Learn about gravity and air resistance as you make a balloon wobble through the air. A simple physics experiment for the classroom.

## Safety

Choking Hazard: Small parts not for children under 3 years. This activity involves marbles, which can be a choking hazard for young children. Adult supervision is required for young children.

• balloons
• marble.

## What to do

1. Inflated wobbly balloon

Blow up a balloon and tie the end.
2. Bounce the balloon into the air with your hand and observe how it moves.
3. Place a marble in another balloon. Blow this balloon up and tie the end.
4. Bounce this balloon into the air with your hand. How does it move?

## What's happening

The first balloon has a relatively small mass compared to its volume. The balloon falls due to gravity, while air resistance slows its fall. Another way of saying this is that the force of air resistance has an effect on its inertia. The materials making up the balloon (the air and the rubber skin) are spread fairly evenly around the middle, making the balloon fall smoothly.

The marble in the second balloon doesn’t behave like the balloon around it. It is denser, meaning the force of air resistance on its movement has less of an effect on its inertia than on the balloon’s inertia. It hits the side of the balloon and bounces back. The balloon also feels an equal force from the marble, but in the opposite direction, pushing it away from the marble.

This is a good example of Newton’s first law of motion on inertia, and third law of motion on opposing forces.

The marble keeps moving and bouncing around inside the balloon, which results in the wobbly motion of the balloon.

## Applications

Air resistance is an important consideration in the design and function of aircraft. The higher up in the atmosphere, the thinner the air. This lowers the air resistance meaning planes require less fuel to achieve the same speed.

In the case of parachutes, a high air resistance is required. The large surface area of a parachute results in a large air resistance. This slows down the person so that they are able to land safely.

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