Questions OBJECTIVE - I and Answer (The Forces) HC Verma Part 1

 Q#1

When Neils Bohr shook hand with Warner Heisenberg, what kind of force he exerted?
(a) Gravitational                           
(b) Electromagnetic 
(c) Nuclear                                     
(d) Weak.     

Answer:
(b) Electromagnetic 
When two surfaces are in contact with each other, the atoms at the two surfaces come close to each other. The charged constituents of the atoms of the two bodies exert great forces on each other. So these forces are Electromagnetic forces.      

Q#2
Let E, G and N represent the magnitudes of electromagnetic, gravitational and nuclear forces between the two electrons at a given separation. Then

(a) N > E> G   (b) E > N > G   (c) G > N > E   (d) E > G > N.   

Answer: (d) E > G > N.
Nuclear forces are close range forces and they are appreciable only within nucleus otherwise it is negligible. So, in this case, it is negligible. Gravitational forces depend upon the mass of the particles and electromagnetic forces upon charge. For an electron, its charge is equal to a proton but the mass is much less, so the electromagnetic forces between these electrons will be greater than the gravitational forces.

Q#3
The sum of all electromagnetic forces between different particles of a system of charged particles is zero.
(a) only if all the particles are positively charged
(b) only if all the particles are negatively charged
(c)  only if half the particles are positively charged and half are negatively charged
(d) irrespective of the signs of the charges. 

Answer: (d) irrespective of the signs of the charges. 

Between any two particles either of the same charge or different, the forces occur in action-reaction pairs according to Newton's third law of motion. The forces of these pairs are equal in magnitude but opposite in direction. So their sum is zero.

Q#4
A 60 kg man pushes a 40 kg man by a force of 60 N. The 40 kg man has pushed the other man with a force of
(a) 40 N        (b) 0        (c) 60 N       (d) 20 N.   

Answer: (c) 60 N

If we take the force exerted by the 60 kg man as action then according to Newton's third law of motion equal and opposite force of reaction will be exerted by the 40 kg man on the other man.