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OCR Depth in Physics Question 1

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Reply 20
Avatar for nwmyname
nwmyname
OP
13
Original post by ombtom
Extend the arrow for weight until it passes through the arrow for tension.

You know that the reaction force must pass through the point where the rod meets the wall.

+ That's a good attitude to have. :tongue:



So the blue line is the one I need to draw.
Screen Shot 2016-03-29 at 14.31.31.png29.2KB
Reply 21
Avatar for ombtom
ombtom
17
Original post by nwmyname
So the blue line is the one I need to draw.


Yes, that looks right! :smile:
Reply 22
Avatar for nwmyname
nwmyname
OP
13
Original post by ombtom
Yes, that looks right! :smile:

Great! Sorted!
ooh.
Can I ask why the arrow is in that direction? Just as a theory question that I'm interesting in knowing?
Reply 23
Avatar for ombtom
ombtom
17
Original post by nwmyname
Great! Sorted!
ooh.
Can I ask why the arrow is in that direction? Just as a theory question that I'm interesting in knowing?


To make equilibrium. Your diagram with the red and blue arrows is actually great for visualising this; the blue arrow is equal and opposite to the resultant of the red arrows.

Think about a triangle of the forces, where tails are joined to heads of the arrows. The reaction force has to be in that direction to ensure that the triangle works.
Reply 24
Avatar for nwmyname
nwmyname
OP
13
Original post by ombtom
To make equilibrium. Your diagram with the red and blue arrows is actually great for visualising this; the blue arrow is equal and opposite to the resultant of the red arrows.

Think about a triangle of the forces, where tails are joined to heads of the arrows. The reaction force has to be in that direction to ensure that the triangle works.


So simply by extending the line of weight we have made it easier to draw the blue arrow and therefore create the triangle of equilibrium?
Reply 25
Avatar for ombtom
ombtom
17
Original post by nwmyname
So simply by extending the line of weight we have made it easier to draw the blue arrow and therefore create the triangle of equilibrium?


I think so.
Original post by nwmyname
Hi,
This question attached is really confusing me.
Why on earth is the vertical force exerted by the rod 60N??
It is really confusing me?
And the tension question too??
I've tried it but not got to either conclusions correctly!
Thanks.

Update I have got part b but part a is the one I do not understand.

τ⃗=r⃗×F⃗\vec{\tau} = \vec{r} \times \vec{F}
τ1⃗+τ2⃗=0 \vec{\tau_1} + \vec{\tau_2} =0
τ1⃗=−120L/2x⃗×y⃗=−60Lz⃗ \vec{\tau_1} = -120L/2 \vec{x}\times \vec{y} =-60L \vec{z}
∴τ2⃗=60Lz⃗ \therefore \vec{\tau_2}= 60L \vec{z}
F⃗=+60y⃗ \vec{F} =+60\vec{y}
Reply 27
Avatar for ombtom
ombtom
17
Original post by langlitz
τ⃗=r⃗×F⃗\vec{\tau} = \vec{r} \times \vec{F}
τ1⃗+τ2⃗=0 \vec{\tau_1} + \vec{\tau_2} =0
τ1⃗=−120L/2x⃗×y⃗=−60Lz⃗ \vec{\tau_1} = -120L/2 \vec{x}\times \vec{y} =-60L \vec{z}
∴τ2⃗=60Lz⃗ \therefore \vec{\tau_2}= 60L \vec{z}
F⃗=+60y⃗ \vec{F} =+60\vec{y}



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