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Kinetic Energy – Numerical Problem

a running man has half the kinetic energy that a boy of half his mass has.the man speed up by 1m/s and then has the same kinetic energy as the body. what were the original speeds of man and boy?

Dual nature of light and related questions

How light has a dual nature?

A photon has zero rest mass. Explain how ?

(These questions were posted by Shubham SInha  and Ritika Chatak)


The dual nature of light has been established through the various theories in an attempt to find out the real nature of light. The first theory on the nature of light was by Sir Isaac Newton. He said that light consists of tiny spherical an perfectly elastic particles called corpuscles which travel through space with an enormous speed. This theory could explain various phenomena exhibited by light like, reflection, refraction, rectilinear propagation etc. But could not explain color vision, colors in thin films, dispersion etc.

Then came Huygen’s wave theory which considered light as mechanical waves. This theory could explain many phenomena but could not explain how light can travel through vacuum, as a mechanical wave requires a medium for propagation.

Then Electromagnetic Wave Theory  by James Clark Maxwell was used to explain the nature of light and it was experimentally established. The velocity of electromagnetic waves predicted theoretically matched closely with the actual velocity of light determined experimentally by different scientists through different methods.

But, the discovery of photoelectric effect made us rethink. Photoelectric effect, in which electrons are emitted out when light strikes the surface of metals, could be explained well only if we consider the particle nature of light.

Max Planck in his Quantum theory considers radiations as packets of energy called quanta (plural of quantum) or photons. These photos can behave like a particle during its interaction with matter and posses wave nature as basicllay light is an electromagnetic wave.

Thus scientists have concluded that light possess both wave nature and particle nature at the same time. In order to explain some phenomena we may consider its wave nature whereas for some other , we have to consider it as particle.

Motion of connected system – A Numerical problem

A mass 5kg descending vertically,draws up a mass of3kg by means of a light string passing over a pulley.At the end of 4 seconds,the string breaks.How much higher 3 kg mass would go?

Post your solutions as comments to this post

Answer will be posted soon once we get responses from visitors

Horizontal projectile and freely falling body

A rifle at a height H aimed horizontally fires a bullet to the ground. At the same time , a bullet with the same mass in dropped from the same height. Neglecting air resistance, which one hits the ground first?Explain.
ED posted
Both will hit the ground simultaneously.
When a body is projected horizontally, its initial vertical velocity […]

Matter, Antimatter and Einstein’s equation

It is my understanding that when antimatter comes in contact with matter that there is nothing left but pure energy.
And since energy and matter are interchangeable (E=Mc2) doesn’t that mean you are actually left with all matter and no antimatter?
Asked Ken
The equation E = mc2 tells about mass – energy relation and both matter and […]

How can we define one Newton ?

How can we define one Newton? (Asked Pawas Rituja)

One newton is defined as that force which when acts on a body of mass 1 kg produces an acceleration of 1 m/s^2 in it

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A simple numerical on momentum

Car mass is 1200 kg moving velocity is 108 km/hr. On applying brakes velocity reduced to 36 km/hr. Find change in momentum?

(posted by Aditi)

m=1200 kg

u=108 x 5/18=30m/s


Change in momentum = m(v-u) = 1200 x (30-10)=1200 x 20= 24000 kg m/s

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