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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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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A fast moving train cross a person standing near a railway track

Farringdon_station_MMB_22_S-Stock

When a fast moving train cross a person standing near a railway track, the person has a tendency to fall towards the train. This is because

a) a gravitational force
b) of dizziness related to vestibular system dysfunction
c) vibrations of air created by the train
d) a fast moving train creates large velocity of air between the person and the train
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Human-Powered Helicopter

HPHA student builds a human-powered helicopter (HPH). It is a helicopter powered by one person carried on board and the power is generated by pedaling.
This human-powered helicopter has the following parameters: diameter of helicopter rotor blades is 15 meters and total mass is 100 kg. The air density 1.3 kg/m^3.

Can the HPH take off?

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Why do golf balls need dimples?

GolfBall

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Why do track bicycles sometimes have a disk wheel?

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