Question

Flash and thunder are produced simultaneously . but thunder id heard a few second after is seen,why how does the speed of sound in air vary with rise in density of the medium?

give one ex. Where kinetic energy is transferred from one object to others.


Archimedes could find the purity of king's crown using buoyancy. How?




How do u define mechanical work ?
Name and define SI unit of work.
What do you mean positive and negative work ? Give example

Answer 1

The light waves and sound waves travel a few km.  Their speeds are 300,000 km/s and 340 m/sec.  So there is a difference of seconds of time between seeing a flash and hearing a thunder.
The speed of sound increases with the density of the medium.  The particles vibrate in the medium to transfer sound energy.  If the particles are closer to each other, then the vibration of one particle induces more easily the vibration in the next particle. Then sound energy is transferred quicker. 
When a moving particle collides with a stationary particle, part or full of the kinetic energy of the moving particle is transferred to the stationary particle.

Archimedes immersed the crown in water.  He noted the difference in water levels to find the volume of water displaced.  He measured the weight by a balance.  Then found the density of the crown.  The pure gold crown is denser and weighs more than impure crown as impurities such as silver, copper are lighter than gold.
He calculated the density of pure gold and density of gold in the crown.

Mechanical work = Force acting on an object * displacement of the object.
                          = torque acting on an object * angle through which object rotates
               = the change in the kinetic energy of the object. (initial - final)
         The SI unit is Joules.

Positive work means the system or the body in context  has done work.  Negative work means some external system or environment has done work on the system or body.
 
 If a body falls from a height then gravity does work on the body.  So the work done by the body is negative.  So its energy increases.
If a moving ball on a floor comes to rest, then the work done by the ball is positive.  So its energy decreases and final kinetic energy is less than the initial.

Answer 2

Light travels faster than sound. So there is a difference of seconds of time between seeing a flash and hearing a thunder.

The speed of sound decreases with the increase in density of the medium. In rainy season, when the air is moist and its density is less, sound travels faster than in winter.

When a moving particle collides with a stationary particle, part or full of the kinetic energy of the moving particle is transferred to the stationary particle.
Archimedes immersed the crown in water.  He noted the difference in water levels to find the volume of water displaced.  He measured the weight by a balance, using which he found the density of the crown. The pure gold crown is denser and weighs more than impure crown as impurities such as silver, copper are lighter than gold. He calculated the density of pure gold and density of gold in the crown.

Mechanical work = Force acting on an object * displacement of the object.
= torque acting on an object * angle through which object rotates
= the change in the kinetic energy of the object. (initial - final)
Its SI unit is Joules.

Positive work means the system or the body does work.  Negative work means some external system or environment has done work on the system or body.
 
 If a body falls from a height then gravity does work on the body.  So the work done by the body is negative.  So its energy increases.

If a moving ball on a floor comes to rest, then the work done by the ball is positive.  So its energy decreases and final kinetic energy is less than the initial.