Challenge Question

Some of these are easy, some are hard. Some may require experiments, some can be done purely by thought, and some may be impossible.

Which is which?

Something to reflect on ...

A friend of mine wanted to see the inside of a photocopier while it was working, so she photocopied a mirror.

What was the result?

... lonely as a cloud ...

Think of a nice, big, fluffy, cotton-wool cloud. You probably know that it's made of of water droplets. But water is heavy, so why doesn't the cloud fall down?

The wind beneath my wings.

Common knowledge and standard dogma says that the lift from an aeroplane's wings comes from the difference in air pressure above and below, which in turn is caused by the air travelling at different speed. This difference in speed of the air flow is, in turn, caused by the upper surface being more curved.

So how do 'planes fly upside down?

An irresistible force.

In its short form:

 The Moon is getting further away. What provides the force to pull it forward?

OK, now for the more complete version.

We know that the moon causes tides on Earth. Tides create friction, and hence dissipate energy. That energy is the rotational energy of the Earth, so the Earth's rotation is slowing.

And we account for that with the occasional leap-second.

But that means the angular momentum of the Earth is decreasing, but the angular momentum of the Earth-Moon system must be constant. Therefore the angular momentum of the Moon must be increasing. If you do your sums, the Moon must be getting further away.

And it is - about 3.5cm per year.

But if it's getting further away, something must be pulling on it. Obviously the Earth's gravity holds it in orbit, but if it's going into a higher orbit it must have more energy, so it must be being sped up.

How?

Suppose I have ten cards, one of which represents a prize of great value. I shuffle them, spread them out face down, and let you choose any two. You take them, but you don't look at them.

I take the remaining eight cards and look at them. One by one, I show you six of them. I want to keep the tension high, so I definitely don't reveal the card of great value. After all, I might not have it.

After revealing those six of the cards, I have two, you have two, and I make you this offer:

 You can choose and keep one, but only one, of my cards, if you give me both of yours.

Assuming this isn't a card trick, and that everything is as I've described - should you accept?

Weighing the options

Moved to Weighing The Options ...

Straight up!

 If you look at the moon near one of its quarters, neither full nor new, but somewhere in between, you can clearly see the line separating the light side from the dark side. Sometimes the moon is seen as a crescent, either recently departed, or now approaching the New Moon. Sometimes the moon is seen as a Gibbous moon, close to the Full Moon, and either approaching or receding.

 And there is a point when the moon is exactly a half moon. Sometimes the exact moment comes when it's on the other side of the Earth and you don't see it, sometimes it's overhead. This is the moment when the terminator, the line separating light from dark is a straight line from your vantage point.

So the question is this:

• From which parts of the Earth's surface can you ever see the terminator as a vertical line?

• Under what conditions?

On the level

• From which parts of the Earth's surface can you ever see the terminator as a horizontal line?

• Under what conditions?

Moon and tides

Everyone knows that the tides are caused by the moon. But if that's true, why is it that the moon goes overhead once a day, and yet there are two tides per day?

How can one moon cause two tides?

Coffee Cup Physics

 Where to tap the coffee cup Take a coffee cup and tap it in each of the two places shown. If you get it right there is a clear difference in the "clinks" that result. Why? How would you test if your explanation is right? Now use your explanation, your "theory", to make some predictions. Don't cheat - make the predictions first!

What are your predictions for the following questions:

1. Will it work on every cup?
2. Will all cups give the same pitch difference?
3. Will it matter whether the cup is full or empty?
4. What happens when you tap opposite the handle?
5. What if there is no handle?

Now test your predictions - are you right? The act of making predictions and then testing them with experiments is the heart and soul of science. By testing your predictions you are testing your theory, which is the basis of your understanding. Without testing your predictions you may easily be fooled by coincidence.

So, how did you get on? Let us know!

Like a candle in the wind ...

A candle has its distinctive flame shape because of the convection currents set up as it burns. It heats the air which expands and rises, drawing the flame upward with it. Cooler air is drawn in from the bottom, feeding oxygen to the flame and shaping the lower part.

So what happens in zero gravity?

Be careful. The steady state solution is easy to find, but is it stable? Maybe the steady state solution is not what happens in practice. If not, what does happen, and how can you tell? How can you test your prediction?