Find the force between 2 blocks

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Homework Help Overview

The problem involves two blocks, A and B, where block A moves towards block B and collides with it. Block A is frictionless, while block B has a coefficient of friction of 0.2. After the collision, both blocks move together at a speed of 2 m/s. The objective is to find the force between the two blocks, denoted as C.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants discuss the forces acting on both blocks and the implications of friction. There are attempts to set up equations based on Newton's laws and momentum conservation. Questions arise about the correct force systems for each block and the role of friction in the scenario.

Discussion Status

Participants are exploring different interpretations of the problem, particularly regarding the forces acting on block A after the collision. Some guidance has been offered regarding the absence of friction for block A and the need to combine equations for both blocks to find the solution.

Contextual Notes

There is some confusion about whether the entire problem statement has been presented, and participants are questioning the completeness of the information provided. The discussion reflects uncertainty about the forces involved and the nature of the collision.

Helly123
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1. The problem statement, all variables and given/known data
20180503_214535.jpg


Homework Equations

The Attempt at a Solution


B at rest. A moves 7m/s and bump into B. Object A is frictionless. Object B not, coeff of friction = 0.2.
g = 9.8 m/s2
After collision both objects moves together with speed 2 m/s to the right.

Let refer C for force between objects. (What we have to find)

Object B as system
##C - F_{friction} ## = ##mass_B## . a

Object A as system
##F_A## - C = ##mass_A## . a

I am still confused to make fhe force system for B or A. How is the correct force system?

How to find that C? When we don't know ##F_A##?
Please help me..
 

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This looks tricky.

Hint: work out what would happen if there were no friction. Assume an inelastic collision. What result do you get?
 
PeroK said:
This looks tricky.

Hint: work out what would happen if there were no friction. Assume an inelastic collision. What result do you get?

Sigma F = Sigma m * a
F_A = (m_A + m_B) * a
Is it?
 
Helly123 said:
Sigma F = Sigma m * a
F_A = (m_A + m_B) * a
Is it?

That's not an answer. Do you know how to solve the problem of an inelastic collision?

Also, what do you know about momentum?
 
PeroK said:
That's not an answer. Do you know how to solve the problem of an inelastic collision?

Also, what do you know about momentum?
Ok let me think.
Momentum?

M1V1 + M2V2 = M1V1' + M2V2'
M1V1 + M2V2 = (M1 + M2)V'
 
I have to say I'm confused as well. Are you sure this is the whole question?
 
PeroK said:
I have to say I'm confused as well. Are you sure this is the whole question?
http://[url=https://ibb.co/diBMon]https://preview.ibb.co/jd8dZS/20180504_004628.jpgthis is the whole question. Hope it shows clear
[ATTACH=full]225086[/ATTACH]
 

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Okay. That makes sense now. So, you've solved part 1).

What determines what happens in part 2)?
 
PeroK said:
Okay. That makes sense now. So, you've solved part 1).

What determines what happens in part 2)?
Well... they both moving together.. now.. there is friction working and acceleration slow them down
 
  • #10
Helly123 said:
Well... they both moving together.. now.. there is friction working and acceleration slow them down

What slows them down?
 
  • #11
PeroK said:
What slows them down?
Friction at mass 2 right?
 
  • #12
Helly123 said:
Friction at mass 2 right?
Right. So evaluate that frictional force.
 
  • #13
haruspex said:
Right. So evaluate that frictional force.
5*9.8*0.2 = 9.8 N
 
  • #14
B as system
C - F_friction = Mass_B * a
 
  • #15
Helly123 said:
5*9.8*0.2 = 9.8 N
Yes.
Helly123 said:
B as system
C - F_friction = Mass_B * a
Yes.
What about the other block?
 
  • #16
haruspex said:
Yes.

Yes.
What about the other block?
F_A - C = Mass_A * a
 
  • #17
Helly123 said:
F_A - C = Mass_A * a
What is F_A?
 
  • #18
haruspex said:
What is F_A?
The force makes A move?
 
  • #19
Helly123 said:
The force makes A move?
That sounds rather pre-Newtonian. Forces make things accelerate.
What forces are acting on block A after the collision?
 
  • #20
haruspex said:
That sounds rather pre-Newtonian. Forces make things accelerate.
What forces are acting on block A after the collision?
Force of contact with b because of friction too?
Pre-Newtonian?
 
  • #21
Helly123 said:
Force of contact with b because of friction too?
You have the force of contact with B represented by C in your equation in post #16. I am asking about this mysterious FA in that equation. What is it? Where does it come from? We are told there is no friction between A and ground.
 
  • #22
haruspex said:
You have the force of contact with B represented by C in your equation in post #16. I am asking about this mysterious FA in that equation. What is it? Where does it come from? We are told there is no friction between A and ground.
Yes.. the force that moves A
Something that accelerate A to move into b?
 
  • #23
Maybe there is no F_A since we focus on the collision?
 
  • #24
Helly123 said:
Maybe there is no F_A since we focus on the collision?
The collision has happened. We are dealing with the subsequent acceleration. There is no FA.
So combine your equations for the two blocks and solve.
 
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  • #25
haruspex said:
The collision has happened. We are dealing with the subsequent acceleration. There is no FA.
So combine your equations for the two blocks and solve.
Ok. I will try
 

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