# mechanics

Two 20kg weights are hung from light strings that pass over frictionless pulleys. The ends of the strings are connected to a small, light spring balance, S,
calibrated in kg. The pulley axes are mounted on the wall of a lift in a 50-storey
building.
What is the reading on the spring balance when the lift is at rest?
Make a qualitative sketch of the variation with time of the reading on S as the lift
makes a round trip, delivering passengers from the ground floor to the top floor and
then returning to the ground floor.
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