Inevitable
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#1
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Albert and Bella are both standing in a lift. The mass of the lift is 250 kg. As the lift moves upward with constant acceleration, the floor of the lift exerts forces of magnitude 678 N and 452 N respectively on Albert and Bella. The tension in the cable which is pulling the lift upwards is 3955 N.

Why do the forces of 678N and 452N that the floor of the lift exerts on Albert and Bella act downwards and not upwards?
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ghostwalker
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(Original post by Inevitable)
Albert and Bella are both standing in a lift. The mass of the lift is 250 kg. As the lift moves upward with constant acceleration, the floor of the lift exerts forces of magnitude 678 N and 452 N respectively on Albert and Bella. The tension in the cable which is pulling the lift upwards is 3955 N.

Why do the forces of 678N and 452N that the floor of the lift exerts on Albert and Bella act downwards and not upwards?
They don't.

The forces that the lift exerts on Albert and Bella act upwards.

The forces that Albert and Bella exert on the lift act downwards.
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Inevitable
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(Original post by ghostwalker)
They don't.

The forces that the lift exerts on Albert and Bella act upwards.

The forces that Albert and Bella exert on the lift act downwards.
part a) work out the accleration

part b) work out the masses of albert and bella

Meaning that 672N and 452N act upwards along with the tension which is 3955N. The only force that acts downwards in this case is the the mass of the lift which is 250kg.

for part a) In the textbook, the two forces (672N and 452N) which you say act upwards, actually act downwards?

They worked out acceleration by 3955 - 672 - 452 - 250g = 250a
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ghostwalker
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(Original post by Inevitable)

for part a) In the textbook, the two forces (672N and 452N) which you say act upwards, actually act downwards?

They worked out acceleration by 3955 - 672 - 452 - 250g = 250a
They're not the same forces.

The forces in the diagram, are the forces acting on the lift floor, and they are downwards.
Recall, action and reaction are equal and opposite.

When dealing with the lift, you are interested in the forces acting on the lift.
There is the cable's tension going up, the weight of the lift going down, and the forces due to the two people in the lift going down.
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jm4425
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The upward forces are the reaction of the lift on the people. They act on the people not the lift. But by every action having an equal and opposite reaction we can infer that they are also the magnitudes of the people's weights (which act downwards)


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