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# Tag Archives: relative motion

## Heating up of water due to shaking

Would shaking a bottle of chilled water, cause it to warm up, even slightly, due to some kind of friction?

Yes, This is due to friction (Viscous forces). The different layers of water resist relative motion and the work done in producing motion against the viscous force appears  as heat.

## Problem from relative velocity

A Boat covers certain distance between two spots in a river taking t1 hrs going downstream and t2 hrs going upstream. What time will be taken by boat to cover same distance in still water?

posted by STUTI

$t=\frac{2t_{1}t_{2}}{t_{1}+t_{2}}$

## Relative position of a helicopter

If a helicopter takes off from a point on earth.(at certain altitude) and its still at that point in air.After certain time interval the observer whose position was same as that of helicopter’s take off point will see the helicopter right above him.
The question arises is that how it is possible if earth is rotating hence the observer is also, but the helicopter is in air.How is it possible?
Thanks…

## Relative velocity in free fall

If a person jumps out of a plane, and a few seconds later, another person jumps, why is it that the difference in their velocities stays the same? Shouldn’t it decrease because eventually, the first diver will reach maximum velocity, but the other diver will be slower at first, but then he reaches max. velocity as well…I’m not wording it the best way, but any help would be greatly appreciated :]

[Posted by Iris]

Both are examples for free fall. The velocity at any instant is given by v = gt

If for the first one, time = t and for the second who jumps after n seconds, time=t-n

Therefore, velocity of the first one at any instant, v1=gt

for second one, v2=g(t-n)

The difference in velocities,

v1-v2 = gn, which shows that the difference in velocity remains constant.

This is because, both are moving with the same acceleration and their velocities are increasing at the same rate

## Doppler Effect

“What is Doppler Effect?”

Doppler Effect is the apparent change in frequency of a wave due to the relative motion between the source and the listener.

(Please come back soon for more details with figures and videos)

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