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Question: March 2012 Archives

When you pump up a bike tire with a bicycle pump, the valve will heat up. Why is that? (If it because of the air passing through the valve, why doesn't the valve heat up when you fill your car tire at the gas station?)

When air is compressed it heas up, so when you use the hand pump, the air is compressed as you go and that heat is passed along to the valve. At the gas station, however, the air has been compressed for hours so the residual heat has already dissapated.

When an orchestra warms up, the pitch of the wind instruments increases and the pitch of the strings instruments decreases. Why is that?

In both cases, it is because the instrument warms up. The warmth from the player's breath warms the air in the wind instrument and increases the speed of sound, which increases the pitch. With string instruments, the strings expand and slacken slightly, which lowers the pitch.

Why will a heavily loaded raft drift more quickly with the current in a river?

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