Question

An object of mass 1 kg thrown vertically upward
with a speed of 35 m/s. The net work done by
force of gravity during 4th second of its motion is
(Take g = 10 m/s2)
(1) -12.5 J
(2) 12.5 J
(3) 25 J
(4) Zero​

Answer 1

answer : option (4) zero

explanation : mass of object, m = 1kg

initial velocity of object , u = 35 m/s [ upward direction ]

Acceleration due to gravity, g = 10m/s² [ downward direction ]

now displacement covered during nth second is given by, $S_n=u+\frac{1}{2}a(2n-1)$

displacement covered during 4th second by object, S = 35 + 1/2 (-10) (2 × 4 - 1)

= 35 - 5 × 7 = 0 [ here a = g = -10m/s² is taken because g is acting just opposite direction of its motion ]

hence, displacement covered during 4th second is zero.

but we know, workdone = force × displacement

then, workdone by force of gravity during 4th second = weight of body × displacement covered during 4th second

= 1 × 10m/s² × 0

= 0.