# Finding weight of a mass on an accelerating and braking elavator

#### alexas

1. Homework Statement

Zach, whose mass is 78 kg, is in an elevator descending at 11 m/s. The elevator takes 3.2 s to brake to a stop at the first floor.

What is Zach's weight before the elevator starts braking?

What is Zach's weight while the elevator is braking?

2. Homework Equations

F = mg

F = ma ??

3. The Attempt at a Solution

What is Zach's weight before the elevator starts braking?
I was thinking....

F = mg

So F = (78kg)*(11m/s) ????

The second question i was completely clueless on....
but an idea of mine was...
F = ma

a = (11m/s)/(3.2s) = 3.4375 m/(s^2)

F = (78kg)(3.4475) ????

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#### LowlyPion

Homework Helper
1. Homework Statement

Zach, whose mass is 78 kg, is in an elevator descending at 11 m/s. The elevator takes 3.2 s to brake to a stop at the first floor.

What is Zach's weight before the elevator starts braking?

What is Zach's weight while the elevator is braking?

2. Homework Equations

F = mg

F = ma ??

What is Zach's weight before the elevator starts braking?
I was thinking....

F = mg

So F = (78kg)*(11m/s) ????

The second question i was completely clueless on....
but an idea of mine was...
F = ma

a = (11m/s)/(3.2s) = 3.4375 m/(s^2)

F = (78kg)(3.4475) ????
Your final idea gave you the right acceleration (deceleration) for the elevator, but not the force that some scales would measure.

What about his original weight standing still or moving at constant velocity? It's not m*v as you've shown in part a)

#### tiny-tim

Homework Helper
What is Zach's weight before the elevator starts braking?
I was thinking....

F = mg

So F = (78kg)*(11m/s) ????
uhh?
The second question i was completely clueless on....
but an idea of mine was...
F = ma

a = (11m/s)/(3.2s) = 3.4375 m/(s^2)

F = (78kg)(3.4475) ????
if the braking deceleration was 0, would the weight be 0?

#### djeitnstine

Gold Member
Try drawing a free body diagram to see the forces involved. That will make your life simpler.

#### alexas

I figure for the first question that if the speed is constant that accleration is zero so it would simply just be mass*gravity.

But for the second one since i need to factor in the deceleration i was thinking
(9.8 + 3.4375)*(78kg)
Is this correct?

#### LowlyPion

Homework Helper
I figure for the first question that if the speed is constant that accleration is zero so it would simply just be mass*gravity.

But for the second one since i need to factor in the deceleration i was thinking
(9.8 + 3.4375)*(78kg)
Is this correct?
That is correct.

#### alexas

I dont get it. I type in the answer and it says i have a rounding error. lolll

So 1032.52 or 1032.53 neither one works.

#### tiny-tim

Homework Helper

So 1032.52 or 1032.53 neither one works.
erm … that's six sig figs!

#### alexas

Yeah they wanted the answer as 1000, :P

Thanks for the help everyone.

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