Scarf modelled as a pulley system / F=ma exercise

In summary, the "Scarf modelled as a pulley system / F=ma exercise" illustrates the principles of Newton's second law of motion (F=ma) through a practical scenario involving a scarf as a pulley. This exercise demonstrates how forces acting on the scarf can be analyzed to understand the relationship between mass, acceleration, and force, effectively integrating physics concepts with hands-on experimentation.
  • #1
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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  • #2
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##.
 
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  • #3
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.
 
  • #4
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 … 🤔
 

FAQ: Scarf modelled as a pulley system / F=ma exercise

What is the basic setup of a scarf modeled as a pulley system in an F=ma exercise?

The basic setup involves a scarf looped over a pulley with masses attached to both ends. The system is used to analyze forces and acceleration, where the pulley is assumed to be frictionless and the scarf is considered massless for simplicity.

How do you calculate the tension in the scarf when it is modeled as a pulley system?

The tension in the scarf can be calculated by analyzing the forces acting on each mass. Using Newton's second law (F=ma), you set up equations for each mass and solve for the tension. The tension is the same throughout the scarf if the pulley is frictionless and the scarf is massless.

What assumptions are made in the scarf as a pulley system model?

The primary assumptions are that the pulley is frictionless, the scarf is massless and inextensible, and the system is in a uniform gravitational field. These assumptions simplify the calculations by ensuring that the tension is constant throughout the scarf and that the only forces considered are gravitational and tension forces.

How do you determine the acceleration of the masses in the system?

The acceleration of the masses can be determined by applying Newton's second law to each mass and solving the resulting system of equations. The net force on each mass is equal to the mass times its acceleration, and the accelerations of the masses are related because they are connected by the scarf.

What happens if the masses on either side of the pulley are different?

If the masses are different, the system will accelerate in the direction of the heavier mass. The tension in the scarf and the acceleration of the masses can still be calculated using Newton's second law, but the equations will reflect the difference in mass, resulting in an acceleration that is proportional to the difference in the masses divided by the total mass of the system.

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