Satellites & Trays in Motion

Curriculum links:

• Physics – forces – conservation of momentum – conservation of angular momentum

Learning objectives:

1. Explore how satellites and planets orbit one another.
2. Learn about conservation of momentum and how we experience it in our daily lives

Materials:

• A tray or a bucket with a strong handle
• Water
• Rope
• Plastic cups

Safety:

• Take care not to swing something too heavy.
• To prevent anything from getting hit and broken during the experiment, do the experiment in an area with a lot of room – Maybe try it outside!

Instructions:

You can do this with a tray or a bucket of water that has a strong handle. Either way you’re going to have to have something to swing!

If using a tray:

• Start out by making the tray. The base can be made from a square piece of plastic or wood measuring approximately 30cm. Drill holes in all four corners large enough to hold a piece of rope. Attach the ropes to each corner of the platform and tie big secure knots to stop them coming out. Join the ropes together in a knot about half a meter away from the tray.

• Practice swinging the tray around without the cups in order to get the feel of a smooth, circular motion.
• Then add a plastic cup filled half full with water. The liquid adds mass to the cup and helps to keep the cup in place. You shouldn’t have to make the board below sticky but you can if you like.
• Swing the tray and plastic cup (several plastic cups if you’re feeling brave!) around in a complete circle without spilling the liquid or flinging materials around the room. 

If using a bucket:

• Tie rope around the bucket handle and part-fill the bucket with water.

• Swing the bucket around in a complete circle without spilling the water.

WORDS OF WARNING In order for the bucket to keep falling with the water, the bucket must travel fast enough to keep up with the water. If you spin the bucket too slowly, you get wet – which is hilarious… so consider it!

How it works:

• In the absence of a force an object will continue to move in a straight line.
• In order to change the direction of a moving object you have to apply a force.
• Gravity is a force that is applied to all things on earth which causes things to change direction towards the ground.
• The person swinging the object is applying a force through the string that is stronger than gravity and stops the water from falling out of the cup/ bucket.

Satellites

You may think of a satellite as a type of machine that orbits Earth, taking pictures and collecting information. You’d be right there are thousands of satellites orbiting Earth but did you know that the Earth is a satellite too? It orbits the sun going around and around in much the same way. How do they keep doing that —and why don’t our satellites just fall out of the sky?

If you throw a ball up into the air, the ball comes right back down. That’s because of gravity—the same force that holds us on Earth and keeps us all from floating away.

To get into orbit, satellites first have to launch on a rocket. That’s fast enough to overcome the strong pull of gravity and leave Earth’s atmosphere. Once the rocket reaches the right location above Earth, it lets go of the satellite.

The satellite uses the energy it picked up from the rocket to stay in motion. That motion is called momentum.

But how does the satellite stay in orbit? Wouldn’t it just fly off in a straight line out into space?

Not quite. You see, even when a satellite is thousands of miles away, Earth’s gravity is still tugging on it. That tug toward Earth – combined with the momentum from the rocket – causes the satellite to follow a circular path around Earth: an orbit. When a satellite is in orbit, it has a perfect balance between its momentum and Earth’s gravity.