Just want to see if I did everything right. I'm notorious for simple mistakes.(adsbygoogle = window.adsbygoogle || []).push({});

1. The problem statement, all variables and given/known data

A 100kg man concerned about his weight decides to weigh himself in an elevator. He stand on a bathroom scale in an elevator that is moving upward at 3.0 m/s. As the elevator reaches his floor, it slows to a stop.

If the elevator slows to a stop over a distance of 2.0m, what is the reading on the bathroom scale?

2. Relevant equations

I used

F = ma

v2 = v1 + a12(t2 - t1)

r2 = r1 + v1(t2 - t1) + (1/2)(a12)((t2-t1)^2)

3. The attempt at a solution

Ok I know that I needed to find the force of the scale. to find that I needed the net force, since I have the force of the gravity on the guy (100*9.8 = 980N)

But to find that I needed the acceleration.

thus with v2 = v1 + a12(t2-t1) I got this

0 = 3 + a12(t2 - 0)

-3 = a12(t2)

a12 = -3/t2

I plugged that into r2 = r1 + v1(t2-t1) + (1/2)(a12)((t2-t1)^2)

2 = 0 + 3(t2 - 0) + (1/2)(-3/t2)((t2-0)^2)

2 = 3t2 - (3/2)(1/t2)(t2^2)

*Canceling I get

2 = 3t2 - 3t2/2

4 = 6t2 - 3t2

3t2 = 4

t2 = 4/3

Plugging back into a12 I get

a12 = -3/(4/3)

a12 = -9/4

Thus the net Force = 100(-9/4) = -225N

Which means Fsacle = -225 - 980 = -1205N

Making the scale read 1205N since it can only do positive right?

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# Need a check on a elevator going up problem

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