I am a little bit confused. while doing questions I can see direction of normal forces are different. In Some Cases, It is perpendicular to the contact surface and towards the object on which normal is applied. In Other Cases, I can see it is perpendicular but opposite direction…
In The Above Image If You Draw The FBD of man you can see the direction of normal is different from the below image if we draw the fbd of B.
Then How will i determine the direction of normal force upwards or downwards ?
Best Answer
A normal force acts at right angles to surface.
When considering the direction of a normal force one should consider what would happen if it is not there.
For your second example here are the FBDs with the coloured forces (other than black) being Newton third law pairs.
For block $B$ if the normal force due to $A$ was not there block $B$ would accelerate downwards due to the gravitational force on it.
This means that the normal force on $B$ due to $A$ must be upwards and hence (N3L) the normal force on $A$ due to $B$ must be upwards.
In The Above Image If You Draw The FBD of man you can see the direction of normal is different from the below image if we draw the fbd of B.
I do not agree with this statement.
Your first example is slightly more complicated and depends on whether or not the person is anchored to the platform.
If the person is not anchored then the only direction possible for the normal force on the person due to the platform is upwards and hence (N3L) the normal force on the platform due to the person is downwards.
So the normal force on the person is in the same direction as that on block $B$ due to block $A$.
If the person is not anchored to the platform then, without any further information, the direction of the normal force on the person due to the platform is indeterminate so you could put either up or down but then making sure that the direction of the normal force on the platform due to the person is shown to be in the opposite direction.