Solving Suspended Weight Forces: Common Mistakes and How to Correct Them

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SUMMARY

The discussion focuses on solving suspended weight forces, specifically addressing common mistakes in calculating resultant forces. Participants clarify that the forces acting on a system, such as a chain and a box, should be treated as downward forces measured in Newtons. The correct approach involves summing the weights directly without multiplying by gravity again, as the weights are already expressed in Newtons. The consensus is that the resultant force for a system at rest must equal zero, confirming the necessity of proper force addition.

PREREQUISITES
  • Understanding of Newton's laws of motion
  • Familiarity with force calculations in physics
  • Knowledge of weight as a force measured in Newtons
  • Basic principles of static equilibrium
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Students of physics, educators teaching mechanics, and anyone involved in engineering or physical sciences who seeks to understand the principles of force calculations in suspended systems.

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We usually only dealt with blocks suspended from a weightless string, so this is new to me. What I did to find F for the first one, which I thing I did wrong is:
F[chain]=mg=2.5(9.8)=24.5
F[box]=1.0(9.8)=9.8
Then I added the two forces together. Is this correct, if not, where have I gone wrong?
 
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I believe you have it correct. The measurement point is insensitive to the location of the mass. Otherwise, you would need to consider such nonsense as the top half of each 1.0 N box versus its bottom half. Consider that your string is connected to the top of the heavy chain, light chain or spring. In each case, the 'string' remains weightless.
 
No offense at all, I just want to make sure for sure that its right, but can someone with more forum experience comment on this?
 
Your method is correct, all systems are in rest so the resultant force has to be zero. The only forces are gravity and the opposite force F. But you are given the weights (in Newtons) which are forces already: mass times g. Do not multiply them with g.

ehild
 
Wait so is my answer right?
 
No. The numerical results are wrong.

ehild
 
So weight=mg, so if they say the weight is 5N, they already made that calculation with gravity? So then I'd just add 2.5+1, or subtract? Which one and why?
 
Yes they have made the calculation with gravity. That is why the unity is N. If you multiply Newtons with m/s^2 you do not get force.

So the force of gravity are given for the chain and for the block, both point downward. You need the resultant force for the whole system: what is the resultant of two downward forces? Why do you want to subtract them?

ehild

ehild
 
Yes they have made the calculation with gravity. That is why the unity is N. If you multiply Newtons with m/s^2 you do not get force.

So the force of gravity are given for the chain and for the block, both point downward. You need the resultant force for the whole system: what is the resultant of two downward forces? Why do you want to subtract them?

ehild
 

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