Problem involving Newton's second law

Click For Summary

Homework Help Overview

The discussion revolves around a problem involving Newton's second law, specifically addressing the forces acting on a block in a system described in a provided image link. Participants are exploring the implications of gravitational forces and tension in the context of the problem setup.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to apply a force balance equation but is confused about the omission of gravitational force in the solution provided. Other participants clarify the orientation of the setup, suggesting that the gravitational force is balanced by the normal force and does not need to be included in the force balance for the horizontal analysis.

Discussion Status

The discussion is active, with participants providing clarifications regarding the problem's setup and addressing the original poster's confusion about the forces involved. There is a productive exchange of ideas, and some participants express understanding after further explanation.

Contextual Notes

Participants note that the problem is stated to be oriented horizontally, which influences the consideration of forces acting on the block. This context is crucial for understanding the balance of forces in the system.

Clever_name
Messages
125
Reaction score
0

Homework Statement



see here for a situation of the problem http://i40.tinypic.com/6fbblk.jpg

Homework Equations



F=ma

The Attempt at a Solution



Ok well I've solved this problem but one thing about the solution process troubles me.
Namely when trying to find the force in the bar.

Say for instance i have the acceleration of the block B.
I don't understand why i can't do a force balance such that,

-Mb*g -Tsin(a) = Mb * Ab (1),

instead the solution is

-Tsin(a) = Mb*Ab (2)

I don't understand why the weight of the object is not accounted for in equation 2. Can someone please clear this up for me, thanks!
 
Physics news on Phys.org
The whole setup is stated to the horizontal! The gravitational force is balanced by the normal force.
 
thanks for the prompt reply, however i don't understand what you mean stated to the horizontal can you please elaborate for a slow poke like me.
to me it looks like B is hanging from the vertical.
 
Last edited:
What you are seeing is the TOP VIEW. Think that the whole system is on a table with you looking at it from above it. The gravitational force acts toward the legs of the table but is balanced by the reaction force offered by the table. So we don't need to worry about these forces. And everyone gets confused sometimes. (never think low of yourself).
 
Ah i see now - its stated in the problem that it is to the horizontal! thanks for that.
 

Similar threads

Newton's second law (My turtle named Newton being accelerated)
  • · Replies 9 ·
Replies
9
Views
2K
Newton's 2nd Law: Why is Resistance Considered a Positive Force?
  • · Replies 3 ·
Replies
3
Views
2K
Newton's Third Law for book on a table
  • · Replies 8 ·
Replies
8
Views
4K
Newton's law problem: Pushing 2 stacked blocks on a horizontal table
  • · Replies 13 ·
Replies
13
Views
3K
Mechanics question: Three masses and a pulley on a tabletop
  • · Replies 3 ·
Replies
3
Views
1K
Circular Motion - Newton's Second Law: Bead on a Rotating Hoop
  • · Replies 39 ·
2
Replies
39
Views
6K
Explanation for Newton II giving negative mass in my Physics lab results please
  • · Replies 44 ·
2
Replies
44
Views
4K
Proving Newton's 3rd law from conservation of momentum
  • · Replies 2 ·
Replies
2
Views
997
Verifying that Newton's Equations are equivalent to the EL equations
  • · Replies 15 ·
Replies
15
Views
2K
Two blocks in contact -- find the force between them
  • · Replies 14 ·
Replies
14
Views
3K