A student stands on a bathroom scale in an elevator at rest on the 64th floor of ...

Ally385

1. The problem statement, all variables and given/known data
A student stands on a bathroom scale in an elevator at rest on the 64th floor of a building. The scale reads 828 N.
(a) As the elevator moves up, the scale reading increases to 919 N. Find the acceleration of the elevator.
(b) As the elevator approaches the 74th floor, the scale reading drops to 782 N. What is the acceleration of the elevator?
(c) Using your results from parts a and b, select which change in velocity, starting or stopping, takes the longer time. Explain.


2. Relevant equations

f=ma

3. The attempt at a solution

I've done all of these problems twice and I can't figure out the solution. I thought you would just find the average acceleration but I guess not.

Doc Al

Start by identifying the forces acting on the student, then apply Newton's 2nd law: ƩF = ma.

Ally385

I know that the scale is exerting a force but I'm not sure how much. Also, would the elevator be exerting a force? I also know that gravity is a force but I think that's it.

Doc Al

Right, the scale exerts an upward force. The force it exerts is the reading on the scale.
No, not directly. (It exerts its force by pushing on the scale. All we care about are forces on the person, and the scale is the only thing touching the person.)
Good. Only two forces act. Set up an equation from Newton's 2nd law, then you can solve for the acceleration.

Can you figure out the person's mass?

Ally385

Ok so that would get you 828 N/ 9.8 m/s/s = 84.4898. Is that in kg? So that's the mass of the person so then do 919 N / 84.4898 = 10.88 which is your acceleration?

And then for the last part it would be 728/84.4898 = -8.62 the negative because it's slowing down. Negative acceleration. Correct?

HallsofIvy

I think it would be easier to use the difference in force 919- 828= 91 N. Since The 828 N was the weight of the person standing still, the 91 N must be the result of the upward acceleration of the elevator. NOW use "f= ma". What is m, the mass of the person?

Ally385

Ok so acceleration would be 9.29 m/s/s and then for the second part it would be -4.70! I get it thank you :D

Doc Al

You're off by a factor of 10. I suggest redoing them more carefully (especially the second part).

Don't skip steps. Write out the equation:
ƩF = ma
scale force - weight = ma