Unknown mass on a pulley system - Help

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Homework Help Overview

The problem involves a pulley system with a known mass of 100 kg and an unknown mass on the other side. The scenario describes the motion of the 100 kg block falling 1.0 m in 6.0 seconds after being released from rest, prompting the need to find the mass on the left side of the pulley.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to apply Newton's laws and kinematic equations but expresses confusion about the initial steps and calculations. Some participants suggest calculating acceleration first and then applying Newton's second law to both masses. Others question the setup of the forces acting on the masses, particularly the signs in the equations.

Discussion Status

The discussion is ongoing, with participants providing guidance on recalculating acceleration and re-evaluating the force equations. There is an acknowledgment of errors in the original poster's reasoning, particularly regarding the direction of forces, but no consensus has been reached on the correct approach yet.

Contextual Notes

The original poster indicates a lack of understanding due to missing class and relies on textbook knowledge, which may not be fully grasped. The discussion highlights the need for clarity on the application of Newton's laws in the context of the pulley system.

dazedconfused
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Homework Statement


The 100 kg block shown in the diagram takes 6.0s to reach the floor 1.0m below after being released from rest. What is the mass of the block on the left?

Homework Equations


Newton's Third Law
F=ma
F (a on b) = -F (b on a)

The Attempt at a Solution



I don't even know where to start, I was sick last class and can't figure out how to do this from the textbook. I've tried so far:
T1 = tension rope 1
T2 = tension rope 2
t0= 0s
t1= 6s
d=1.0m
g=9.8m/s^2
m1=unknown, find
m2=100kg
mass M: Fy = 0
T1-mg=0
T1=mg

T2= mg = (100kg)(9.8m/s^2)
= 980N

And now I'm lost.
 

Attachments

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Start over.

(1) Calculate the acceleration from the given information.

(2) Apply Newton's 2nd law to each mass. This will allow you to solve for the tension in the rope and for the unknown mass.
 
By using Newton's 2nd law,

d=0.5at2 (since v0 is zero),

I got an a=0.0556 m/s^2

Now, since they're on a pulley,

a= (m2g-m1g) / (m2+m1)

Plugging in, I get:

0.0556 = [(100kg*9.8m/s^2)-(m1*9.8m/s^2)] / 100kg + m1

rearranging, 5.56 + 0.0556 m1 = 980 - 9.8 m1
m1 = 100 kg

I know that's not the right answer, where am I going wrong?
 
The only error I see is here:
dazedconfused said:
a= (m2g-m1g) / (m2+m1)
You have the signs of the two weights reversed. (Take positive to be in the direction of the acceleration.)
 

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