What is the direction of forces exerted on a plank by a wall in statics?

[nahanksh]

Homework Statement

http://online.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys211/oldexams/exam3/sp08/fig15.gif

The force exerted on the plank by the wall is upwards. (True/False)

Homework Equations

The Attempt at a Solution

In statics, when using \sumFy = 0, i thought the forces exerting on the wall should cancel the force of gravity (weight : mg).. That's the reason why i thought the statement to be TRUE. But it's actually FALSE...

How to decide the direction of forces?
(I am always confused with this..)

Please could someone help me out here?

 

[tiny-tim]

Hi nahanksh!

In statics, when using \sumFy = 0, i thought the forces exerting on the wall should cancel the force of gravity (weight : mg).

But you're forgetting the force from the pointy thingy … that's three forces that have to cancel, and you have no idea what that extra force is.

Hint: when you don't know what a force is, and you don't want to know, always take moments about a point which you choose so that that force has zero moment.

 

[nahanksh]

I don't quite get the hint you have given...

Are you saying that there is both horizontal and vertical forces in the point thing?
(Then, the answer is FALSE because it has upward AND downward forces?)

I am so confused...T_T

 

[tiny-tim]

Are you saying that there is both horizontal and vertical forces in the point thing?

Hi nahanksh!

(just got up :zzz: …)

I'm not interested in the forces at the point thing.

Just follow my hint! ​

 

[nahanksh]

Hi nahanksh!

(just got up :zzz: …)

I'm not interested in the forces at the point thing.

Just follow my hint! ​

I don't understand still...

We ARE interested in the point thing because it's only point related to the WALL..

How do we know the direction of the forces by the wall without considering that point..?

Please give me a little bit more of a hint...

 

[tiny-tim]

How do we know the direction of the forces by the wall without considering that point..?

By taking moments about that point, so that the force at that point is multiplied by zero.