Question

1. A chain of mass 2 kg is placed on a smooth table
with 1/4th of its length hanging over the edge.
If the length of chain is 4 m then the work done in
pulling the hanging portion of the chain back to the
surface of the table is
[NCERT Pg. 118]
(1) 5 J
(2) 2.5 J
(3) 2 J
(4) 4J​

Answer 1

Answer:

length and weight of 1/4 part is 1m and 1/2kg

work done =F*d

F=mg= 1/2*9.8=4.9N

W= 4.9*1= 4.9J

if g=10 then ans will be 5J

Explanation:

Answer 2

Potential energy of the part of the chain hanging =MgH…..(where M is the mass, g is the acceleration due to gravity and H is the perpendicular height of of centre of mass of the body from point of reference)

Here mass of the chain is M

The length of the hanging portion of the chain =L 》Centre of mass of the hanging portion of the chain =L/2

Taking the surface of the table as reference

P (int)=MgH=Mg(L/2)=MgL/2 J

P (fin)=0 J………..(because L=0 when the whole chain is on the table)

W.D=P (int)-P (fin)

=MgL/2 - 0

= MgL / 2 J