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# OCR M3: Systems of Rigid Objects watch

1. Hi, the answer to the following question is 200x + 450 = R, but I can't see why?

Two uniform planks AB and BC of weights 400N and 300N respectively are arranged to facilitate the crossing of a stream. The heavier plank is 3m and the lighter plank is 2m long. The plank AB rests horizontally on two supports, one at each end of the beam. The plank BC rests horizontally on a support at C and on the plank AB, which it slightly overlaps at B. A woman of weight 600N walks slowly across the planks. (The diagram shows her at a distance x metres from A.)

a) Find R in terms of x, where R is the upward force exerted by the support at B on the plank AB.
2. (Original post by HashBrowns)
Hi, the answer to the following question is 200x + 450 = R, but I can't see why?

a) Find R in terms of x, where R is the upward force exerted by the support at B on the plank AB.
Neither can I, I get R=200x+350
3. How did you get that? I think that is correct.
4. (Original post by ghostwalker)
Neither can I, I get R=200x+350
Sorry that is what I meant to write.
5. (Original post by HashBrowns)
How did you get that? I think that is correct.
Either:

Take moments about C, for the plank BC.

And then take moments about A, for the plank AB and the woman.

Then the reaction at B will be the sum of the two.

Or:

Consider the reaction at B due to the two planks alone.

Then just consider the reaction at B, due to the woman - take moments.

6. (Original post by ghostwalker)
Either:

Take moments about C, for the plank BC.

And then take moments about A, for the plank AB and the woman.

Then the reaction at B will be the sum of the two.

Or:

Consider the reaction at B due to the two planks alone.

Then just consider the reaction at B, due to the woman - take moments.

Could you help me with this one?

Two uniform rods XY and YZ, each of weight 20N and length 80cm, are connected by a smooth pin joint at Y. They rest symmetrically on two smooth pegs P and Q at the same level, with angle XYZ =90 degrees. Show that P and Q are approximately 28 cm apart and find the magnitude of the force from the pin on each rod.
Thanks
7. (Original post by HashBrowns)
Could you help me with this one?

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