# Did anyone here sit OCR Mechanics 2?

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#1
If you did, could you do the last question (the childs toy made up of a hemisphere and cone)? I'm convinced that it actually wasn't on the syllabus, or at best that the syllabus is ambiguous and that question jsut shouldn't have been there. Rest of the paper was OK, but the circular motion one was a little weird. Thoughts?
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16 years ago
#2
Rah, you have to do circular motion and centres of mass in 3D for OCR M2?
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#3
Well, I didn't think so, and the syllabus says "Splitting complex shapes into smaller shapes." The shape was a cone with hemisphere on the end. We hadn't covered finding the centre's of mass of these shapes in class, but had looked at others like squares, circles (not semi circles!), triangles etc. I guess the board considered a hemisphere and cone to be simple enough to fit on the syllabus and my teacher (who is normally very good at covering EVERYTHING) didn't. So the question was impossible for everyone in my class.
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16 years ago
#4
Those are found by integration though, at least that's how its done in Edexcel's M3.
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#5
Yeah, I had a look in the revision guide, and finding centers of mass by integration is on M3/4 on OCR. I think you were supposed to do it by treating the 3D solid as a 2D triangle (for the cone) and a 2D semi circle (for the hemisphere). Still, the triangle bit was OK, but we were never shown how to do it for semi circles - my teacher didn't consider this to be a simple enough shape to be covered by the M2 syllabus (syllabus is very ambiguous for this point and semi circles had not come up before AFAIK). We did circles (obviously at the dead centre), but not semi circles.
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16 years ago
#6
Semi-circles are treated as sectors. For a sector, radius r and of angle 2a at the centre, the c of m is 2rsin a/3a (a measured in radians). So, if the angle at the centre is pi/6, you halve it to find a. For a semi-circle, you just use 2a = pi.
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16 years ago
#7
2rsin a/3a from the centre, that is.
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16 years ago
#8
The centre of mass for uniform shapes are in the formula booklet!
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