How Is the Contact Force Between the 7kg and 11kg Blocks Calculated?

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Homework Help Overview

The problem involves calculating the contact force between two blocks of mass 7kg and 11kg that are part of a system of four blocks being pushed on a frictionless surface. The total force applied to the system is 190N, and the arrangement of the blocks is specified.

Discussion Character

  • Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the calculation of forces acting on the blocks, particularly focusing on the signs of the forces based on the chosen direction of acceleration. There is an exploration of how to correctly apply the equations of motion to determine the contact forces.

Discussion Status

Some participants have pointed out potential sign errors in the calculations, leading to a clarification of the direction of forces based on the chosen coordinate system. The conversation is ongoing, with participants verifying each other's reasoning and calculations.

Contextual Notes

There is an emphasis on the convention of direction for forces and acceleration, which may affect the interpretation of the results. The problem is set in a frictionless environment, which is a critical assumption in the calculations.

Nicolaus
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Homework Statement


You are pushing four blocks of masses 4, 7, 11, and 17kg positioned next to each other on a frictionless surface.
Find the force of contact between the 7 and 11kg blocks when you push the blocks with a 190N force to the left.
They are arranged as such: |17||11||7||4|


Homework Equations


F = 190N = (17+11+7+4)a
a = 4.87

The Attempt at a Solution


m4a = F - F7on4
F7on4 = 190 - (4x4.87) = 170.52N
F4on7 = -170.52N
m7a = F4on7 - F11on7
F11on7 = -170.52 - (7x4.87) = -204.12N
 
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You're making a sign error. Note that acceleration is to the left.
 
So F7on4 should be -170.52 given that I have chosen the left direction as positive?
 
Nicolaus said:
So F7on4 should be -170.52 given that I have chosen the left direction as positive?
Yes. F7on4 points to the right, which is negative in your convention.
 

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