Scarf modelled as a pulley system / F=ma exercise

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SUMMARY

The discussion centers on the application of the scarf model as a pulley system in physics, specifically analyzing the proportion of mass hanging over the edge represented by the formula $$\frac{m_1}{m_1+m_2}$$. Participants express confusion regarding the expected behavior of this ratio when comparing masses m1 and m2. It is established that the ratio accurately reflects the proportion of the scarf hanging over the edge, with m1 being the mass that directly influences this proportion. The conclusion emphasizes that the ratio cannot exceed 1, clarifying misconceptions about its behavior under varying mass conditions.

PREREQUISITES
  • Understanding of basic physics concepts, particularly Newton's laws of motion.
  • Familiarity with pulley systems and their mechanics.
  • Knowledge of mass ratios and their implications in physical systems.
  • Ability to interpret mathematical expressions in a physics context.
NEXT STEPS
  • Study the principles of Newton's second law, F=ma, in relation to pulley systems.
  • Explore the concept of mass ratios in mechanical systems and their practical applications.
  • Learn about the implications of mass distribution in systems involving pulleys.
  • Investigate common misconceptions in physics related to mass and force interactions.
USEFUL FOR

This discussion is beneficial for physics students, educators, and anyone interested in understanding the dynamics of pulley systems and the mathematical principles governing them.

laser
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Homework Statement
Consider a scarf draped over a table. Model it as two particles of mass m1 and m2 joined by a model string passing over the edges of the table modelled as a model pulley. Assume the masses are proportional to the corresponding lengths of the scarf, i.e. the scarf’s mass is uniformly distributed. If the coefficient of static friction between the scarf and the table surface is μ, what proportion of the scarf can hang over the table before the scarf slips off the table?
Relevant Equations
F=ma
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My teacher gave the above answer as a solution. However, I am not convinced that the proportion is really $$\frac{m_1}{m_1+m2}$$. If m2 << m1the proportion would be really big, right? But intuition tells me that it should be the opposite. Furthermore, if m2 >> m1, then one would expect the proportion to be "big". But it's the opposite :/.

What am I missing here?
 
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The ratio ##m_1/(m_1+m_2)## is by definition the proportion of the scarf hanging over the edge. Since ##m_1## is the mass hanging over the edge, this should be expected to be large whenever ##m_1 \gg m_2##.
 
laser said:
If m2 << m1the proportion would be really big, right? But intuition tells me that it should be the opposite. Furthermore, if m2 >> m1, then one would expect the proportion to be "big". But it's the opposite :/.
Intuition is telling you the same thing for both m2>>m1 and m2<<m1, yet is wrong both times? That does not sound possible.
Besides, it can never be "really big". It clearly cannot exceed 1.
 
haruspex said:
Besides, it can never be "really big". It clearly cannot exceed 1.
I’d say 1 is a “really big” fraction of the scarf … it is all of it … 🤔
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

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