What is the magnitude of the elevators acceleration

[catmd13]

Homework Statement

When you weigh yourself on good old terra firma (solid ground), your weight is 160lb . In an elevator your apparent weight is 140lb. What is the magnitude of the elevators acceleration.

Homework Equations

For this problem would we use the equation Fn= mg+ma

The Attempt at a Solution

 

[LowlyPion]

Homework Statement

When you weigh yourself on good old terra firma (solid ground), your weight is 160lb . In an elevator your apparent weight is 140lb. What is the magnitude of the elevators acceleration.

Homework Equations

For this problem would we use the equation Fn= mg+ma

The Attempt at a Solution

Welcome to PF.

That equation is ok.

Be careful what you use for g since your givens are in lbs.

 

[catmd13]

i used the equation listed above but i get keep getting a wrong answer.

i used 160lb for Fn and 123 for mg i not really sure what to use for ma. I was also wondering if I should change the units from pounds

 

[selter01]

ma = -mg + Fn

 

[selter01]

they gave you mg and Fn, Fn is your apparent weight. mg is normal weight

 

[catmd13]

so they give me Fn= 63.5kg and mg= 72.6kg i plugged it into find ma and i came up with .123 and it is still wrong. Can you help me out and see where i am making my mistake

 

[Redbelly98]

so they give me Fn= 63.5kg and mg= 72.6kg

Note that Fn is a force, while "63.5 kg" is a mass. So they cannot be equal.

Did you use selter01's equation,

ma = -mg + Fn​

?

m=72.6 kg, and you're trying to find "a".

 

[catmd13]

I used that equation how would i express 63.5kg as a force so i could use it for Fn I am stuck and do not know where to go from here

 

[Redbelly98]

I would convert the 140 pounds-force (the apparent weight, as given in the problem statement) into Newtons. Then you'll have Fn in Newtons, and Newtons are force units, so that's good.

EDIT:
You can use Google to do unit conversions, if they are not in your textbook:
http://www.google.com/search?hl=en&q=1+lbf+in+Newtons&btnG=Google+Search&aq=f&oq=

 

[bahar123123]

as we know that the equation relating the Apparent and the Real weight is given as
Fnet=W(real)-W(apparent)...1

but
W(real)=Mg....... ...2
and F(net)=Ma........ ...3