Jar of Flies

jar-of-fliesA scientist weighs a jar of sleeping flies. Then, he shakes the jar to wake them up. While flies are flying in a closed jar, the scientist weighs the jar again. Does the jar weigh less when the flies are flying?

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Why are wet things darker?

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When something gets wet, it usually appears darker. Why?
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A ball is thrown vertically upwards

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A ball is thrown from ground level vertically upwards with an initial velocity of vi. Draw a velocity-time graph of the ball’s motion. Consider collisions with the ground are absolutely elastic and neglect air resistance.

Image By Michael Maggs Edit by Richard Bartz – Own work, CC BY-SA 3.0, Link
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How long would it take to fall through the center of the Earth?

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We have a hole that leads straight through the center of the Earth. We neglect air resistance and assume that the Earth has uniform density ρ. How long would it take to fall through the center of the Earth and reach the other side of the planet?

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What is the surface temperature of the planet?

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A planet of radius r and mass M has a homogeneous atmosphere (density independent of height). The atmosphere has a height h and the atmosphere’s gas has a molar mass μ.
What is the surface temperature of the planet?

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Shooting a bullet horizontally and dropping a bullet

fireddroppedbulletIf you fire a bullet horizontally and drop one from your hand at the same height and time, which will hit the ground first?

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