Finding acceleration of elevator using mass

[mjolnir80]

Homework Statement

one day you take your bathroom scale with you into an elevator. the scale reads 150 lbs as the door closes. the reading varies between 120 lbs and 170 lbs as the elevator travels up. what is the acceleration of the elevator while its accelerating and decelerating?

Homework Equations

dynamics equations

The Attempt at a Solution

from the first part we know that the scale is telling us the normal force.Fg=Fn Fn=150lbs
for the accelerating phase F= Fn-Fg, ma=170lbs - mg
this is where i get stuck. where should i go from here?

 

[Doc Al]

from the first part we know that the scale is telling us the normal force.Fg=Fn Fn=150lbs

The scale always gives the normal force. The initial reading tells you your weight (mg).

for the accelerating phase F= Fn-Fg, ma=170lbs - mg
this is where i get stuck. where should i go from here?

You're doing fine. Substitute the value of mg (the weight) and solve for "a". (Figure out your mass.)

 

[mjolnir80]

Fn = mg (for the first part) m= Fn/ g is that the mass? the units don't really match though

and what do i substitute in for mg ? 150 lbs?

 

[Doc Al]

Fn = mg (for the first part) m= Fn/ g is that the mass?

Correct.

the units don't really match though

It's up to you to use consistent units. What's the value of g in the English system? What's the unit of mass?

and what do i substitute in for mg ? 150 lbs?

Yes.

 

[mjolnir80]

thanks for the help