# Force and Motion

## Homework Statement

I'm quite confused with the material that has been presented to me in my physics class.
The scenario is:
There is a person that is riding an elevator that is traveling upwards.
Draw a free body diagram for the person.
(Is Fnorm larger than Fgrav?)

Fnorm, Fgrav.

## The Attempt at a Solution

Well I'm just generally confused. Can Fgrav change, since gravity technically stays the same, assuming we stay on Earth?
Some clarification would be great.

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cepheid
Staff Emeritus
Gold Member
Well I'm just generally confused. Can Fgrav change, since gravity technically stays the same, assuming we stay on Earth?
Some clarification would be great.
Assuming that the elevator remains close to the Earth's surface, Fgrav doesn't change. It is equal to mg, and you can take g to be a constant.

The key to understanding this problem is to use Newton's 2nd Law. Unfortunately,
There is a person that is riding an elevator that is traveling upwards.
is too vague a statement. Did the problem mean that the person is accelerating upwards? If so, what does Newton's 2nd Law say about the net force on that person?

Now consider the two forces acting on the person that together comprise that net force. For there to be a net force, these two forces must obviously be unbalanced. The question is just, in what manner?

It's actually accelerating at 2m/s, upwards. The person weighs 80kg. What would be greater; Fgrav or Fss. And if I have one of the forces, fgrav or fss, how could I obtain the missing force?

cepheid
Staff Emeritus
Gold Member
It's actually accelerating at 2m/s, upwards. The person weighs 80kg. What would be greater; Fgrav or Fss. And if I have one of the forces, fgrav or fss, how could I obtain the missing force?
What does Newton's 2nd Law say about the direction of the net force? It has to be upwards, since the body is accelerating upwards, right? (You can also get the magnitude, since you know the mass, and F = ma).

Now, if two forces are acting on this body, and their vector sum has to produce a net upward force, which of the two forces must be greater in magnitude? The one that points up or the one that points down?

That's how we reason our way to the answer. More systematically: draw a free body diagram for the object, and from this you have an inventory of all the forces acting on it. Then write down the equation you get from:

sum of all forces = net force = ma.