Question

how does the tension in a string to which some object is attached in a lift moving upwards is equal to mg+ma? How is t=mg+ma?

Answer 1

According to Newton’s second law of motion, the net force equals mass times acceleration. We are going to use a free body diagram (force diagram) to show that the equation of the motion is given by

T – mg = – ma

Thereby,

T = mg – ma

and the answer is: (d)

the tension less than mg.

Answer 2

When an object of mass “m” is attached to string it’s weight (mg) acts vertically downwards.
Tension in the string is acting upwards.
When lift was at rest ==> T = mg
______________________

When lift moves upwards with constant acceleration “a” then
Object also moves upwards along with lift with acceleration “a”
a = F_net / m
a = (T - mg) / m
T = mg + ma

F_net = T - mg because tension in the string is acting vertically upwards and weight of object is acting vertically downwards. As the object is accelerating upwards F_net = upward force - downward force.