Moments around pivots (Physics)

Phalanx

Phalanx

Phalanx

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Yeah, i've been revising a lot and I just can't seem to do this. The question is:

Fig2.1 shows a painter's plank resting on two supports A and B

questionow9.gif


(fig2.1)

(i) Show that the force acting on the plank at the support B is approximately 540N by taking moments of all the forces about the support A.

(ii)Calculate the force acting on the plank at Support A

This is the only stuff I suck at, I dunno how to work it out and the only thing I have is that the clockwise moments are equal to the anti clockwise moments (ala The Principle of moments)
 
Phalanx said:
*snip*
Click to expand...

Work out the overall moment about pivot A as if B isn't there, and then work out what force is needed at point B to cancel out that moment. Rinse and repeat. (the two questions are the same but for different points)
 
OCdt Stringy said:
Work out the overall moment about pivot A as if B isn't there, and then work out what force is needed at point B to cancel out that moment. Rinse and repeat. (the two questions are the same but for different points)
Click to expand...

thanks for the reply, the thing is, I don't really know how to work out the moments around a pivot either (you'd think i've never seen this question before).

Is the equation "Moment = Force x Distance" because i read that on a website but Force x Distance is also work done :\
 
Well, for part (i), you are taking moments about A, and following the principle the other forces are equal.

So: (80N x Distance) + (650N x Distance) = (B x Distance)

You should be able to fill in the gaps from here, and then you just do the same for part (ii).
 
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Phalanx said:
thanks for the reply, the thing is, I don't really know how to work out the moments around a pivot either (you'd think i've never seen this question before).

Is the equation "Moment = Force x Distance" because i read that on a website but Force x Distance is also work done :\
Click to expand...
Moment = Force x Distance from the Pivot
SimonTW said:
*snip*
Click to expand...
Oh come on, where's the learning :p
We can show him the door, but only he can walk through it
 
Yeh I know I shouldn't have given him as much info! But I think he knew the laws, it was just how to apply it he was stuck on, so I decided to be kind! :p

I know how frustrating it can be if you are stuck, and often realising how to do the question helps because you can then go on and practice it in other examples (one way I learn anyway!).
 
^^^ i think I just suck, sorry

ok so B is 1.7m away from A so I do

Force x Distance = 80n x 1.7 = 136

Force x distance = 1.7 x 650n = 1105

urggh, so confused
 
Phalanx said:
ok so B is 1.7m away from A so I do

Force x Distance = 80n x 1.7 = 136

Force x distance = 1.7 x 650n = 1105

urggh, so confused
Click to expand...
No no no!

The "distance from the pivot" is the distance away from the pivot that the force is acting.

So in the 80N case, it's 0.85m, etc
Ignore point B for the time being. Once you have worked out the total moment, then you think about point B.


Imagine it like this (imagine point B doesn't exist)



Once you have worked out the total moment for the plank and the man acting about point A, then work out what force would be needed at point B to balance it.
 
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OCdt Stringy said:
No no no!

The "distance from the pivot" is the distance away from the pivot that the force is acting.

So in the 80N case, it's 0.85m, etc
Ignore point B for the time being. Once you have worked out the total moment, then you think about point B.
Click to expand...

oh, wait a minute, the distance away from the pivot to where the force is acting?

so it's 0.85m x 80 = 68

and 0.40m x 650 = 260

yea, well, clearly no but kinda?
 
FrostedNipple said:
lol, i was just doing this paper, tut tut for leaving it so late, i assume your tests are tomorrow also! :D
Click to expand...

hey, i've been revising all fudgeing day and this is the only question I really am stuck on
 
OCdt Stringy said:
No no no!

The "distance from the pivot" is the distance away from the pivot that the force is acting.

So in the 80N case, it's 0.85m, etc
Ignore point B for the time being. Once you have worked out the total moment, then you think about point B.


Imagine it like this (imagine point B doesn't exist)



Once you have worked out the total moment for the plank and the man acting about point A, then work out what force would be needed at point B to balance it.
Click to expand...

ok, so

0.85 x 80 = 68

1.3 x 650 = 845

yeah?
 
I assume you doing this for 'A' level from your birthday. Now I know standards are slipping as I did this at 'O' level :D

If you think this is bad in 1995 I did the assessment of the Forth Bridge, it had 22,000 degrees of freedom your's is 1 degree - since the amount of extra work is a square of the increase it was about 484 million times harder!

i (0.85*80+1.30*650)/1.7 = 537.1N

ii 650+80-537.1=192.9N
 
Bomag said:
I assume you doing this for 'A' level from your birthday. Now I know standards are slipping as I did this at 'O' level :D

If you think this is bad in 1995 I did the assessment of the Forth Bridge, it had 22,000 degrees of freedom your's is 1 degree - since the amount of extra work is a square of the increase it was about 484 million times harder!

i (0.85*80+1.30*650)/1.7 = 537.1N

ii 650+80-537.1=192.9N
Click to expand...
Doesn't really teach the method to help in tomorrows exam does it?
Phalanx said:
ok, so

0.85 x 80 = 68

1.3 x 650 = 845

yeah?
Click to expand...
Yep. Now add them together - and thinking about the equation to work out moments, how would you work out what force you need at B to balance it?
 
Bomag said:
I assume you doing this for 'A' level from your birthday. Now I know standards are slipping as I did this at 'O' level :D

If you think this is bad in 1995 I did the assessment of the Forth Bridge, it had 22,000 degrees of freedom your's is 1 degree - since the amount of extra work is a square of the increase it was about 484 million times harder!

i (0.85*80+1.30*650)/1.7 = 537.1N

ii 650+80-537.1=192.9N
Click to expand...

in part i, why do you divide by 1.7???
 
OCdt Stringy said:
Doesn't really teach the method to help in tomorrows exam does it?

Yep. Now add them together - and thinking about the equation to work out moments, how would you work out what force you need at B to balance it?
Click to expand...

ok, so added, i get 913.

So this number means that the total Force acting around pivot A = 913 yeah?
 
Phalanx said:
ok, so added, i get 913.

So this number means that the total Force acting around pivot A = 913 yeah?
Click to expand...
The moment about pivot a is 913Nm yes.

Now, to cancel that moment out there is a force acting at point B.

Given that
Moment = Force x Distance from the Pivot

How could you use that equation to work out the force at point B?
 
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