Two blocks, a fixed pulley and friction

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SUMMARY

This discussion focuses on a physics problem involving two masses, m1 and m2, connected by a massless string over a fixed pulley. The key equations used include Σf=ma, fsmax=μs*fn, and vf2=vi2+2aΔx. The minimum mass m2,min required for the system to start moving is calculated as m2=(μs*m1g)/g. When m2 is doubled, the discussion emphasizes the transition from static to kinetic friction and the determination of final velocity using vf=√(2aH).

PREREQUISITES
  • Understanding of Newton's laws of motion
  • Knowledge of static and kinetic friction coefficients (μs and μk)
  • Familiarity with basic kinematic equations
  • Concept of massless, unstretchable strings in pulley systems
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  • Explore advanced applications of Newton's laws in multi-body systems
  • Study the effects of friction on motion in different scenarios
  • Learn about energy conservation in pulley systems
  • Investigate the dynamics of systems with varying mass ratios
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Students studying physics, particularly those focusing on mechanics, as well as educators looking for practical examples of pulley systems and friction in motion.

SakuRERE
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Homework Statement


a mass m1 is attached to a second mass m2 by an acme (massless, unstretchable) string. m1 sits on a table with which it has coefficients of static and dynamic friction μs and μk respectively. m2 is hanging over the ends of a table, suspended by the taut string from an acme pulley. at time t=0 both masses are released.
1. what is the minimum mass m2,min such that the two masses begin to move?
2. If m2= 2m2,min, determine how fast the two blocks are moving when mass m2 has fallen a height H (assuming the m1 hasn't yet hit the pulley)?

Homework Equations


Σf=ma
fsmax=μs*fn
vf2=vi2+2aΔx[/B]

The Attempt at a Solution


for a:
I am not sure about how I understand the question, but what comes to my mind when reading ( Begin to move) is that still the system acceleration is zero and that they are asking for the Fsmax (the maximum static friction) just before the system starts to move and the friction becomes Kinetic friction of Fk.
so:
for block m1
Σƒ= m*a=0
fsmax=T
μs*Fn= T
μs*m1g=T
now for block m2:
Σƒ=m*a=0
m2g=T

and by this m2g=μs*m1g so m2=(μs*m1g)/g

section b)
I guess since the mass of m2 is doubled, that here we will deal with the kinetic friction rather than the static friction.
since both are linked by a rope with the same pulley than they must have the same acceleration, and we should find it.
and after finding it then we will use this equation:
vf2=vi2+2aΔx
where vi= 0
so vf=√(2aΔx).
am I right??
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SakuRERE said:
so m2=(μs*m1g)/g
Yes, but you can simplify a little.
SakuRERE said:
section b)
I guess since the mass of m2 is doubled, that here we will deal with the kinetic friction rather than the static friction.
since both are linked by a rope with the same pulley than they must have the same acceleration, and we should find it.
and after finding it then we will use this equation:
vf2=vi2+2aΔx
where vi= 0
so vf=√(2aΔx).
am I right??
Yes.
 
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haruspex said:
but what is a
yes, of course, I will be substituting a in term of the given variables but I just didn't because I wasn't sure from my solution for section a at the first place. but now after your response, I will continue the solution. and regarding the H and Δx, I will be substituting Δx with H simply, but I wanted to write the equation in its original form so that everyone easily identifies it.
I will be happy if you grant me another help and answered this question:
for a moment I wondered, will the final velocity be the same for both blocks, I know they have the same acceleration and H will be the same as well but doesn't the friction affect the velocity of that block in the incline?
 
SakuRERE said:
I will be substituting a in term of the given variables
Yes, sorry, I realized that after rereading your original post, so I edited my reply.
SakuRERE said:
will the final velocity be the same for both blocks, I know they have the same acceleration
Same acceleration over the same period of time, so...
 
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haruspex said:
Same acceleration over the same period of time, so...
Thanks! I just wanted to make sure and to move away any doubts that are used to appear out of nowhere :)
 
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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