# I Comfused about Normal Force

1. Mar 18, 2016

### John Pang

Why is normal force always perpendicular to the surface?
I understand that normal force is the upward reaction force by a surface due to an external action force pushing the surface. Then, if I apply an extermal force on a non inclined surface at an angle, let's say 30 degrees, shouldnt the normal force have the exactly opposite direction and point 30 degrees above the ground, instead of 90 degrees? Does this imply that any force acting on a surface must be perpendicular to the surface? Then if i apply a force at an angle on the ground, and the force acting on the ground is only the y component,then where does the x component go? This question is really frustrating.

2. Mar 18, 2016

### John Pang

Sorry for the typo xD
Should be "confused" and "external"

3. Mar 18, 2016

### sophiecentaur

Well - that's what it is. It's the force that's normal to a surface (definition). The question could be "why use the normal force for calculations?" It's often useful to resolve forces into two orthogonal directions and that can often mean taking the force parallel to a surface and the normal force. Which directions to choose and why is usually based on eliminating some force(s) for one of the equations you write. For instance, in calculating trajectories, we usually talk in terms of horizontal and vertical components because there is no g in the horizontal direction. (Entirely different example, I know but it illustrates my point.)
Why would one use the normal force in your case? The coefficient of friction is the ratio of the parallel and normal forces so, when there's slippage, that can help to solve your equations because there's another simple equation that you can write down.
Definitely not. If you push a carpet along with a broom, there's a normal and a horizontal force.
If you want a less vague comment then you could draw the diagram and state the actual problem and we could work with it.
Your problem is a very common one for students because the teacher or the guy who wrote the worked example just chose directions in an apparently arbitrary way. They will have used their vast experience to make their choice (same as when you re-arrange an equation in a particular way and suddenly everything cancels and the answer just fall out). The fact is that you don't actually have to choose it right in the first place to get the right answer. The only thing is that choosing the axes and even the co ordinates to be most suitable reduces the donkey work of manipulating the Maths. The more simple examples you do, the more obvious it gets to choose the right way through. (That's a terrible answer for you, I know, but it's true.) You can really get to hate smartarse people who have lots of experience.
Show us the actual problem and we may be able to help.

4. Mar 18, 2016

### drvrm

I think its a problem of visualization:

As some body applies a force - on a surface one does it by contact so there exists a plane or region of contact-
so the surface and the body can push each other normally;
there is no other way than the normal direction for the surface to react to the push- though it appears as normal to the surface but it is normal to the "surfaces in contact". suppose a cyclist tilts his cycle on the road at an angle say 30 degree- he is pushing at an angular position but the surface of tire and road contact is horizontal plane -so the road will react normal to it by a normal reaction.

Suppose you are pushing a body kept on inclined plane horizontally by a force F -the inclined plane will not feel a push equal to F .
if you resolve the force along the plane and normal to the plane , the normal reaction should be normal to the plane of contact and that is perpendicular to the plane. the other part of the impressed force can try to move the body along the plane.

5. Mar 18, 2016

### PhanthomJay

that is the resultant force, not the normal force
no
no, the perpendicular component is the normal force
the parallel component is the parallel force like friction force.
By definition, the normal force is the force or component of the force that is perpendicular to the surface. When one body is in contact with the other, there exists the possibility of a force perpendicular to the surface and a force parallel to the surface. The perpendicular force is called the normal force and the parallel force is usually a friction force or some other force like a pin that causes a restraint in that direction.

6. Mar 18, 2016

### John Pang

Thanks all ! I think i get it. Maybe it is just the matter of definition for calculation purpose.

7. Mar 18, 2016

### sophiecentaur

Exactly. Start the job with a view to making it easier.