# Homework Help: Normal force reading on a scale -- How and why?

1. Oct 10, 2015

### Elvis 123456789

1. The problem statement, all variables and given/known data
let's say you have a regular bathroom scale, which we know measures the normal force that it exerts on a person aka. apparent weight. Now let's say that we didn't know that it measured the normal force when a object was on it. Obviously we still know that the only two forces acting on the object are the normal force and the force of gravity. We also cannot perform any experiments with the scale, such as accelerating it to see if the scale reading changes. Do you have enough information to deduce that the scale reads the normal force and why? ( This wasn't really a homework question, just a misunderstanding that I've had with this topic, and i wanted to see if somebody can help clear it up for me)

2. Relevant equations

3. The attempt at a solution

2. Oct 10, 2015

### J Hann

What does Newton's 3'd Law say and what is the definition of the normal force?

3. Oct 10, 2015

### Elvis 123456789

Newton's 3rd Law states that in every interaction there are equal and opposite force pairs that act on different objects. The normal force is the force that a surface applies perpendicularly to the surface on an object. I don't see how either of those two things will give me the answer to the question.

4. Oct 11, 2015

### J Hann

Isn't the normal force considered to be the "normal reaction" of the scale?
What else can the scale reading be?

5. Mar 23, 2018

### Mustafa Mahfouz

i still dont get it , could u please explain more ? and some pictures would really help

6. Mar 23, 2018

### jbriggs444

We do not know this. Because it is obviously false. There is a third force.

7. Mar 23, 2018

### Staff: Mentor

I'm having trouble figuring out the third force you are referring to. Please tell me you are not talking about air buoyancy.

8. Mar 23, 2018

### jbriggs444

The two forces already identified are the normal force (acting on the top of the scale) and gravity (acting on the bulk of the scale). What holds it up?

The idea that "gravity" acts on the top of the scale would be incorrect. Your feet act on the top of the scale, not gravity.

9. Mar 23, 2018

### Staff: Mentor

I thought the OP was referring to the forces acting on the object.

10. Mar 23, 2018

### jbriggs444

Right, you are.

We are concerned with what the scale is measuring. So forces acting on the scale are naturally the relevant things to consider.