Tension vs weight tells you power direction. In a rope lift: if the rope’s pull is upward and the load moves up, the machine is doing positive power on the load. If the load moves down while the rope still pulls up (you’re lowering gently), the load does negative power on the rope—energy flows back to the machine or is dissipated.
Two everyday pictures
Elevator:
Going up → motor delivers power to the car.
Coming down gently → motor can take power back (modern lifts feed it to the building’s electrical system), or waste it in a brake if regen isn’t available.
Lowering a bucket by hand: your hands feel the rope pulling up while the bucket moves down → your muscles are absorbing energy (they heat up). That’s negative power done by the rope on the bucket and positive power done by the bucket on you.
Question ;-
You rise through the same height H in three different ways. Start and finish at rest in all cases; ignore air drag and don’t hold the hand-rail.
Ride an upward escalator, standing still on a step.
Climb a stationary staircase at a steady pace.
Walk up an upward escalator so that your speed relative to the ground is the same as in case 2 (you just reach the top sooner).
Let “contact forces” mean only the forces from the steps/floor on you (normal + any friction). Over the ascent, which statement about the net work done by these contact forces on you is correct?
A. Same in all three cases: zero, because normal forces do no work. B. Case 1 is positive and equals your gain in gravitational energy; Case 2 is zero; Case 3 is greater than Case 1. C. Case 1 is positive and equals your gain in gravitational energy; Case 2 is zero; Case 3 is equal to Case 1. D. Case 1 is zero; Case 2 is positive and equals your gain in gravitational energy; Case 3 is zero.
CRACKNEETPhysics
Did you know :
Tension vs weight tells you power direction.
In a rope lift: if the rope’s pull is upward and the load moves up, the machine is doing positive power on the load. If the load moves down while the rope still pulls up (you’re lowering gently), the load does negative power on the rope—energy flows back to the machine or is dissipated.
Two everyday pictures
Elevator:
Going up → motor delivers power to the car.
Coming down gently → motor can take power back (modern lifts feed it to the building’s electrical system), or waste it in a brake if regen isn’t available.
Lowering a bucket by hand: your hands feel the rope pulling up while the bucket moves down → your muscles are absorbing energy (they heat up). That’s negative power done by the rope on the bucket and positive power done by the bucket on you.
Question ;-
You rise through the same height H in three different ways. Start and finish at rest in all cases; ignore air drag and don’t hold the hand-rail.
Ride an upward escalator, standing still on a step.
Climb a stationary staircase at a steady pace.
Walk up an upward escalator so that your speed relative to the ground is the same as in case 2 (you just reach the top sooner).
Let “contact forces” mean only the forces from the steps/floor on you (normal + any friction). Over the ascent, which statement about the net work done by these contact forces on you is correct?
A. Same in all three cases: zero, because normal forces do no work.
B. Case 1 is positive and equals your gain in gravitational energy; Case 2 is zero; Case 3 is greater than Case 1.
C. Case 1 is positive and equals your gain in gravitational energy; Case 2 is zero; Case 3 is equal to Case 1.
D. Case 1 is zero; Case 2 is positive and equals your gain in gravitational energy; Case 3 is zero.
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