JEE physics Assertion & Reason

(a) The rise in temperature of the soft steel is an example of transferring energy into a system by work and having it appear as an increase in the internal energy of the system. This works well for the soft steel because it is soft. This softness results in a deformation of the steel under blow of the hammer. Thus the point of application of the force is displaced by the hammer and positive work is done on the steel. With the hard steel, less deformation occur, thus, there is less displacement of point of application of the force and less work done on the steel. The soft steel is therefore better in absorbing energy from the hammer by means of work and its temperature rises more rapidly.

(d) Work done in the motion of a body over a closed loop is zero only when the body is moving under the action of conservative forces (like gravitational or electrostatic forces). i.e. work done depends upon the nature of force.

(b) If two protons are brought near one another, work has to be done against electrostatic force because same charge repel each other. This work done is stored as potential energy in the system.

(c) When two bodies of same mass undergo an elastic collision, theirvelocitiesgetinterchanged after collision. Water and heavy water are hydrogenic materials containing protons having approximately the same mass as that of a neutron. When fast moving neutrons collide with protons, the neutrons come to rest and protons move with the velocity of that of neutrons.

(c) When we supply current through the cell, chemical reactions takes place, sochemicalenergy of cell is converted into electrical energy. If a large amount of current is drawn from wire for a long time only then wire get heated.

(a) From, definition, work done in moving a body against a conservative force is independent of the path followed.

(e) Work done and power developed is zero in uniform circular motion only.

(d) A body can have energy without having momentum if it possess potential energy but if body possess momentum then it must posses kinetic energy. Momentumand energy have different dimensions.

(d) In an elastic collision both the momentum and kinetic energy remains conserved. But this rule is not for individual bodies, but for the systemofbodiesbeforeandafterthecollision.While collision in which there occurs some loss of kinetic energy is called inelastic collision. Collision in daily life are generally inelastic. The collision is saidtobeperfectlyinelastic, if two bodies stick to each other.

(e) Rate of change of momentum is proportional to external forces acting on the system. The total momentum of whole system remain constant when no external force is acteduponit.Internal forces can change the kinetic energy of the system.

(c) The gravitational force on the comet due to the sun is a conservative force. Since the work done by a conservative force over a closed path is always zero (irrespective ofthenature of path), the work done by the gravitational forces over every complete orbit of the comet is zero.

(a) The work done on the spring against the restoring force is stored as potential energy in both conditions when it is compressed or stretched.

(c) Change in kinetic energy = work done by net force.This relationship is valid for particle as well as system of particles.

(d) In a round trip work done is zero only when the force is conservative in nature.Force is always required to move a body in a conservative or non-conservative field