A book rests at an angle against one side of a bookshelf...

AI Thread Summary
The discussion revolves around solving a physics problem involving forces on a book resting at an angle on a bookshelf. The user presents equations related to vertical and horizontal forces but struggles to find the correct values for the forces involved. Respondents highlight the need for clarity regarding the origin of the numbers used, such as the force of 1.52, the angle of 31, and the mass of 0.305, which are unclear. They emphasize the importance of using consistent symbols and values to avoid confusion and ensure accurate problem-solving. The conversation underscores the necessity of understanding the problem's parameters to provide effective assistance.
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Homework Statement
A 0.135 kg
book rests at an angle against one side of a bookshelf. The magnitude and direction of the total force exerted on the book by the left side of the bookshelf are given by

|𝐹L|=0.575 N𝜃L=55.0°

What must the magnitude |𝐹B|
and direction 𝜃B
of the total force exerted on the book by the bottom of the bookshelf be in order for the book to remain in this position?
Relevant Equations
Fnet=ma
download.png


1.52cos(31)+Fbsin(Θb) = (0.305)(9.8)
Fbsin(Θb) = 1.69

Now for horizontal direction:
Fbcos(Θb) = FLsin(ΘL)
cos(Θb) = FLsin(ΘL)/Fb
cos(Θb) = (1.52 x sin31)/1.69

cosΘb=0.464
Θb = 62.35

I thought to find Fb I would just plug the Θb value into one of the trig functions, but apparently both of my answers are not right. What am I doing wrong?
 
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It would help us help you if you used symbols instead of numbers. Failing that, at least you could tell us where the numbers you are using come from. For example, your first equation
1.52cos(31)+Fbsin(Θb) = (0.305)(9.8)
is totally mysterious to me.
Where did the force of 1.52, the angle of 31 and the mass of 0.305 come from? Put yourself in our position. We cannot help you find what you are doing wrong if we don't know what you are doing.
 
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Likes jbriggs444 and erobz
You seem to be working with two versions of the problem: different masses, different angles, different forces. One way that can happen is that a student cribs a solution to what looks like the same problem but fails to spot the differences in the numbers.
 
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