# Elevator Problem Normal Force

1. May 4, 2017

### TheFlemster

1. The problem statement, all variables and given/known data
A 74.0 kg person is standing inside an elevator. The elevator is moving from the 3rd floor to the 21st floor. As the elevator passes the 4th floor it is moving at 2.30 m/s and is increasing speed at a rate of 1.43 m/s2 . At this moment, what is the normal force that acts on the person?

2. Relevant equations
normal Force, N = m(a+g)
F = ma

3. The attempt at a solution
The attempt at solution is on the attached image. Am I working it correct or should I have used F=ma?

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2. May 4, 2017

### jbriggs444

Remember that the F in F=ma denotes the vector sum of all external forces. There are two external forces on the person.

3. May 4, 2017

### TSny

I believe your answer is correct. The formula N = m(a+g) looks like something pulled out of thin air. If you want to show that you understand the physics, start with Fnet = ma.

4. May 4, 2017

### TheFlemster

So then I ignore the acceleration of 1.43 m/s^2? I am still confused as to which way I am supposed to work it

5. May 4, 2017

### TSny

No, don't ignore it. The acceleration of 1.43 m/s2 is the "a" in F = ma. Start with F = ma and fill in the left-hand side as suggested by @jbriggs444.

6. May 4, 2017

### TheFlemster

F=mg + ma
F=(74)(9.8) + (74)(1.43)
F=831.02 N
Wouldn't this be the total force acting on the person?

7. May 4, 2017

### TSny

No, ma does not represent a force acting on the person. mg + ma does not represent the total force acting on the person.

What are the two actual forces acting on the person?

8. May 4, 2017

### TheFlemster

The normal force and the force due to gravity?

9. May 4, 2017

### TSny

Yes, good. So, how would you combine these two forces to represent (symbolically) the net force acting on the person?

Your answer can then be used for the left side of Fnet = ma.

10. May 4, 2017

### TheFlemster

N=mg

#### Attached Files:

• ###### IMG_3426.JPG
File size:
21.3 KB
Views:
15
11. May 4, 2017

### TSny

Your diagram of the forces looks good. But there is no reason why the two forces should equal one another. The net force is the combination of the two forces. How would you combine the normal force N (which is upward) with the gravitational force mg (which is downward) to get an expression for the total force Fnet?

12. May 4, 2017

### TheFlemster

Fnet=N-mg
Fnet=ma
combining the two, ma=N-mg
then, N=ma + mg
N=(74)(1.43) + (74)(9.8)
N=831.02

13. May 4, 2017

### TSny

Yes, good. Include units.

14. May 4, 2017

### TheFlemster

so the correct answer would be a 831.02 N normal force acting on the person?

15. May 4, 2017

### TSny

Yes. Do you have the correct number of significant figures in you answer?