What did I do wrong? (Newton's Second Law)

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Homework Help Overview

The discussion revolves around a problem involving Newton's Second Law, specifically analyzing the forces acting on a 3.0 kg body subjected to two horizontal forces: one of 9.0 N due east and another of 8.0 N at an angle of 62 degrees north of west. Participants are attempting to determine the body's acceleration based on these forces.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to apply Newton's Second Law by calculating the net force and subsequently the acceleration, but questions arise regarding the treatment of the forces, particularly the inclusion of vertical components despite the problem stating only horizontal forces are acting.

Discussion Status

Participants are exploring different interpretations of the problem, particularly the implications of the angle of the second force and its contribution to vertical acceleration. Some guidance has been offered regarding the calculation of acceleration, but confusion remains about the terminology used in the problem statement.

Contextual Notes

There is a noted discrepancy in the problem's phrasing regarding horizontal forces and the actual calculations involving vertical components, leading to questions about the clarity and intent of the problem setup.

th77
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Only two horizontal forces act on a 3.0 kg body. One force is 9.0N, acting due east, and the othr is 8N, acting 62 degrees north of west. What is the magnitude of the body's acceleration?
Fnet= ma ...so I took that to mean F1x + F2x = ma
F1x = 9 cos 0 = 9
f2x = 8 cos 62 = 3.76
a = (9 + 3.76)/3 = 4.25
The solution says 2.9
 
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East and west are opposite directions. What does that tell you?
 
I think it tells me that it should be F1 - F2 instead. If yes then

a = (9 - 3.76)/3 = 1.75

It still doesn't come to 2.9
 
Oops, sorry to mislead you a bit.
The MAGNITUDE of the acceleration evidently includes the contribution from the VERTICAL acceleration the body also expriences.
That is, you are to find [itex]a=\sqrt{a_{x}^{2}+a_{y}^{2}}[/itex]
 
You're right because I got the answer using it, but I don't know why. The problem says that only 2 'horizontal' forces act on the object. Why is the vertical involved? I know F2 is making an angle, but then what do they mean by horizontal forces? Shouldn't it just say 'forces'? I have a sample problem in the book that specifies there is one-dimensional motion yet 1 of the forces is making an angle.
 
th77 said:
You're right because I got the answer using it, but I don't know why. The problem says that only 2 'horizontal' forces act on the object. Why is the vertical involved? I know F2 is making an angle, but then what do they mean by horizontal forces? Shouldn't it just say 'forces'? I have a sample problem in the book that specifies there is one-dimensional motion yet 1 of the forces is making an angle.
The phrasing in the book is just dumb, that's all there is to it.
 
thanks for your help
 

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