Question

True or false for each statement :

1)Without friction or work by an external force, the sum of the potential and kinetic energies of a body is constant.

2)Work is done when the form of energy changes.

3)Work can be done in the absence of motion.

4)Less power is required while slowly lifting a box than while lifting it up quickly.

5)Energy conservation law for a projectile (no friction): Potential energy decrease equals the kinetic energy increase.

6)A source of energy is required to do work.

Answer 1

(1) When external force is not acting on the system then the total mechanical energy remains constant. so sum of kinetic and potential energies of a system reamins constant. Thus it is True

(2) Work is done when the form of energy changes. In general when work is done only the energy gets changes into one form to other. It is also True

(3) Work done is defined as   W = force *dispalcement

when there is no motion then work done is zero. Then it is false

(4) Power P =W/t

Since power is inversly proportional to the time so when the time requirement is small then power is more. Then more power is required while lifting the box quickly. Thus it is true.

(5) Energy conservation law for a projectile (no friction): Potential energy increase equals the kinetic energy decrease. False

Since when a projected upwards with initial velocity having maximum kinetic energy and when it reaches maximum height it have potential energy its velocity becomes zero.

(6) Energy is defined as ability to do the work. So a source of energy is required to do the work. Thus it is True.