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a) When the lift is at rest.
This implies that the resultant force on the lift is zero i.e. action and reaction are equal in size. The force acting on the lift is the weight of the person standing in the lift. This is balanced by the reaction by the floor of the lift.
Therefore, weight mg=- reaction R,
Or simply; mg+R=0.
b) When the lift descends with an acceleration a
For the lift to move downwards, the weight of the occupant must be greater than the reaction by the floor of the lift. Therefore, the resultant force pulling the lift downwards is equal to the difference between the weight mg and the reaction R;
Resultant force F= mg-R.
From the second law of motion, the resultant force F=ma.
Therefore, ma=mg-R.
And R=mg-ma =m(g-a).
c) When the lift ascends with an acceleration a
In this case, the reaction by the floor of the lift must be greater than the weight of the occupant. Hence, the resultant force F=ma=R-mg.
And R=ma+mg=m(a+g).
sharon kalunda answered the question on April 24, 2019 at 06:21
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