There are two cases when drawing FBD of a block resting on friction or frictionless surface.We draw at the back of it a force of 20N in the right direction or we draw of force of 20N in the front of the block I.e front side (Not the top side ) in the right direction.Now , I see that in case 1 there is a push force and in case 2 there is a pull force.Is it right to think that.Like we can draw a push force or pull force in either directions.Shouldn’t it affect the FBD because in case 1 , The force goes through the body and case 2 , it is just pull.
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$\begingroup$ When I taught high school physics, I didn't draw FBD's either way. I would put a dot in the center of a rectangle to represent the center of mass, and all force arrows would originate from that dot in whatever direction was appropriate. $\endgroup$– David WhiteCommented Nov 9, 2020 at 18:02
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$\begingroup$ Why is that way not there in textbooks?Your way is nice.It simplifies understanding. $\endgroup$– user279337Commented Nov 9, 2020 at 18:39
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2 Answers
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If the objective is to determine the net force acting on the body, then cases 1 and 2 can be considered equivalent.
If the surface is frictionless, then the net force acting on the body is 20 N to the right, regardless of whether it is a push or pull.
If there is friction then the net force acting on the body will be 20 N to the right minus the friction force acting to the left. Again, it doesn't matter if the 20 N applied force is a push or pull.
Hope this helps.
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$\begingroup$ Ok thank you very much. $\endgroup$– user279337Commented Nov 9, 2020 at 14:59
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$\begingroup$ please check my other question.I have not got a answer yet with which I think I can understand the concept.@Bob D $\endgroup$– user279337Commented Nov 9, 2020 at 17:30
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$\begingroup$ @user279337 Which other question are you referring to? $\endgroup$– Bob DCommented Nov 9, 2020 at 17:47
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$\begingroup$ @user279337 I see the other question now. Since it is a separate question, I would first like to know whether or not you accept my answer to the current question. $\endgroup$– Bob DCommented Nov 9, 2020 at 17:52
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$\begingroup$ I do accept sir.That’s why I asked you. $\endgroup$– user279337Commented Nov 9, 2020 at 18:37
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In general it doesn't matter which side of the object you draw the arrow on. For example, in the case of your block resting on a surface there is a normal force that the surface exerts on the block. You could draw this arrow underneath the block (pointing into it) or coming out of the top of the block (pointing away from it).
For the case of an applied force it might be better to draw the arrow on the side that the force is being applied on (eg on the left if it is a push and on the right if it is a pull) but this is just a matter of preference.
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$\begingroup$ So It is just your choice.Merci $\endgroup$– user279337Commented Nov 9, 2020 at 15:00