# Homework Help: Forces [F=ma]

1. Oct 24, 2007

### Heat

1. The problem statement, all variables and given/known data

The upward normal force exerted by the floor is 620N on an elevator passenger who weighs 650N.

2. Relevant equations

F=ma

3. The attempt at a solution

I am reviewing all the Homework assigned so, I do got the answers. :)

Anyways, the way I set this problem up was F=ma.

First we need to know the mass, which we get by w=mg.

650 = m9.8
m = 66.33

now we plug into F=ma.

Fnet = 66.33a

I managed to get the acceleration of .455 , but my question here is , why is the net force 620-650 and not 650-620.

Do we always subtract from the smaller force first? My first instinct was since gravity was larger it would be 650-620.

What if it was this instead: "The upward normal force exerted by the floor is 700N on an elevator passenger who weighs 100N."?

Would it be normal force-w or will it still be w-n. ?

I just need a clarification.

Last edited: Oct 24, 2007
2. Oct 24, 2007

### sapiental

what exactly does the problem ask you to find? The normal force is less because the elevaotr is accelerating in what direction to make the passenger weight less?

3. Oct 24, 2007

### Heat

It tells me to find acceleration.

I did of -.455

I know because of it being negative it goes down.

but

what if I subtracted 650-620 = 30
then it would be 30/66.33 = .455

a would be positive.

4. Oct 24, 2007

### sapiental

"My first instinct was since gravity was larger it would be 650-620"

the normal force acts against the downward direction.

so u can think of it as 620N + (-650N) .

"Do we always subtract from the smaller force first? My first instinct was since gravity was larger it would be 650-620. "

no, newtons law is addition of forces so u need to keep your +/- sings with respect to the directions.

Last edited: Oct 24, 2007
5. Oct 24, 2007

### Tlocc

Your first problem is that w=mg not ma.

6. Oct 24, 2007

### sapiental

Gravity is commonly measured in units of m s-2, (metres per second squared). hmmm