does it take for it to get tired?
• Hold your arm out again but this time use the other arm to support it at the elbow
• How long does it take for your horizontal arm to get tired?
• Repeat the experiment but this time ask your partner to push your arm down gently each time.
• Is it easier to move your arm when unsupported or when supported with the triangular bracing supplied from your other arm?

With legs

• Stand with your legs together.
• Ask your partner to gently push you from the side until you go off balance.
• Now stand with your legs apart.
• Ask your partner to push again from the side.
• Is it easier to push you off balance when your legs are together or when you brace yourself by forming your legs into a triangle? 

• centre of gravity
• forces

For Teachers

Harbour Bridge

Racing car frame