# Knock Down Clown

The clown gets knocked down, but it keeps getting back up! Make a toy that refuses to be pushed around and always comes back for more, thanks to its low centre of gravity.

• Science > Physical Sciences > Year 2 > ACSSU033
• Science > Physical Sciences > Year 4 > ACSSU076
• Science > Physical Sciences > Year 7 > ACSSU117
• Science > Physical Sciences > Year 10 > ACSSU229

## You'll need

• bouncy ball
• craft knife
• ruler
• pencil
• paper
• sticky tape
• scissors
• coloured pencils

## Try this

1. With adult supervision or assistance, use the craft knife to cut the bouncy ball in half.
2. Place one half of the bouncy ball on a flat surface with the curved side touching the flat surface.
3. Push down on one side of the ball.
4. Observe that the ball always returns to its starting position.
5. Using a pencil and a ruler, draw a square (10 cm x 10 cm) on the paper.
6. Use scissors to cut the square out of the paper.
7. Roll the square to form a tube that fits on the top flat edge of the ball. Make sure that the round part of the ball is sticking out of the bottom of the tube.
8. Use sticky tape to hold the paper tube in place.
9. Push the paper tube using a finger to see whether the paper tube returns to an upright position when it’s knocked over.
10. Use the scissors to cut a small section off of the top of the paper tube.
11. Repeat Step 9.
12. Repeat Step 10 and 9 until the paper tube returns to an upright position when it’s knocked over.
13. Use coloured pencils to draw a clown on the paper tube. (The toy is now ready for use.)

## What's happening?

The centre of gravity of an object is the point at which the object’s average position of weight is located. The centre of gravity is commonly called an object’s balance point. If you support the object’s centre of gravity (balance point), then you support the entire object.
If an object is symmetrical, the centre of gravity is at its midpoint or its geometrical centre. If the object is oddly shaped, then the centre of gravity is toward the heavier end of the object.

The centre of gravity of a bouncy ball is exactly in the middle of the bouncy ball. When the bouncy ball was cut in half, the centre of gravity of the half-bouncy ball changed. It’s in the middle, just beneath the flat side, which is heavier than the round side.

When you pushed down on one side of the half-ball, you increased the weight on that side of the half-ball, moving its centre of gravity. Once you let go of the half-ball, its centre of gravity returned to its usual position and the half-ball was able to ‘stand’ on its own, with half of its weight on either side of its centre of gravity.

When you attached the paper tube to the half-ball, you added weight to the upper part of the toy. The toy’s support base was the half-ball. When the paper tube was long, the clown couldn’t stand-up after being knocked over because its centre of gravity was too high and laid outside its support base. You shortened the paper tube to lower the centre of gravity of the toy so that the centre of gravity was above its support base. This allowed the toy to return to the upright position after being pushed over.

The Leaning Tower of Pisa is 55.5 m tall and weighs 16 256 800 kg. It has a stone foundation that is only 3 m thick and it sits on soft sand, rubble and clay. This weak support base wasn’t strong enough to support the tower’s magnificent structure, so it began to sink unevenly, so that at one point the tower was leaning more than 5.1 m to one side! The Leaning Tower of Pisa doesn’t topple because its centre of gravity is still above its support base. However, if the tower continues to tilt far enough so that its centre of gravity extends beyond its base, the tower will topple over.

Four Wheel Drives (4WDs) are heavier and sit higher off the ground than sedans. As a result, their centre of gravity is higher than it is in a sedan. The centre of gravity in a 4WD is even higher if the drivers put more weight on the roof of the 4WD by putting heavy items on their roof racks or in storage containers. Because of their high centre of gravity, 4WDs are at a greater risk of toppling over than sedans. For this reason drivers of 4WDs have to be particularly cautious not to make fast, tight turns or swerve dangerously as that would result in the centre of gravity of the 4WD extending beyond its support base.