Force acting on 2 blocks 高手請進

2009-11-01 12:43 am

B block is placed on the ground with A block is putted on its upper surface

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Block A|
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Block B|-------------------------)))applied force acting on Block B
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let the Applied force is samller than
the total sum of dynamic(max.) Friction acting on block B...
i.e. No net force acting on block B

But there is a Friction by block B on block A....
it reveals that block A will move while block B remain sationary...
is it true????????????????????????????

however,i have done some exercise stating that 2 blocks will remain sationary,
plz solve my problem!!! Thank You...

P.S.
in the limiting friction on blockB...
I don&amp;#039;t know whether
Friction by block A on block B or
Friction by Ground on block B
has the highest piority in balancing the Applied force,,

更新1:

Thanks physics8801 for answing my question! However,I don't understand why the determining factor is friction on ground. As we know, Block B has 2 friction acting on it when an applied force is exerted. But in your case,it seems that u ignore the friction by Block A acting on Block B.

更新2:

Why we could ignore it? Is there any limiting friction given by Block A on Block B? What is the case when the Applied force is larger than all the friction on Block B? I hope u can give more details about it.I am grateful to have you help me.

回答 (2)

AAA

2009-11-01 7:18 am

Actually, friction is involved when there is motion, or more accurately, relative motion with respect to a certain frame of reference. In this case, the friction between Block A and B would not cause motion as when Block A has no motion, there is no relative motion between A and B. Hence, Block B would not move as there is no net force exerted on it and originally, it is at rest

參考: Physics devotee

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天同

2009-11-01 1:05 am

If the applied force acting on block B is smaller than the limiting friction (given by the ground) on B. Block B would not move at all.
Since block B has no motion, there would not be any relative motion between block A and block B. As such, no (frictional) force is acting on block A. It is thus clear that block A would not move.

Q: I don&#039;t know whether Friction by block A on block B or Friction by Ground on block B has the highest piority in balancing the Applied force,,

The determining factor is friction on ground.

2009-11-03 21:03:41 補充：
You can view block A and B as a single object. The friction on B (given by the ground) equals to the coefficient of friction multiply by the normal reaction, which is the sum of weights of both A and B. Hence, the effect of A has been accounted for.

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