IIT JEE , AIEEE Forum - Physics , Mechanics

karthik2007

Two bodies A, B of masses m are placed on a smooth horizontal table. The body exerts a force F on the body B. If the experimenter exerts a force on A so that the system accelerates, find the force he exerts.

This is my approach:
Consider A as the system. Now, as it exerts a force F on B, B exerts a force F backwards. Let the force by the exp. be F'.

So, F' - F = m_aa.

Consider B as the system. Now, since A exerts a force F forward on this, the equation is:

F'+F = m_ba.

I do not get the answer through this method, however, if I take the equation for body B as F' = m_ba, I get the answer. My question is, if we take body B, shudn't we include the force on it by A, which is in the forward direction?

swatygupta

can u give the figure? the ques is not clear.. how r the blocks placed?

catch_arnnie

see karthik, i think there's a mistake in the 2nd equation because of the following explanation.

see man, when you are writing the equation on body A then you wrote F' - F = m_aa which is in accordance with the FBD of body A.

Now, when you wrote F'+F = m_ba then this isn't in accordance with the FBD because F' is on body A & not on body B. On body B, there's only one force which is F (or normal reaction) due to which there will be same acceleration on body B...

the correct equation should be F = m_ba.

plz correct me if i'm wrong....

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