AQA A Level Maths Mechanics - Forces - Lift problem

Hello

I just want to clarify my understanding of lift problems as I have seen a few contradicting diagrams on a few websites and it has been confusing me.

Is the following correct?

When solving lift problems, they can ask a variety of questions so you need to split the problem into 3 diagrams.

- The lift system as a whole.
- The forces on the lift only.
- The forces on the object within the lift only.

The lift as a whole has:
- Tension of the cable PULLING up on the lift. (T)
- The weight of the lift and the weight of the box within the lift (Mg + mg)
- The normal reaction force of the BOX on the lift and the normal reaction force of the LIFT on the box (cancel out).

The forces on the lift only are:
- The tension of the cable.
- Weight of the lift and box.
- The normal reaction force of the BOX on the LIFT.

The forces on the object alone are:
- Weight of the box.
- Normal reaction force of the LIFT on the BOX.

A diagram has been included as well. Is this correct because TL maths did not include the normal reaction force of the BOX on the LIFT but did include the normal reaction force of the LIFT on the BOX/person.

Thanks,

Max
Screenshot 2025-04-14 144848.jpg

(edited 1 month ago)

Reply 1

mqb2766

21

for the lift only, if you had both mg and the reaction of the box on the lift, youd be double counting the box weight. The most usual systems to consider are 1) the whole system (lift + box) and 2) box only. As you say, the reactions are irrelevat for 1) and the reaction for 2) is determined by its acceleation (and ...).

Reply 2

SenpaiMicro

OP

6

Original post by mqb2766

for the lift only, if you had both mg and the reaction of the box on the lift, youd be double counting the box weight. The most usual systems to consider are 1) the whole system (lift + box) and 2) box only. As you say, the reactions are irrelevat for 1) and the reaction for 2) is determined by its acceleation (and ...).

I see what you mean, since the normal reaction force of the box on the lift accounts for the weight of the box itself. The reason im a bit confused is because of this picture from save my exams. We are given the normal reaction force of the lift on the box which is the same as that of the box on the lift. Is it interchangeable to use the normal reaction force of the box on the lift instead of the weight of the box or am I interpreting this wrong?

Reply 3

SenpaiMicro

OP

6

Original post by mqb2766

for the lift only, if you had both mg and the reaction of the box on the lift, youd be double counting the box weight. The most usual systems to consider are 1) the whole system (lift + box) and 2) box only. As you say, the reactions are irrelevat for 1) and the reaction for 2) is determined by its acceleation (and ...).

Wait I just got it. I realised now. If we were focusing on the lift itself we would not include the weight of the box as that is a force acting on the box. We would include the normal reaction force of the box on the lift however.

Reply 4

mqb2766

21

Original post by SenpaiMicro

Wait I just got it. I realised now. If we were focusing on the lift itself we would not include the weight of the box as that is a force acting on the box. We would include the normal reaction force of the box on the lift however.

Thats correct.

Reply 5

SenpaiMicro

OP

6

Original post by mqb2766

Thats correct.

Thank you very much for your help, I appreciate it.

Reply 6

mqb2766

21

Original post by SenpaiMicro

Thank you very much for your help, I appreciate it.

One way to be sure is to write the reaction for the box/lift using the box (only) so
ma = mg - R
Here down is positive. Then sub that into the lift (only) force equation (eliminate R)
Ma = Mg + R - T
and you should end up with the force equation for the lift+box as youre eliminating the reaction.

(edited 1 month ago)

AQA A Level Maths Mechanics - Forces - Lift problem

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