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# M2 Rigid Bodies Watch

1) Does the HC of the reaction force a the wall act to the left because the HC of the thrust from the support rod is pushing the pole away from the wall?

2) What would the conditions be for the HC of the reaction force at the wall to be acting to the right?

3) Why isn't there a reaction force at D? Is that where the words 'freely jointed' come into it?

Thanks for any help
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3. (Original post by PhyM23)
1) Does the HC of the reaction force a the wall act to the left because the HC of the thrust from the support rod is pushing the pole away from the wall?
Took me a while to deduce what "HC" meant - I was looking for an HC on the diagram.

Here's how I look at things- first, I'd split every force in the system into horizontal components. I'd observe the direction of each force. If you split the thrust into components in this case, you'll observe that there is only one horizontal force acting in the system - Tsin30 acting to the right.

The rigid body must be in equilibrium so we cannot have any unbalanced forces in the system. What does this tell us? The horizontal component of the reaction force must balance this, so therefore it acts to the left.

2) What would the conditions be for the HC of the reaction force at the wall to be acting to the right?
Precisely the opposite of what I've said above.
4. (Original post by PhyM23)

1) Does the HC of the reaction force a the wall act to the left because the HC of the thrust from the support rod is pushing the pole away from the wall?
Yes. Consider the horizontal rod in equilibrium. Force to the right comes from the supporting strut, so there must be a force to the left, which comes from the wall.

2) What would the conditions be for the HC of the reaction force at the wall to be acting to the right?
It would need a force on the horizontal rod going to the left - can't happen in the pictured situation.

3) Why isn't there a reaction force at D? Is that where the words 'freely jointed' come into it?

Thanks for any help
NO.

There is a reaction force at D. It's just of no interest in/to the question.

"freely jointed" means there is no rotational force at D. Any forces acting will thus be along the struts/rod/etc.
5. (Original post by Ayman!)
Took me a while to deduce what "HC" meant - I was looking for an HC on the diagram.

Here's how I look at things- first, I'd split every force in the system into horizontal components. I'd observe the direction of each force. If you split the thrust into components in this case, you'll observe that there is only one horizontal force acting in the system - Tsin30 acting to the right.

The rigid body must be in equilibrium so we cannot have any unbalanced forces in the system. What does this tell us? The horizontal component of the reaction force must balance this, so therefore it acts to the left.

Precisely the opposite of what I've said above.

My apologies for the confusion; I was just being lazy and couldn't be bothered writing out 'horizontal component'

This is a great response. Thank you
6. (Original post by ghostwalker)
Yes. Consider the horizontal rod in equilibrium. Force to the right comes from the supporting strut, so there must be a force to the left, which comes from the wall.

It would need a force on the horizontal rod going to the left - can't happen in the pictured situation.

NO.

There is a reaction force at D. It's just of no interest in/to the question.

"freely jointed" means there is no rotational force at D. Any forces acting will thus be along the struts/rod/etc.
Thank you very much for this. I am a little confused with your last point though. How would I be able to determine whether the reaction force is of interest of the question. Also, what do you mean by rotational force, and how does this not being present make the forces act elsewhere?
7. (Original post by PhyM23)
Thank you very much for this. I am a little confused with your last point though. How would I be able to determine whether the reaction force is of interest of the question.
Question will tell you or mention it, or if you were asked to draw all the forces.
There is a thrust in the rod CD; by Newton's laws there must be a reactive force.

Also, what do you mean by rotational force, and how does this not being present make the forces act elsewhere?
Rotational forces in this case would be frictional forces. Saying "freely jointed" means there is no turning friction. I'm reasonaly sure that all joints you encounter at A-level are freely jointed.

Not being present (and they're not), doesn't mean the "forces act elsewhere". Don't really understand what you meant by that phrase.

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