# Forces : Man in a lift on a scaler

by davedays
Tags: forces, lift, scaler
 P: 15 1. The problem statement, all variables and given/known data A man of mass 70 kg stands in a lift on a set of weighing scales. The scales are calibrated in newtons. Assuming that g=10N kg -1 What weight will the scales register when the : a) lift is static b)lift accelerates down at 10 ms-2 c)lift accelerates up at 10 ms-2 2. Relevant equations F=ma 3. The attempt at a solution a) F=ma F= 70 x 0 F=70 b) accelerates down at 10ms -2 F=ma F=70 /10 F=7 70-7=63 c) accelerates up at 10 ms -2 70 + 7 = 77 Is this correct ? Thanks, dave
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P: 40,697
 Quote by davedays Is this correct ?
No, your answers are not correct. First identify all the forces acting on the man. (One of those forces will equal the scale reading.) Then apply ΣF = ma to each case.

P: 15

## Forces : Man in a lift on a scaler

I don't understand you, sorry. Can you explain again please ?

Is F=ma all I need here ?

I am not sure, however I really want to solve it :)
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P: 40,697
 Quote by davedays I don't understand you, sorry. Can you explain again please ?
What forces act on the man? (Hint: Two forces act on him.) What direction do they act?

 Is F=ma all I need here ?
Yes. Realize that F stands for the net force.
 P: 15 So I think it would be gravity acting down and also the normal force pushing him up, correct ? Knowing that in the first case I subtract and in the second case I add the forces right ? However I still do not get the connecting between 'g' and the question. :(
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P: 40,697
 Quote by davedays So I think it would be gravity acting down and also the normal force pushing him up, correct ?
Exactly. And realize that the normal force is what the scale measures.

 Knowing that in the first case I subtract and in the second case I add the forces right ?
Just use up as + and down as -. Using that convention, ΣF = N - W.

 However I still do not get the connecting between 'g' and the question.
You are given the mass but need the weight (to apply F=ma). That's where 'g' comes in.
 P: 15 Ill try do a then : if the GF is pushing him down and NF is pushing him up then his weight is 70N cause the gravity force is greater by 70 N Second one: If the lift is going down then the gravity force must he smaller by 10 : 70-10 = 60 N Third one : vice versa 70+10=80 N Am I thinking correct at this time ? By the way I really appreciate your help and patience here.
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P: 40,697
 Quote by davedays Ill try do a then : if the GF is pushing him down and NF is pushing him up then his weight is 70N cause the gravity force is greater by 70 N
First things first: If the man's mass is 70 kg, what is the GF on him? (The gravitational force is the same in all three cases, as gravity isn't changing.)
 P: 15 it must be zero cause hes not going up, right ?
 P: 29 The man is 70 kg, and there's a gravitational force acting down on him, with this in mind F=m(man)*a(???) even though the elevator isn't accelerating what other acceleration is there? Keep in mind gravity is in m/s^2.
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P: 40,697
 Quote by davedays it must be zero cause hes not going up, right ?
No. I asked about the gravitational force acting on the man. That's certainly not zero! How do you compute the force of gravity? (That's where you use 'g'.)

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