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Written by Beth Askham

Make your own climbing frog, using the power of friction.

You will need

• A 20 cm x 20 cm piece of cardboard
• Two 5 cm long pieces of drinking straw
• String
• Sticky tape
• Scissors
• Coloured pens, pencils, crayons, paints or textas
• Other decorations

What to do

1. On your piece of cardboard draw out an outline of a frog, making sure the widest point of its body is at least 7 cm across.
2. Cut out and decorate your frog.
3. Using sticky tape, stick the straws next to each other on the back of the cardboard forming an upside down V. The straws should be about 1 cm apart at one end of your frog, and about 5 cm apart at the other end of your frog.
4. Cut a piece of string 2 m long, then thread it through the straws that are close together so the two ends of the string hang down below your frog.
5. Have a helper stand on a chair or table, and place a pencil through the loop of string at the top of the frog. Hold the ends of the string in your hands and pull down on one end of the string, then pull down on the other end. Keep doing this and watch your frog climb.

   

6. As the frog moves up the string, continue to pump your arms like pistons, and slowly move your arms wider apart.
7. As a challenge for your frog, rub some cooking oil on the string. How does your frog climb now?

What’s happening?

You might expect your friction frog to slide down the string because of gravity, but another force is holding up the frog when you move the strings.

This force is friction, which is between the surfaces of the string and the straw. Friction is a force that works against an object that is moving. Although the string and straws look quite smooth, when viewed under a microscope they are jagged and rough, and the angle of the straws means that they are always touching the string.

When you pull down on one of the strings, it moves through the straw. The microscopically rough surface of the straw grips against the string, stopping the frog from slipping down.

If you used cooking oil, then you would find it much harder for your frog to climb. This is because cooking oil is a lubricant – it smooths out the surfaces of the straw and string. This reduces the friction between them, so very little of your kinetic force is converted to friction force.

The rougher the two surfaces are, the greater the friction between them. You could use tube pasta and wool for this experiment, but if the friction is too strong, then you wouldn’t be able to pull the wool through at all!

Real-life science

Cars require friction to start moving, to stop when the brakes are applied, and to steer. Car engines have hundreds of moving parts and they are constantly rubbing and grinding against each other. The friction from this contact begins to wear down the engine parts, so engine oil is used as a lubricant to reduce this friction so the parts last longer. Tyre manufacturers need friction to stop and start the car, but too much friction would mean the car uses too much fuel.

Friction comes into play when walking along a frosty or icy footpath. The smooth surface of the ice means you cannot grip with your shoes, making you slip and slide. People in cold climates and football players wear shoes with spiky or rough soles that increase the friction between their shoes and the ice or grass.

Even if you are an astronaut you would still have to deal with the force of friction, this time from the atmosphere when your space ship returns to Earth. Because the shuttle travels so quickly a huge amount of heat is produced from the air molecules bombarding the ship. Astronauts are protected from this heat by thermal insulation on the outside of the capsule. Friction often creates heat; just try rubbing your hands together.

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