Lift Problems Watch

dont know it
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#1
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Say Box A lies on top of Box B inside a lift. If you had to find out the force that box B exerts on the lift, would it be the weight of Box A + the weight of Box B?
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phys981
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Depends whether the lift is accelerating or not and it's difficult to give full advice without knowing exactly what info you've got.


The force B exerts on the lift is the same as the force the lift exerts on B (Newton's 3rd Law)

The resultant force on B is F=ma and this is made up of (1) the weight of B (2) The force of A on B and (3) the force of the lift on B. So, assuming you know the masses of A and B you can find (1) and (2) and if you know the acceleration of the lift, you should therefore be able to work out (3)

Your answer is correct IF the lift has zero acceleration
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Notnek
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(Original post by dont know it)
Say Box A lies on top of Box B inside a lift. If you had to find out the force that box B exerts on the lift, would it be the weight of Box A + the weight of Box B?
A simpler question:

If a box is in a lift that is accelerating then do you know the force exerted by the box on the lift? Is it equal to the box's weight?

I'm asking because this is the worst understood topic in M1 so I want to check you understand this part.
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dont know it
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(Original post by Notnek)
A simpler question:

If a box is in a lift that is accelerating then do you know the force exerted by the box on the lift? Is it equal to the box's weight?

I'm asking because this is the worst understood topic in M1 so I want to check you understand this part.
Hoping it's right but yes, I do think the force exerted by the box on the lift is equal to the box's weight.
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Notnek
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(Original post by dont know it)
Hoping it's right but yes, I do think the force exerted by the box on the lift is equal to the box's weight.
No this isn't right but I understand why you think this. I'm going to paste something I posted somewhere else and I recommend understanding it before tackling lift questions. If you have any questions let me know and I can answer them tomorrow. Once you understand it I recommend starting again with simpler lift questions.

If a box is at rest on a horizontal ground then the force that the box exerts on the ground will be equal in magnitude to the box's weight. You already know this. But the weight of the box and the force it exerts on the ground are two different things but it just so happens that they are equal in magnitude when the box is stationary on a ground.

For lift questions you can't assume that the force of the box on the lift is equal in magnitude to the weight of the box. This is a hard concept to understand at first but if you imagine yourself in a lift and the cable breaks then your feet wouldn't be applying much force on the lift so the force you exert on the lift isn’t equal to your weight.

All you know is that the force exerted by the box on the lift is equal to the force exerted by the lift on the box, by Newton's Third Law. Again, the weight of the box is a different force.

The only way to find this unknown force is to do calculations using F = ma using information in the question.

You need to understand all this before tackling lift questions. The key concept here is that the force exerted by an object on a ground is not necessarily equal to the weight of the object. And for lift questions it generally won't be.
Last edited by Notnek; 7 months ago
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I hate maths
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(Original post by Notnek)
No this isn't right but I understand why you think this. I'm going to paste something I posted somewhere else and I recommend understanding it before tackling lift questions. If you have any questions let me know and I can answer them tomorrow. Once you understand it I recommend starting again with simpler lift questions.
"All you know is that the force exerted by the box on the lift is equal to the force exerted by the lift on the box, by Newton's Second Law. Again, the weight of the box is a different force."

Isn't it the third law?
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dont know it
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(Original post by Notnek)
No this isn't right but I understand why you think this. I'm going to paste something I posted somewhere else and I recommend understanding it before tackling lift questions. If you have any questions let me know and I can answer them tomorrow. Once you understand it I recommend starting again with simpler lift questions.
I get what you're saying I think, so you have to incorporate the normal reaction force? I just don't see why that's the case if it is.
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Notnek
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(Original post by I hate maths)
"All you know is that the force exerted by the box on the lift is equal to the force exerted by the lift on the box, by Newton's Second Law. Again, the weight of the box is a different force."

Isn't it the third law?
Oops I'll edit it.
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Notnek
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(Original post by dont know it)
I get what you're saying I think, so you have to incorporate the normal reaction force? I just don't see why that's the case if it is.
Here's a simple lift diagram:



Let's say the lift has mass 200kg and it is accelerating downwards at 3 m/s^2.

Often for lift questions its best to consider the box separately and also consider the combined system of the box and the lift.

So what are the forces just acting on the box? Remember that its accelerating downwards so the force pulling it down must be bigger than the force pulling it up.

What are the forces acting on the combined system of the box and the lift?
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dont know it
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#10
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(Original post by Notnek)
Here's a simple lift diagram:



Let's say the lift has mass 200kg and it is accelerating downwards at 3 m/s^2.

Often for lift questions its best to consider the box separately and also consider the combined system of the box and the lift.

So what are the forces just acting on the box? Remember that its accelerating downwards so the force pulling it down must be bigger than the force pulling it up.

What are the forces acting on the combined system of the box and the lift?
The force on the box would be the normal reaction force exerted by the lift on the box(N3L).

On the combined system, it would just be the tension and the weight of the particle and lift combined. If the mass of the lift was 200kg, the equation would be: T-(55g+200g)=255a.
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dont know it
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#11
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(Original post by Notnek)
No this isn't right but I understand why you think this. I'm going to paste something I posted somewhere else and I recommend understanding it before tackling lift questions. If you have any questions let me know and I can answer them tomorrow. Once you understand it I recommend starting again with simpler lift questions.
I've understood this concept now. So I see that the weight of the box will act as well as the mass*acceleration of something which acts in the opposite direction to the weight. I know this may sound stupid but what does this mass*acceleration tell us? It gives us the force opposing the weight of the box but where does it come from?
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