# Normal force and reaction

It is said that the normal force comes into play any time two bodies are in direct contact with one another, and always acts perpendicular to the body that applies the force. This force is a reactionary force because its value depends on the force the body from which the Normal force is emanating experiences and changes with change in the force applied.

When I apply force on a body at an angle to the normal, even then I am getting a reaction equal to my force. This, too, is emanating from the body that is reacting. So why is the normal special?? When we discuss the free body diagrams of forces applied on bodies, we do not always show this normal - harness attached to a horse, rope on a pulley, wire attached to picture etc.?

This force is a reactionary force because its value depends on the force the body from which the Normal force is emanating experiences and changes with change in the force applied.

No, this is wrong. For example, suppose two cars collide head-on at equal speed. Car A makes a normal force on car B, and B makes a normal force on A. These two forces are Newton's-third-law partners. Newton's-third-law partners are always of the same type, so a normal force is always the "reaction" to a normal force.

So why is the normal special?

It isn't.

When we discuss the free body diagrams of forces applied on bodies, we do not always show this normal

You should.

• The OP is most certainly confuied between reaction force and normal reaction force and you are suggesting him to show normal in case of picture hanging from wall. I would highly appreciate if you could draw a diagram with normal reaction force and post it here in cawe kf picture hanging from wall and please do not draw normal between wall and painting Oct 26, 2013 at 20:06
• @rijulgupta: In your example, if you're imagining the picture as hanging on a nail, then the vertical force that holds the picture up is not a force from the wall, it's a force from the nail. The free-body diagram for the picture would show an upward normal force F1 from the nail on the picture, and a downward gravitational force F2 from the earth on the picture. F1 and F2 are not 3rd-law partners. If you would like more detail on this, please post a separate question and I'll give a more detailed exposition.
– user4552
Oct 26, 2013 at 20:42
• From where do yoh associate a normal from a wall to support the picture to my example is really ridiculous. I did not say anything as such, I clearly mention that it is the tension that holds bhe picture and tension works along the thread, and since it is a force it counteracts gravity to hold the picture Oct 26, 2013 at 20:48

What we call the normal reaction force is a common name given to the reaction forcs which arises between a body and the surface it is kept on under the influence of gravity or other force like centrifugal(in frame of body).

Your claim that we do not show normal force under conditions such as harness of a horse etc is wrong as in these cases we do show the reaction force but in these conditions it is denoted by names such as Tension etc. Same goes for the case of wire attached to picture and rope on pulley

• What we call the normal reaction force is a common name given to the reaction forcs which arises between a body and the surface it is kept on... No, normal forces are not necessarily caused as "reactions" to other forces in the sense of Newton's third law. Newton's-third-law partners are always of the same type, so a normal force is always a reaction to a normal force. As an example where there is no other force besides a normal force, consider a pool ball hitting another pool ball.
– user4552
Oct 26, 2013 at 19:52
• Why is it called normal "reaction" force ? And you mentioned in another comment tension is not type of force, what is it then ? Oct 26, 2013 at 19:58
• In one of my text book NCERT it says "there are mutual contact forces (for each pair of bodies) satisfying the third law. The component of contact force normal to surfaces in contact is called normal reaction" I advice you to think twice before downvoting answers etd. As the book also mentions tension as a type of force Oct 26, 2013 at 20:04
• More sources if needed normal : " The normal force is one of the components of the ground reaction force and may coincide with it" en.m.wikipedia.org/wiki/Normal_force.... tension : "In physics, tension is the pulling force exerted by a string, cable, chain, or similar solid object on another object." en.m.wikipedia.org/wiki/Tension_(physics) Oct 26, 2013 at 20:11
• Why is it called normal "reaction" force ? It isn't, at least not in any competent treatment I've seen. In one of my text book NCERT it says "there are mutual contact forces (for each pair of bodies) satisfying the third law. The component of contact force normal to surfaces in contact is called normal reaction" Books are wrong sometimes. This sounds wrong, although I can't be certain without more context. As the book also mentions tension as a type of force This sounds like an incompetently written book, but I can't tell for sure because I don't have a copy.
– user4552
Oct 26, 2013 at 20:31