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# 2 SUPER QUICK M2 Moments questions watch

1. (Original post by ghostwalker)
Yes.

These things will be clearer when you've covered moments.
I have covered moments

In all honesty, I manage to get the answers right in the end but I consider it a fluke because I don't really understand the theory/reasoning behind it. Like with this question I managed to get the answer right because if when the answer came out negative I knew to flip the direction of the force. I just wanted to understand why the direction of the reaction force acts leftward and up.

Also about the angle question, which angle is that finding exactly?
2. (Original post by creativebuzz)
I have covered moments

In all honesty, I manage to get the answers right in the end but I consider it a fluke because I don't really understand the theory/reasoning behind it. Like with this question I managed to get the answer right because if when the answer came out negative I knew to flip the direction of the force. I just wanted to understand why the direction of the reaction force acts leftward and up.
Thought it was left and down.

Also about the angle question, which angle is that finding exactly?
You just need a reference line. The obvious one to go for is the vertical downards from A. So the angle it makes with that.

You should specify clearly in your answer which angle you're refering to, e.g. 55 degrees clockwise from the downwards vertical.

You could also use the horizontal as a reference line....
3. (Original post by ghostwalker)
Thought it was left and down.

You just need a reference line. The obvious one to go for is the vertical downards from A. So the angle it makes with that.

You should specify clearly in your answer which angle you're refering to, e.g. 55 degrees clockwise from the downwards vertical.

You could also use the horizontal as a reference line....
But isn't the angle made from A and the downward vertical just 90 degrees because there's that right angled triangle by A
4. (Original post by creativebuzz)
But isn't the angle made from A and the downward vertical just 90 degrees because there's that right angled triangle by A
That triangle is just the position of the rod. You need to find the angle the resultant force (combining horizontal and vertical components of the reaction) makes, with the vertical for example.
5. (Original post by ghostwalker)
Thought it was left and down.
I understand why it's down (because the strut is pushing it upwards so the rod must be pushing downwards to keep it straight) but why is the reaction going leftward?
6. (Original post by creativebuzz)
I understand why it's down (because the strut is pushing it upwards so the rod must be pushing downwards to keep it straight)
That's not the reason. If the strut was attached to the rod at a postion beyond the CofG of the rod, the reaction at the wall would have been upwards, not down.

but why is the reaction going leftward?
In both cases I think about what would happen if the hinge wasn't constraining it. Which direction would the strut move in? Then the reaction at the hinge must be in the opposite direction.
7. (Original post by ghostwalker)
That's not the reason. If the strut was attached to the rod at a postion beyond the CofG of the rod, the reaction at the wall would have been upwards, not down.

In both cases I think about what would happen if the hinge wasn't constraining it. Which direction would the strut move in? Then the reaction at the hinge must be in the opposite direction.

Do you mean CofD?

If the hinge wasn't constraining it the rod would just topple in a clockwise movement

Before you reply to this, would you mind spotting where I went wrong in this work done question: http://www.thestudentroom.co.uk/show....php?t=4009241
8. (Original post by creativebuzz)
Do you mean CofD?
What does CofD stand for?

If the hinge wasn't constraining it the rod would just topple in a clockwise movement

I.e. the left end would go right and up, hence the reaction is down and left.

Before you reply to this, would you mind spotting where I went wrong in this work done question: http://www.thestudentroom.co.uk/show....php?t=4009241
9. (Original post by ghostwalker)
What does CofD stand for?

I.e. the left end would go right and up, hence the reaction is down and left.

No worries about the other question! It's been fixed!

But back to the moments question, are you saying that the rod would fall into the strut? (I'm just trying to visualise it properly)

10. (Original post by creativebuzz)
No worries about the other question! It's been fixed!

But back to the moments question, are you saying that the rod would fall into the strut? (I'm just trying to visualise it properly)

Huh! You had it correctly before. Where's this coming from?
11. (Original post by ghostwalker)
Huh! You had it correctly before. Where's this coming from?
Ohh I see! I get that it goes up! But how is that showing

that it is moving rightward?
12. (Original post by creativebuzz)
Ohh I see! I get that it goes up! But how is that showing

that it is moving rightward?
You can either think of a clockwise motion means it's pulling away from the wall, hence rightwards.

Or also think about as it falls, it's rotating about C too, and hence moving away from the wall.

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