Two blocks, a fixed pulley and friction

Click For Summary

Homework Help Overview

The problem involves two masses connected by a massless string over a pulley, with one mass on a table experiencing friction and the other hanging freely. The discussion focuses on determining the minimum mass required for motion and the subsequent speed of the masses after a certain distance has been fallen.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the conditions for the system to begin moving, particularly focusing on static versus kinetic friction. There are attempts to relate the forces acting on both masses and derive expressions for the minimum mass and acceleration. Questions arise regarding the relationship between the final velocities of the two blocks and the effects of friction.

Discussion Status

There is ongoing exploration of the problem with participants clarifying their understanding and confirming assumptions. Some guidance has been offered regarding the simplification of expressions and the relationship between acceleration and velocity, but no consensus has been reached on the final outcomes.

Contextual Notes

Participants are working under the constraints of homework rules, which may limit the information they can use or the methods they can apply. There is also a focus on ensuring clarity in the equations used and the variables involved.

SakuRERE
Messages
68
Reaction score
5

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??
upload_2018-10-4_23-7-19.png
 

Attachments

  • upload_2018-10-4_23-7-19.png
    upload_2018-10-4_23-7-19.png
    6.6 KB · Views: 849
Physics news on Phys.org
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.
 
  • Like
Likes   Reactions: SakuRERE
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...
 
  • Like
Likes   Reactions: SakuRERE
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 :)
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
2K
Replies
14
Views
3K
  • · Replies 13 ·
Replies
13
Views
2K
Replies
61
Views
4K
  • · Replies 4 ·
Replies
4
Views
2K
Replies
2
Views
2K
  • · Replies 23 ·
Replies
23
Views
3K
  • · Replies 13 ·
Replies
13
Views
8K
  • · Replies 10 ·
Replies
10
Views
4K
  • · Replies 15 ·
Replies
15
Views
7K