M1: Dynamics help! Watch

chardabest
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A particle A of mass 5kg rests on a rough plane inclined at an angle of 30degrees to the horizontal. a string attached to A lies along a line of the greatest slope of the plane and passes over a smooth pulley at the top of the plane. A particle B of mass 6kg hangs vertically from the string 1m above the horizontal plane. the system is released from rest with a string taut. If B takes 2s to reach the horizontal plane, find the coefficient of the friction between A and the inclined plane.

i used suvat to find the acceleration which i got to be 0.5, then considered particle A and B and found the tension to equal 55.8 and the Friction to equal 28.8.

but when i come to but it into F=mR than its wrong! anyone see what i've done wrong? the answer is 0.679 (3sf)

Thanks :>
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generalebriety
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(Original post by chardabest)
A particle A of mass 5kg rests on a rough plane inclined at an angle of 30degrees to the horizontal. a string attached to A lies along a line of the greatest slope of the plane and passes over a smooth pulley at the top of the plane. A particle B of mass 6kg hangs vertically from the string 1m above the horizontal plane. the system is released from rest with a string taut. If B takes 2s to reach the horizontal plane, find the coefficient of the friction between A and the inclined plane.

i used suvat to find the acceleration which i got to be 0.5, then considered particle A and B and found the tension to equal 55.8 and the Friction to equal 28.8.

but when i come to but it into F=mR than its wrong! anyone see what i've done wrong? the answer is 0.679 (3sf)

Thanks :>
I get 0.679. You've almost certainly just worked out R wrongly, because I get 28.8 for the friction too.
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chardabest
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i resolved perpendicularly n got r-5gsin30=0.....
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generalebriety
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(Original post by chardabest)
i resolved perpendicularly n got r-5gsin30=0.....
Isn't that what you got for the component of the weight down the plane?

Try cos, not sin.
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chardabest
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omg...im sooooo stupid! what the bob did i use sin for! lolol thanks!!! ill try tht!
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generalebriety
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Happens to us all.
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chardabest
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lol and what happened when i used cos...the right answer came up! thanks a lot....!!
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