# Edexcel Mechanics Hinges

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Hi I’m very confused on a question, if we have a rod that it attached to a vertical wall and this rod is held in horizontal position by a rope, then why does the resultant force where the rod is attached to the wall act diagonally?

For example in question 1

https://www.examsolutions.net/tutori...vel&topic=1645

Solution;

https://youtu.be/3JXw_I9DM6c

Any help would be appreciated, thanks!

For example in question 1

https://www.examsolutions.net/tutori...vel&topic=1645

Solution;

https://youtu.be/3JXw_I9DM6c

Any help would be appreciated, thanks!

Last edited by REUSueeufn#2848; 1 year ago

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#2

(Original post by

Hi I’m very confused on a question, if we have a rod that it attached to a vertical wall and this rod is held in horizontal position by a rope, then why does the resultant force where the rod is attached to the wall act diagonally?

For example in question 1

https://www.examsolutions.net/tutori...vel&topic=1645

Solution;

https://youtu.be/3JXw_I9DM6c

Any help with be appreciate, thanks!

**penelopehills**)Hi I’m very confused on a question, if we have a rod that it attached to a vertical wall and this rod is held in horizontal position by a rope, then why does the resultant force where the rod is attached to the wall act diagonally?

For example in question 1

https://www.examsolutions.net/tutori...vel&topic=1645

Solution;

https://youtu.be/3JXw_I9DM6c

Any help with be appreciate, thanks!

Horizontal: The rope's tension pulls the plank into the wall at A (this must be the case as its the perpendicular force which causes the vertical friction) and hence the wall pushes back with the same force at A

So at A, the wall is reacting upwards and to the right, so the reaction force at A is diagonal in quadrant 1.

Last edited by mqb2766; 1 year ago

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#3

Let's look at it like this. As the plank isn't rotating there must be a moment due to the tension in the rope. But the vertical component of the tension isn't equal to the mass of P plus the mass of plank (times g, of course). Therefore it must be made up at the hinge. Now there is also a horizontal component from the rope, pulling the plank towards the wall. The only thing that can oppose this is a horizontal component from the hinge.

So the hinge has both a horizontal and a vertical component. When we add these the resultant force at the hinge acts diagonally.

So the hinge has both a horizontal and a vertical component. When we add these the resultant force at the hinge acts diagonally.

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(Original post by

Let's look at it like this. As the plank isn't rotating there must be a moment due to the tension in the rope. But the vertical component of the tension isn't equal to the mass of P plus the mass of plank (times g, of course). Therefore it must be made up at the hinge. Now there is also a horizontal component from the rope, pulling the plank towards the wall. The only thing that can oppose this is a horizontal component from the hinge.

So the hinge has both a horizontal and a vertical component. When we add these the resultant force at the hinge acts diagonally.

**David Getling**)Let's look at it like this. As the plank isn't rotating there must be a moment due to the tension in the rope. But the vertical component of the tension isn't equal to the mass of P plus the mass of plank (times g, of course). Therefore it must be made up at the hinge. Now there is also a horizontal component from the rope, pulling the plank towards the wall. The only thing that can oppose this is a horizontal component from the hinge.

So the hinge has both a horizontal and a vertical component. When we add these the resultant force at the hinge acts diagonally.

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#5

(Original post by

Ah ok I get the bit about the force opposing the horizontal component of the tension, however why can the vertical component tension not be equal to the mass of plank and P? Probably a silly question, but could this ever be the case? Thanks!

**penelopehills**)Ah ok I get the bit about the force opposing the horizontal component of the tension, however why can the vertical component tension not be equal to the mass of plank and P? Probably a silly question, but could this ever be the case? Thanks!

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**penelopehills**)

Ah ok I get the bit about the force opposing the horizontal component of the tension, however why can the vertical component tension not be equal to the mass of plank and P? Probably a silly question, but could this ever be the case? Thanks!

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So the resultant diagonal at A comes from resolving the frictional and reaction force of the wall at A then? Ah I’m sorry for how stupid I come across I really struggle with mechanics /::

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#8

(Original post by

So the resultant diagonal at A comes from resolving the frictional and reaction force of the wall at A then? Ah I’m sorry for how stupid I come across I really struggle with mechanics /::

**penelopehills**)So the resultant diagonal at A comes from resolving the frictional and reaction force of the wall at A then? Ah I’m sorry for how stupid I come across I really struggle with mechanics /::

**TWO**components, a

**horizontal**one (representing the

__normal reaction__of the wall on the rod) and a

**vertical**one (representing the

__frictional force__between the rod and the wall)

In the question's part (c), this diagonal force is at an angle to the horizontal. By drawing out the components, you construct a right-angled triangle, and then should quickly realise that

Last edited by RDKGames; 1 year ago

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**penelopehills**)

So the resultant diagonal at A comes from resolving the frictional and reaction force of the wall at A then? Ah I’m sorry for how stupid I come across I really struggle with mechanics /::

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(Original post by

The resultant force at A is acting diagonally. It is made up of

In the question's part (c), this diagonal force is at an angle to the horizontal. By drawing out the components, you construct a right-angled triangle, and then should quickly realise that

**RDKGames**)The resultant force at A is acting diagonally. It is made up of

**TWO**components, a**horizontal**one (representing the__normal reaction__of the wall on the rod) and a**vertical**one (representing the__frictional force__between the rod and the wall)In the question's part (c), this diagonal force is at an angle to the horizontal. By drawing out the components, you construct a right-angled triangle, and then should quickly realise that

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(Original post by

Don't feel bad, a lot of students struggle with mechanics, and often it's not taught very well. You are right now. We get vertical friction because of the horizontal reaction. Then when we add the vertical friction and horizontal reaction our resultant is diagonal.

**David Getling**)Don't feel bad, a lot of students struggle with mechanics, and often it's not taught very well. You are right now. We get vertical friction because of the horizontal reaction. Then when we add the vertical friction and horizontal reaction our resultant is diagonal.

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