Can you solve it? Physics puzzles for smart students

Today's puzzles have been suggested by the Department of Physics, University of Oxford, for reasons which will become clear below.

They start with a question that could change your life, that is if you always burn your mouth with hot coffee.

1. Cuppa riddle

You are in a hurry to work. You pour yourself a cup of black coffee, but it's too hot. You intend to add a fixed amount of cold milk to it, but you know that even after that the coffee will need to cool for a few minutes before you can drink it.

In which case does the coffee cool down more:

1) Immediately add milk, then wait a few minutes before drinking.< /em>

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2) Wait a few minutes, then add milk just before drinking.

2. Fly away

A fly landed on the bottom of a glass on a very sensitive digital scale. Suddenly the fly flies away. What happens to the display on the scale?

1) it increases

2) it goes down

3. Bouncing Bullets

A weightless string connects two identical metal balls. The middle of the string is on the edge of a table. You release both balls at the same time. What will happen sooner: ball 1 flies off the table or ball 2 hits the side of the table?

4. Funny fraction

Prove (1/2) x (3/4) x …x (99/100) < 1/10

It sounds complicated, but there is a two-line solution.

Yes, I know. The last one is about math, not physics, but it allows me to follow up on why I posted these puzzles today.

If you're a a former smart sixth grader at a public school in the UK, you are eligible to take part in a fantastic free course run by Oxford University which will teach you maths and physics at a deeper level than the A level curriculum, preparing you to study these subjects in college.

If you're not a smart sixth elder who loves the hard sciences, but you know someone who does is, please let him know. To apply, you must register and submit papers by October 16.

The new course is called Comprehensive Oxford Mathematics and Physics Online School (COMPOS) and is an idea original by Alexander Lvovsky, professor of physics at Oxford. He suggested the puzzles printed above.

He adds that if you can solve the first two puzzles above, you're probably at the right level for the course. The last two questions are a little more complicated and require a spark of ingenuity.

I'll be back at 5pm in the UK with the answers. Please NO SPOILERS

I put a puzzle here every two weeks on a M...