# Forces : Man in a lift on a scaler

1. Nov 9, 2009

### davedays

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

2. Nov 9, 2009

3. Nov 9, 2009

### Staff: Mentor

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.

4. Nov 9, 2009

### davedays

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 :)

5. Nov 9, 2009

### Staff: Mentor

What forces act on the man? (Hint: Two forces act on him.) What direction do they act?

Yes. Realize that F stands for the net force.

6. Nov 9, 2009

### davedays

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. :(

7. Nov 9, 2009

### Staff: Mentor

Exactly. And realize that the normal force is what the scale measures.

Just use up as + and down as -. Using that convention, ΣF = N - W.

You are given the mass but need the weight (to apply F=ma). That's where 'g' comes in.

8. Nov 9, 2009

### 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

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.

9. Nov 9, 2009

### Staff: Mentor

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.)

10. Nov 9, 2009

### davedays

it must be zero cause hes not going up, right ?

11. Nov 9, 2009

### hellomister

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.

12. Nov 9, 2009

### Staff: Mentor

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'.)