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August 10th, 2018, 01:34 AM   #1
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Kinematics Problem

IMG_0042.jpgMorning Everyone

I am teaching Kinematics in September for the first time but just ran into a small problem. Please could anyone help?
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August 10th, 2018, 01:36 AM   #2
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IMG_0042a.jpg

Correct way round!

Last edited by skipjack; August 10th, 2018 at 06:10 AM.
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August 10th, 2018, 01:54 PM   #3
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Thanks for turning it right side up!

B and C are pressing down on A with force 260 N. Since B and C have total mass 39 kg, their combined weight is (39)(9.8 )= 382.2 N. That means that the string must be pulling C up with tension 382.2- 260= 122.2 N. C alone has weight 18(9.8 )= 176.4 N but the string holds up 122.2 N so the force with which C presses on B is 176.4- 122.2= 54.2 N.
Thanks from topsquark

Last edited by Country Boy; August 10th, 2018 at 01:57 PM.
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August 10th, 2018, 11:42 PM   #4
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Or do it in reverse order... if g is the acceleration due to gravity,
the reaction force of B on C has magnitude 260 N - 21 kg × g = 54.2 N,
so the string's tension is 18 kg × g - 54.2 N = 122.2 N.

(I've used g = 9.8 m/s², but use a different value if specified for the problem.)
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August 11th, 2018, 01:58 AM   #5
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Many thanks guys. So the weight of 'A' is irrelevant?

I think I see it. In terms of what is going on physically, weight C is suspended and that is reducing the reaction force between A and B. Would you agree? I'm just predicting that my students may struggle to see what is going on here.
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August 11th, 2018, 05:16 AM   #6
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Yes. The weight of block A isn't relevant - that block can be treated as though it's part of the ground for the purposes of this problem.
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August 12th, 2018, 02:43 AM   #7
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And this is "statics", not really "kinematics" because nothing is moving.
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