Elevator Problem For General College Physics

[Windowmaker]

Homework Statement

A 220 lb man stands on a scale in an elevator. What does the scale read when the elevator accelerates upward at 1.6 m/s^2? What does it read when accelerating downward at the same rate

Homework Equations

F=ma, w=mg,

The Attempt at a Solution

m=w/g
220/9.81 = 22.43 ( Mass of the human?)

ma=F
22.43 * (1.6) = 35.88 ( Force of the upward force?)
mg= w
35.88 * 9.81 = 352 lbs ( New wieght?)

 

[Rayquesto]

I believe that when the elevator accelerates upward, the normal force is actually mass(1.6m/s^2 + 9.81m/s^2), which is the scale you read. Normal force determines what is read on the scale. mass is however not 220lbs/9.8, but 220lbs/some pounds acceleration constant. you get it? like weight can be measured in lbs or Newtons. If they gave you Newtons, then you'd divide by 9.8 and get something in kilograms, but you have lbs. you have to find out how to convert from lbs to Newtons, then find the mass. WHat do you think you'd do next?

 

[Rayquesto]

oh shoot! downward. I'm sorry I thought it said upward. So, the acceleration is actually a=(9.81m/s^2 - 1.6m/s^2) for normal force.

 

[Rayquesto]

oh wait there are two different questions. So, both ways will be right depending on the question.

 

[Rayquesto]

so, hint: 1 pounds is equal to 4.4482216 Newton.

 

[Windowmaker]

So the force of going up is gravity minus 1.6 m/s^2? And the force going down is gravity + 1.6m/s^2?

So then convert pounds to Newtons, then calculate the mass, and I am good to go?

 

[Windowmaker]

I mean acceleration Lol.

 

[Rayquesto]

its the opposite depending on your calculations.

 

[Rayquesto]

:) up=-9.81 - 1.6
down=-9.81 + 1.6

 

[Rayquesto]

but the normal force is positive.

 

[Windowmaker]

Ok thanks. That clarifies a lot. Didnt know about the conversation of lbs to N. And the Acceleration issue. :)

 

[Rayquesto]

If you drew a picture or something like that, or watched movies with a bunch of action and notice that if an elevator is going down, then someone in the elevator has the tendency to accelerate upwards, if the elevator accelerates downward fast enough. basically, the higher the elevator accelerates downward, the less the force is between the person and the elevator. and obviously the opposite is the case when accelerating upwards.