Solve Newtons' Laws Problems with Kinematics Equations for Homework

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Homework Help Overview

The discussion revolves around applying Newton's Laws and kinematics equations to a problem involving two blocks interacting through friction. Participants are attempting to determine the correct acceleration and distance traveled based on the forces acting on the blocks.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the setup of equations for the blocks, questioning the correctness of their time and acceleration values. There is exploration of the implications of using total mass versus individual masses in calculating acceleration.

Discussion Status

Some participants have provided guidance on the correct approach to calculating forces and accelerations, while others are still grappling with the implications of their assumptions. There is a mix of understanding and confusion regarding the interaction between the blocks and the forces at play.

Contextual Notes

Participants are working under the constraints of a homework assignment, which may limit the information available or the methods they can use. The discussion reflects uncertainty about the assumptions made regarding the system's mass and the forces acting on it.

whoareyou
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Homework Statement



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Homework Equations



Newtons' Laws
Kinematics Equations

The Attempt at a Solution



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I don't know what I'm doing wrong, but I'm not getting the correct answer of 17cm ... :cry:
 
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A will not stop. It comes to rest on the surface of B, that is they move together with the same velocity.

ehild
 
Does that imply that the time is wrong? Do I set up 2 equations, one for block A and block B and determine when their speeds will be the same?
 
Yes, the time is wrong. The force of friction decelerates A and accelerate B till their velocity becomes equal.

ehild
 
so...

3.5 - 5.88T = 4.704T
T = 0.331

d = (1/2)(4.704)(0.331)^2 = 0.257m --> still the wrong answer :(
 
You use wrong value for the acceleration of B. It should be the force of friction divided by 1 kg.

ehild
 
23.52N (the reaction force from the friction of block A) = ma

Now I assumed that it was the total mass of the system, since block A is on top of it and its moving with it. Was that assumption wrong?

23.52N = (m1 + m2)a
a = 23.52 / (m1+m2) = 23.52 / (4 +1 ) = 4.704m/s^2
 
Oh, okay I get the right answer now. But how come you don't use the total mass when finding the acceleration for B? It's directly on top of it ...
 
whoareyou said:
23.52N (the reaction force from the friction of block A) = ma

Now I assumed that it was the total mass of the system, since block A is on top of it and its moving with it. Was that assumption wrong?

23.52N = (m1 + m2)a
a = 23.52 / (m1+m2) = 23.52 / (4 +1 ) = 4.704m/s^2

It is wrong. Zero external force acts on the whole system. The blocks interact, the force of friction is an internal force. A acts B and B acts A through the friction. So block B moves according to the force exerted on it by block A: F= 23.52 N.

ehild
 
  • #10
OH ! okay, makes sense. Thanks :)
 

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