April 9th, 2012, 11:20 AM  #1 
Newbie Joined: Feb 2012 Posts: 22 Thanks: 0  Stuck with a multibody problem
[attachment=0:1exzofe3]IMG_20120409_111544.jpg[/attachment:1exzofe3] I don't know why there's 3 different masses on that picture. 
April 9th, 2012, 01:20 PM  #2 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 464 Math Focus: Calculus/ODEs  Re: Stuck with a multibody problem
Let's assume block A has a mass of 6 kg and block B has a mass of 2 kg. Assuming no kinetic friction, the acceleration of the blocks, from Newton's 2nd law of motion is: Since the blocks are connected, their accelerations must be equal. Therefore, the force F block A must exert on block B is: A confusing diagram with the wrong result given. That's just wrong! If we average the two masses given for block A, then we get the result shown. 
April 9th, 2012, 02:23 PM  #3 
Newbie Joined: Feb 2012 Posts: 22 Thanks: 0  Re: Stuck with a multibody problem
Yep, I had a feeling the answer's wrong. :P

April 9th, 2012, 02:42 PM  #4 
Newbie Joined: Feb 2012 Posts: 22 Thanks: 0  Re: Stuck with a multibody problem
Wait, on the second part, the mass isn't 6 kg? Isn't that block A's mass?

April 9th, 2012, 02:58 PM  #5 
Newbie Joined: Feb 2012 Posts: 22 Thanks: 0  Re: Stuck with a multibody problem
My friend said the total mass of block A is 8 kg.

April 9th, 2012, 04:11 PM  #6  
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 464 Math Focus: Calculus/ODEs  Re: Stuck with a multibody problem Quote:
If the mass of block A is 8 kg then the force on Block B will be even smaller. It would be (mass of block B)/(mass of block A + mass of block B) * the applied force. If the mass of block A was 4 kg, then the force of block A onto block B would be 15 N.  
April 9th, 2012, 10:24 PM  #7 
Newbie Joined: Feb 2012 Posts: 22 Thanks: 0  Re: Stuck with a multibody problem
That image is a really stupid one, in my opinion. My professor himself said that some of the questions in the book are wrong. I'll just think about what you said and what my friend said for now. He said that the 6 kg is like putting another item on top of block A, making the total mass of block A to be 8 kg. Which kinda doesn't make sense, considering that if I follow that logic, then that would mean the two blocks would have the same mass, but block A obviously looks bigger.

April 9th, 2012, 10:31 PM  #8 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 464 Math Focus: Calculus/ODEs  Re: Stuck with a multibody problem
Suppose there is a small insect in between the blocks. Would the bug be more likely to survive if force was exerted on the smaller block or on the larger block and why?

April 10th, 2012, 12:31 AM  #9  
Newbie Joined: Feb 2012 Posts: 22 Thanks: 0  Re: Stuck with a multibody problem Quote:
 
April 10th, 2012, 12:38 AM  #10 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 464 Math Focus: Calculus/ODEs  Re: Stuck with a multibody problem
Yes, since the larger block is 3 times as massive as the smaller block, the contact force would be 3 times as great if the force was exerted on the smaller block.


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