Question

An object weighing 70 kg is kept in a lift. Find its weight as a recorded by a spring
balance when the lift
a) Moves upwards with a uniform velocity of ms-1
b) Moves upwards with a uniform acceleration of 2.2 ms-2
c) Moves downwards with a uniform acceleration of 2.8 ms-2
d) Falls freely under gravity

Answer 1

Given :
mass=70kg
g=9.8 m/s²

a) Moves upwards with a uniform velocity of 5ms-1 [i have taken as 5m/s]:

When a lift is moving upwards with a uniform velcoity 5m/s ,the reaction force R is
R=mg=70x9.8
=686N

ii) When lift is moving upwards with uniform acceleration 2.2m/s² the reaction force is

R =m(g+a)=70(9.8 +2.2)=840N

iii) When the lift descent with a uniform acceleration of 2.8m/s² the reaction R is given by 
=m(g-a)=70(9.8-2.8)=490N

iv) when the lift falls freely under gravity the reaction R is given by
m(g-g)=0