Finding the Horizontal Force on a Bracket Supported by a Single Screw

[Zynoakib]

Homework Statement

One side of a plant shelf
is supported by a bracket
mounted on a vertical wall
by a single screw. Ignore
the weight of the bracket.
Find the horizontal
component of the force
that the screw exerts on
the bracket when an 80.0 N
vertical force is applied as
shown

Homework Equations

The Attempt at a Solution

If I choose A as rotational axis, then it should be calculated as

net torque = 0
(80N)(0.05m) = (x)(0.03m)

but if a choose B as the rotational axis, then it should be calculated as

net torque = 0
(80N)(0.05m) = (x)(0.06m)

So which one is correct?

Thanks!

 

[paisiello2]

Is there a reaction @ B?

 

[Zynoakib]

I don't think so. What does it mean?

 

[paisiello2]

Well, why don't you prove it and sum forces in each direction?

 

[Zynoakib]

Well, why don't you prove it and sum forces in each direction?

That's what I can think of.
(80N)(0.05m) = (n)(0.03m)
n = 133N

but this is incorrect and I don't know why

 

[paisiello2]

You're taking moments. I want you to sum forces in each direction: ΣF_x = 0 and ΣF_y=0.

 

[Zynoakib]

You're taking moments. I want you to sum forces in each direction: ΣF_x = 0 and ΣF_y=0.

Just Fx = 80 from the nail? I don't think there is a horizontal force.

 

[paisiello2]

Just apply Newton's Laws and you will see that there must be a horizontal force.