The necessary inclined force to rotate an object around an axis

[chane]

Homework Statement

I just want to ask if is it possible to find the force F, the angle "alpha" and the distance between the force F and the pivot just with the door weight. Or I have to make an assumption of the angle or the distance.
Btw the object lenght is 2150mm

Relevant Equations

Fy = F*sin(alpha)

 

[PeroK]

Can you explain what you are trying to do?

 

[chane]

I am trying to rotate the door around the pivot (red dot) using an inclined force

 

[erobz]

I am trying to rotate the door around the pivot (red dot) using an inclined force

Then you have to specify $\alpha$ if you are to find the force $F$

 

[PeroK]

... The required force depends on both the angle $\alpha$ and the point at which the force is applied. For physical reasons it's optimum to have $\alpha = \frac \pi 2$ and $F$ applied as far from the pivot as possible. Your equations should lead to the same conclusion.

 

[chane]

Then you have to specify $\alpha$ if you are to find the force $F$

So I have to suppose alpha and make my calculation around it

 

[chane]

... The required force depends on both the angle $\alpha$ and the point at which the force is applied. For physical reasons it's optimum to have $\alpha = \frac \pi 2$ and $F$ applied as far from the pivot as possible. Your equations should lead to the same conclusion.

Thanks now i get it

 

[chane]

Last question how do i write "alpha" as a the symbole (i'm new )

 

[PeroK]

Last question how do i write "alpha" as a the symbole (i'm new )

https://www.physicsforums.com/help/latexhelp/

If you "reply" to a post then you will see the Latex that was used and be able to copy it.

 

[chane]

https://www.physicsforums.com/help/latexhelp/

If you "reply" to a post then you will see the Latex that was used and be able to copy it.

Thanks

 

[kuruman]

For physical reasons it's optimum to have $\alpha = \frac \pi 2$ and $F$ applied as far from the pivot as possible.

That is why people who design doors cleverly put doorknobs and handles as far away from the hinges as possible. Quite often in public places one also sees instructions of use, e.g. "PUSH" or "PULL".

 

[PeroK]

That is why people who design doors cleverly put doorknobs and handles as far away from the hinges as possible.

They're not quite so clever when it comes to the height of the doorknobs, which are usually far too low!

 

[kuruman]

They're not quite so clever when it comes to the height of the doorknobs, which are usually far too low!

I am not so sure. It's possible that the height of doorknobs has been influenced by universal design principles. They have to be accessible to all, including persons with dwarfism, children and perhaps dogs trained to bring in one's newspaper.

 

[Lnewqban]

I am not so sure. It's possible that the height of doorknobs has been influenced by universal design principles. They have to be accessible to all, including persons with dwarfism, children and perhaps dogs trained to bring in one's newspaper.

 

[haruspex]

is it possible to find the force F, the angle "alpha" and the distance between the force F and the pivot just with the door weight.

Just checking... your diagram implies this is a trapdoor, not an upright door, yes? So the further from the hinge the force is applied, the smaller the angle will necessarily be. I.e. $d\sec(\alpha)$ is fixed as the string or arm length.
If you want to take friction into account, note that $F\cos(\alpha)$ contributes to the normal load at the pivot.