- #1
Dko
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Just want to see if I did everything right. I'm notorious for simple mistakes.
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?
I used
F = ma
v2 = v1 + a12(t2 - t1)
r2 = r1 + v1(t2 - t1) + (1/2)(a12)((t2-t1)^2)
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?
Homework Statement
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?
Homework Equations
I used
F = ma
v2 = v1 + a12(t2 - t1)
r2 = r1 + v1(t2 - t1) + (1/2)(a12)((t2-t1)^2)
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?