My Math Forum Kinematics Problem

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 August 10th, 2018, 02:34 AM #1 Newbie   Joined: Feb 2015 From: uk Posts: 11 Thanks: 0 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?
 August 10th, 2018, 02:36 AM #2 Newbie   Joined: Feb 2015 From: uk Posts: 11 Thanks: 0 IMG_0042a.jpg Correct way round! Last edited by skipjack; August 10th, 2018 at 07:10 AM.
 August 10th, 2018, 02:54 PM #3 Math Team   Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 896 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 02:57 PM.
 August 11th, 2018, 12:42 AM #4 Global Moderator   Joined: Dec 2006 Posts: 20,302 Thanks: 1974 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.)
 August 11th, 2018, 02:58 AM #5 Newbie   Joined: Feb 2015 From: uk Posts: 11 Thanks: 0 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.
 August 11th, 2018, 06:16 AM #6 Global Moderator   Joined: Dec 2006 Posts: 20,302 Thanks: 1974 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.
 August 12th, 2018, 03:43 AM #7 Math Team   Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 896 And this is "statics", not really "kinematics" because nothing is moving.

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