Wednesday, November 21, 2007

Gravity Experiments, Reducing Gravity, and Amplifying Gravity

What does a cup full of water have in common with an astronaut? Not much, apart from the way gravity works on them both. Find out how with this experiment.

What You Need :
- A styrofoam or plastic cup
- Something to make a hole in the cup
- A bucket
- A garden or grassy area
- Water
What To Do :

1. Poke a hole in the side of the cup. If you use a plastic cup, you might need an adult's help because it'll be a bit trickier.

2. Cover the hole with your thumb and fill the cup with water.

3. Hold the cup up high and uncover the hole. You'll see that the water gushes out steadily.

This is one reason why it's good to do this experiment in the garden!

What do you think would happen if you let go of the cup? Would the water flow faster or slower out of the cup?

4. Hold the cup up high again and this time, let it drop! Now watch what happens.

Whats Going On:
When you let go of the cup the water stops coming out of the hole. It stays put until it hits the ground. The reason for this is that gravity works differently in different conditions.

When you're holding the cup, gravity pulls down on both the cup and water. But the only thing that moves is the water, because you keep the cup in place.

The water gets pushed against the bottom of the cup and the weight of the water forces it through the hole.

It's a different story when you drop the full cup from a height. Gravity pulls down on the cup and water equally and they fall at the same speed. As they descend together, there is no force pushing the water through the hole.

The same principle is at work when astronauts are in space. They appear to be weightless, but really gravity accelerates them at the same speed as the spacecraft. When the spacecraft is on the ground though, it's gravity that pushes on the astronauts and keeps them in their seats.

Find out how you can defy gravity and cause objects to roll uphill.

What You Need :
- A lightweight cylindrical container (like a plastic storage jar or beaker)
- Rubber bands
- A lump of plasticine
- A tray and a some books to make a ramp where you can adjust the steepness.
What To Do :

1. Put some rubber bands around the beaker or storage jar to give it some grip on your ramp.

2. Build a simple ramp. Rest one end of your tray on the ground and the other end on a pile of books. You might need to adjust the steepness to get this experiment to work.

3. Roll a piece of plasticine so it is the same length as your container.

4. Stick the placticine down one side of the container. Now you're ready to roll!

5. Put the container on the ramp with the plasticine at the top.

6. Roll the container about a quarter of a turn, without letting go of it.

Now let go of the container. You'll see that it rolls up the ramp until the plasticine is at the bottom of the beaker. Spooky huh... Or is it?

Whats Going On:
The container may appear to be defying the laws of gravity when it rolls up the ramp, but it's not. What we are seeing is how gravity and an object's centre of mass or balance work together.

Every object has a centre of mass, the point that it would balance on. When the beaker is empty, its centre of mass or gravity is in the centre of the container. If you placed it on the ramp, gravity would pull through the centre and the beaker would roll down to the floor.

The plasticine is heavier than the container, so when it's added to the beaker the centre of mass changes to where the plasticine is.

By putting the container on the ramp with the plasticine at the top, you were also pointing the centre of mass at the top too (step 1). And while your beaker rolled up the tray, the centre of mass actually rolled downwards (step 2).

So the laws of gravity still hold true!

Center of Gravity

Purpose: To determine how the height of the center of gravity of an object affects its ability its mechanical stability (falling over).


Two empty 2-liter soda bottles with lids
Tap water

Fill one bottle to overflowing with water, and fill the other bottle about one-fourth full. Seal the bottles with their caps.
Stand the bottles side by side on a table.
Tilt each bottle slightly, supporting the top of the bottle with your finger, and then release it. Does the bottle tip over or does it return to a standing position?
Continue increasing the amount of the tilt until one of the bottles falls over.
Predict: How will the amount of water affect its center of gravity and stability? Which bottle will likely fall first?

True or False? A short person is less likely to fall than a tall person? Why?

Try this: Does the length of your feet affect how far you can lean forward without falling?

Center of Gravity

Results: The center of gravity of an object is the point where the weight of the object appears to be concentrated. An object is stable when its center of gravity, the place where the weight seems to be concentrated, is located over its base. The lower an object's center of gravity is compared to its height, the less likely it is to fall over. This ability to resist falling over is known as mechanical stability.

Why? An object is in a state of mechanical stability when it falls back to its original position when tilted slightly. The bottle with the lesser amount of water has a lower center of gravity. The lower the center of gravity, the greater the stability of the bottle, so it was able to be tilted farther without falling over.

True or False? True! The height of a person doesn't affect whether or not they might trip, but it does affect the mechanical stability of the person. Generally, shorter people have a lower center of gravity and greater mechanical stability than a tall person.