When analysing the forces on people in lifts, it is important to remember:Newton's Third Law – to every action there is an equal and opposite reaction, this means that the reaction of anybody in the lift on the lift is the same as the reaction of the the reaction of the lift on anybody in the lift. In the diagrams below
is the tension in the lift cable,
is the mass of the person and
is the mass of the lift
We apply
For the Person:
(1)
For the lift:
(2)
Typically,andare known quantities, and
can be measured. We can find
and from the above simultaneous equations.
From (1)
(1) +(2) gives