Find the force between 2 blocks

[Helly123]

1. The problem statement, all variables and given/known data

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..

 

[PeroK]

This looks tricky.

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

 

[Helly123]

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?

 

[PeroK]

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?

 

[Helly123]

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'

 

[PeroK]

I have to say I'm confused as well. Are you sure this is the whole question?

 

[Helly123]

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]

 

[PeroK]

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

What determines what happens in part 2)?

 

[Helly123]

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

 

[PeroK]

Well... they both moving together.. now.. there is friction working and acceleration slow them down

What slows them down?

 

[Helly123]

What slows them down?

Friction at mass 2 right?

 

[haruspex]

Friction at mass 2 right?

Right. So evaluate that frictional force.

 

[Helly123]

Right. So evaluate that frictional force.

5*9.8*0.2 = 9.8 N

 

[Helly123]

B as system
C - F_friction = Mass_B * a

 

[haruspex]

5*9.8*0.2 = 9.8 N

Yes.

B as system
C - F_friction = Mass_B * a

Yes.
What about the other block?

 

[Helly123]

Yes.

Yes.
What about the other block?

F_A - C = Mass_A * a

 

[haruspex]

F_A - C = Mass_A * a

What is F_A?

 

[Helly123]

What is F_A?

The force makes A move?

 

[haruspex]

The force makes A move?

That sounds rather pre-Newtonian. Forces make things accelerate.
What forces are acting on block A after the collision?

 

[Helly123]

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?

 

[haruspex]

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 F_A in that equation. What is it? Where does it come from? We are told there is no friction between A and ground.

 

[Helly123]

You have the force of contact with B represented by C in your equation in post #16. I am asking about this mysterious F_A 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?

 

[Helly123]

Maybe there is no F_A since we focus on the collision?

 

[haruspex]

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 F_A.
So combine your equations for the two blocks and solve.

 

[Helly123]

The collision has happened. We are dealing with the subsequent acceleration. There is no F_A.
So combine your equations for the two blocks and solve.

Ok. I will try