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Extinguishing fire : Which is better? – Hot water or cold water

“What’s the difference if we add hot water or cold
water to extinguish fire?”

Sreeda asks:

Answer: The thing to be noted here is that the fire extinguishing involves absorption of heat and the absorption of heat in converting hot water to steam is very much more than the heat absorbed in heating cold water to the boiling temperature.  Hot water readily converts to steam absorbing enormous amount of heat and therefore hot water is better for fire extinguishing purpose than cold water.

It is true that cold water absorbs more heat than hot water (say about 1% more than boiling water, but it takes more time than the cooling effect produced by  vaporisation of hot water.

Convert CGPA to Percentage

You can use the following link to convert your CGPA to percentage using the formula given by CBSE


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Gravity and Speed of light – for discussion

Greg Carter posted a question:

“I’ve been reading a few physics books lately and theres a question that i just cant seem to answer. Is the speed gravity is applied to an object faster then the speed of light. that might be the wrong way of explaining it an example ive used is that if the sun blew up randomly, since it takes eight minutes for light from the sun to reach earth, we should not experience anything for eight minutes because everything ive read says nothing travels faster then the speed of light but it just makes since to me that if that happened since f=m1*m2*G/R^2 is the force between the two objects that the earth(along with other planets) would be set off their orbit because there is no long that force acting there. what is wrong with my thinking?” 

I am leaving it for open discussion.

A Numerical from Kinematics

Advait Asks:

“A test projectile is fired horizontally into a viscous liquid with a velocity u.The retarding force is propotional to the square of the velocity, so that the acceleration becomes a=-k(v)^2 .v is the velocity at that point. Derive expressions for the distance D travelled in the liquid and corresponding time t required to reduce the velocity to u/2.
Neglect any vertical  motion”


Conventional Current and Real Current

Manishankar Asks:

“What is the difference between conventional current and real current?”



Conventionally. the direction of current is defined as the direction of flow of positive charges. But in real case, the flowing charges are not positive (in the case of a metal); but the current carriers are electrons which are negatively charged. So, the real current is (the word current means flow) in a direction opposite to the conventional current.

So, if you are asked to find the direction of current in  a circuit, you have to find the direction which is opposite to the direction of flow of electrons.

Difference between dot product and cross product

Shashi Kant asked:

“What is difference between cross product and dot product?”



If the product of two vectors is a scalar quantity it is called scalar product or dot product.

If the product of two vectors is a vector quantity it is called vector product or cross product.

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May 2011

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