How does kinetic energy of a body changes when the mass of the body is doubled but velocity is reduced to half?

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1. Work

(i) Work is said to be done by a force when it causes displacement in a body in its own direction. It is a scalar quantity.

(ii) SI unit of work is joule J. It is defined as the amount of work done when a force of1 N causes a displacement of 1 m in its own direction.

(iii) The conditions to be satisfied for work to be done are (a) a force should act on an object, and (b) the object must be displaced.

(iv) The work done is zero when the displacement is perpendicular to the direction of the force.

2. Energy

(i) Energy is the capacity (ability) to do work.

(ii) Unit of energy is the same as work, i.e., joule J.

(iii) Potential energy is the energy possessed by a body on account of its position (gravitational potential energy) or configuration. Gravitational potential energy is formulated as PE=mgh where m is the mass of the object, h is the height from the reference point (where we take potential energy as zero) of the Earth and g is the acceleration due to gravity.

(iv) Kinetic energy is the energy possessed by a body by virtue of its motion. It is formulated as KE=12mv2 where m is the mass of the body and v is its velocity.

(v) The sum of kinetic energy and potential energy is termed as Mechanical energy.

(vi) Law of conservation of energy states that energy in a system can neither be created nor destroyed. It may be transformed from one form to another form, but the total energy of the system remains constant.

3. Rate of Doing Work:

(i) Power is the rate of doing work and is a scalar quantity. It can also be defined as the rate at which energy is consumed.

(ii) Average power, P=Wt or P=Et where W,E and t are work, energy (both in joules) and time (in seconds) respectively. 

(iii) SI unit of power is watt W. When a work of 1 J is done in 1 s, the power is said to be 1 W.

(iv) The energy used in one hour at the rate of 1 kW is called 1 kW h. It is the commercial unit of energy and is equivalent to 3.6×106 J.

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