# Homework Help: Where is the rotational axis?

1. Sep 13, 2015

### Zynoakib

1. The problem statement, all variables and given/known data
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

2. Relevant equations

3. 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!

2. Sep 13, 2015

### paisiello2

Is there a reaction @ B?

3. Sep 13, 2015

### Zynoakib

I dont think so. What does it mean?

4. Sep 13, 2015

### paisiello2

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

5. Sep 13, 2015

### Zynoakib

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

6. Sep 13, 2015

### paisiello2

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

7. Sep 17, 2015

### Zynoakib

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

8. Sep 17, 2015

### paisiello2

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