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Mechanics Help

Hello Please can some Genius help me,

The Question Paper is June 2012, M1 found here, Q7, Part 3 (6 Marks) Im stuck on the very last bit, where you are trying to find the "acceleration" for the Lower Block which ends up = 0. I got the top block correct. The question goes on about Tracktive forces and what not which I do not understand.

http://pdf.ocr.org.uk/download/pp_10_jun/ocr_57454_pp_10_jun_gce_4728.pdf?

The examiners report says this:

"When analysing the behaviour of the lower part of the block, the effect of the
frictional force exerted on it by the upper portion was usually ignored, so
missing an application of another of Newton’s Laws of Motion. Where this
force was included, it was usually given the same direction as the frictional
force on the lower face of the 3 N block; a few candidates did however add its
effect to the 4 N force."


I don't understand tracktive forces then. If anyone can explain to me how this works, and why the Friction on the Top block acted on the lower block suddenly turns around then please explain (I imagine its Newtons third law but I still don't understand it properly)
(edited 11 years ago)
Original post by TheRenaissanceMan
...


At the interface between the top and bottom blocks, the frictional force acting on the top block is to the right. Then by Newton's Third law, the force acting on the bottom block is to the left. The bottom block exerts a force to the right on the top block, and by Newton's Thrid Law, the top block exerts an equal and opposite force on the lower block.

Alternatively, and somewhat more difficult to see, IMHO, relative to the bottom block, the top block is trying to move to the left, so the frictional force acting on the top block is to the right. Similarly, relative to the top block, the bottom block is moving towards the right, so the frictional force acting on bottom block is to the left.
(edited 11 years ago)
Original post by ghostwalker
At the interface between the top and bottom blocks, the frictional force acting on the top block is to the right. Then by Newton's Third law, the force acting on the bottom block is to the left. The bottom block exerts a force to the right on the top block, and by Newton's Thrid Law, the top block exerts an equal and opposite force on the lower block.

Alternatively, and somewhat more difficult to see, IMHO, relative to the bottom block, the top block is trying to move to the left, so the frictional force acting on the top block is to the right. Similarly, relative to the top block, the bottom block is moving towards the right, so the frictional force acting on bottom block is to the left.


Ah perfect, cheers, after I read this I was did the question again and got it!

I just stumbled upon this as well when I was stumped by a question regarding Contact Forces being the result of Friction and the Normal Contact force which I was going to ask you for help for.

http://cnx.org/content/m14072/latest/

I wish Teachers would emphasise the things to help you understand and answer all questions, it would save alot of time, and understanding would give you transferable skills to all the sciences.

Thanks again !!!

Edit** Having problems with Q7 ii on this paper: http://pdf.ocr.org.uk/download/pp_11_jan/ocr_61432_pp_11_jan_gce_472801.pdf?

If anybody can help. Not sure about the angles, for part (a)
(edited 11 years ago)

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