A 49 kg lady stands on a spring scale in an elevator. During the first 5 sec, starting from rest, the scale reads 69 kg. The velocity of the elevator will be

ANS:C - 20 m/sec

Initail weight before moving on lift(m) =49 kg.

We know as we move upwards increase in weight occurs i.e acceleration increase by (g+a).
New weight (m1) = 69 kg.
Time(t) =5 sec.
Weight(W)= m(g+a) =m1 * g.
49(9.81+a) =69*9.81.
a=4m/sec2.
We know v=u+at.
u =0(as starts from rest).
v = 4*5=20 m /sec2.