Q. 34.4( 5 Votes )

Identify the wrong statement among the following. Rewrite them by making necessary corrections.

a) Work and energy have different units.

b) When an aero plane takes off, the work done by its weight is positive.

c) The potential energy of spring increases when it is extended and decreases when it is compressed.

d) If the work done by external forces on a system is negative then the energy of the system decreases.

e) When a body is falling freely from a height, its kinetic energy remains constant.

f) The unit of power is watt.

Answer :

a) Work and Energy have same unit joule(J),


Work is defined as W = Force Displacement ,its unit came out be Newton-metre or precisely Kg.--- (i)
Same thing with Energy , let us take Kinetic Energy ,it is given by the formula
m


where m is mass of the object in kg


v is the velocity of the object in m/s,


now putting the value of m and v in SI units we will get Kinetic Energy Units as Kg


which is same as of the units of work given in Eq-(i) ,hence proved they have same units.


b) Work done by its weight is negative as the weight is acting downward and plane is going upward.


c) When we try to extend or compress spring we increases its energy and this energy is stored as potential energy and when we release spring it loses its potential energy to do some work.


d) External negative force acting on a system decreases it energy because it acts opposite to the direction of motion of object .for example when friction acts on a object it gradually lowers its velocity .


e) freely falling body loses its potential energy into kinetic energy. So, as potential energy decreases kinetic energy increases .


f) unit of power is joule/second(Watt). Power is defined as rate of change of energy per unit time.


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