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Energy savings calculation!!

An energy saver light bulb of 11 W gives the same illumination as a conventional 60W light bulb. (a) If a typical light bulb is used on average for five hours a day, how much energy is saved per year by one light-bulb? (b) If each household used eight such light-bulbs, ad there are 20 million households in a country, how much energy would that country save annually?

Asked SNB

led(Answers to this question invited from all users and visitors)

Time dilation and Length contraction

Is time dilation and length contraction hypothetical or is it really an observable fact.

for example,consider a rectangular sheet in a spacecraft moving at some high velocity.

Would an observer in the spacecraft be ever able to see the sheet as a square due to length contraction along its breadth(if the velocity is so so adjusted).
Would a stationary observer in the earth be able to observe the same.
In other words who experiences length contraction?

Also who experiences time dilation?


relativityAsked Ajmal

Friction – Numerical Problem

A block of mass 1 kg lies on a horizontal surface in a truck . The coefficient of static friction between the block and the surface is 0.6 .if the acceleration of the truct is 5m/s² calculate the frictional force acting on the block.

motion_friction1_240Asked  Mrinmoy Das

Helmholtz theorem

What is Helmholtz theorem?? Use it to show only transverse component of current density exist in wave equation??

Yuvrajj Vatt asked


Please refer to https://en.wikipedia.org/wiki/Helmholtz_theorem_(classical_mechanics)




Will the mass of earth reduce due to extraction of fossil fuels?

Is it possible that the earth mass would be reducing due to extraction of fossil fuels? 
If so, would it affect to its movement and position in solar system?


fossilfuelandearthThere won’t be any considerable loss of mass. The amount of mass irrecoverably lost is negligibly small on its conversion to energy .

It’s just like arguing that constructing more and more buildings will increase the mass of earth. !! No such thing happens. There is only a rearrangement of matter and inter conversion of mass and energy.

Newtonian movement

If a physically fit 30 kg boy were at rest and pushes off a wall forcefully in zero-G space with two hands, how many meters will he travel in one second?

Mechanics Problem

Pranav posted the following question.

Two Plates each of mass m are connected by a massless spring as shown in diagram.


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