Do we specify in FBD whether the force is pull or push?

Question

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.

Bob D · Accepted Answer

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.

Framazu · Answer

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.

Do we specify in FBD whether the force is pull or push?

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