
#1
May608, 12:51 PM

P: 54

1. The problem statement, all variables and given/known data
Blocks A and B are sitting side by side (touching) on a horizontal surface. A constant horizontal force Fa is applied to block A, which pushes against block B with a 32.0 N force directed horizontally to the right. The same force Fa is applied to block B; now block A pushes on block B with a 16.0 N force directed horizontally to the left. The blocks have a combined mass of 30.0 kg. (a) What is the magnitude of the acceleration of the blocks in figure (a)? (b) What is the magnitude of the force Fa? 2. Relevant equations Fnet = ma 3. The attempt at a solution Part 1: Fa pushes on block A: Block A has Fa pushing to the right, and block B pushing to the left so Fnet = massblockA *acceleration = Fa  32N Block B pushes back on block A (only horizontal force on it) Fnet = massblockB *acceleraton = 32N mass blockA + massblockB = 30kg Using these three equations I solved for Fa and got that Fa = 30kg*acceleration... I thought that I could use the next set of data (Fa pushing on block B) to come up with another equation, then I could solve for Fa and acceleration, but the equations are exactly the same, so pretty useless. Any help would be great, thanks! (ps~ I sometimes have trouble with FBDs, so if something is wrong with how I laid out the forces, it would be great to know!) 



#2
May608, 01:05 PM

Mentor
P: 40,890





#3
May608, 01:13 PM

P: 54

Ok, Now I'm really not so sure:
For Fa pusing on block A: massA + massB = 30kg massblockA *acceleration = Fa  32N massblockB *acceleraton = 32N so, (Fa  32)/ acceleration = massA 32/acceleration = mass B (Fa  32) + 32 = 30 * acceleration So Fa = 30 * acceleration For Fa pusing on B: massA = 16N / acceleration masB = (Fa 16) / acceleration 16N + (Fa  16) = 30 * acceleration or, Fa = 30 * acceleration .... am I having a math problem then? 



#5
May608, 01:25 PM

P: 54

Oh.... how did you know to use those equations? (I have a final coming up, and any tips would be great)
Thanks for your help! 



#6
May608, 01:44 PM

Mentor
P: 40,890

That tells me that: 16.0 = massA*a Add those equations to get: 32 + 16 = (massA + massB)*a Then apply: The best thing to do is to write clear FBD diagrams of both blocks for both sets of data. Write all the equations and look for any easy ways to combine them. 



#7
May608, 02:06 PM

P: 54

Thanks for your help :)




#8
Sep2911, 02:57 AM

P: 44





#9
Sep2911, 03:22 AM

P: 607

unloved .... did you draw FBD ?
EDIT:first assume that the acceleration is different in two cases, when you set up equations , you will see that they turn out to be same 


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