# Does Normal force always HAVE to be perpendicular to plane ?

• B
• Likith D
In summary, the conversation discusses the impact of a perfectly spherical and perfectly rigid ball on a perfectly flat and perfectly rigid wall. It is assumed that the only contact between the ball and the wall is at a single point. The experts discuss the force that acts on the ball during the impact, which is considered instantaneous due to the rigid nature of the objects. It is concluded that the force on the ball is normal, meaning it acts perpendicular to the surface. The experts also consider the motion of the ball after the impact, with one stating that the force may not always act perpendicularly and the ball may not always travel along its original path in reverse. However, another expert points out that if the ball is hitting the wall at a 90
Likith D
Say i Have
1. A perfectly spherical and perfectly rigid Ball
2. A perfectly flat and perfectly rigid wall

Say i throw the ball at my FLAT wall also giving the ball a linear velocity
The only contact with the wall and the ball is a point... a POINT !
So, I can as well forget about the rest of the wall except the POINT ( Forget the wall but keep in mind of the point that the ball collides with. I did that since the existence of the rest of the wall has no effect on the after-collision motion of the ball. Here, I'm also assuming the wall and the ball are perfectly rigid so they meet at a single point only )
The point would have resisted the motion of the ball in the opposite direction to the velocity of the ball( makes sense, as the motion is an example of 1D collision ) And the ball also move back on the same path but in opposite direction like stuff in 1D motion do.
The rigid body of the ball is made of a set of points at a certain fixed distance from each other
The point of contact of the ball and the point of contact of the wall are in 1D motion
The point of contact of the ball which has initial velocity v now has a velocity that is in opposite direction, V ( at the instant of collision)
If the other points continue to be in v, the distance between the points change...
I see only two thing the body could do to stay rigid; for all the set of points to instantly have the velocity, V after collision or for the body to rotate
( or does it want to stay rigid at all? )Anyways...
If it does move in V all at once, the ball will retrace it's original path !
so... no matter how the wall was aligned as long as the ball makes a point sized contact,... the force may or may not have been perpendicular and the ball WILL HAVE traveled the same path in reverse
That's my intuition .. but do correct me if I'm wrong
... or is the whole problem is wrong to the base? ( don't say that my assumption of a perfectly rigid/perfectly flat/ perfectly spherical object is impossible... it's fun when such stuff are involved in calculations)
What's your predictions of what will happen after collision?

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Likith D said:
If the ball does retrace it's original path, which force pushed it that way... It obviously cannot be a "normal" force...or is it ?

Certainly. The force on the ball during the impact (considered instantaneous in this case since we are dealing with perfectly rigid bodies) is the normal force. The word "normal" just means that the force acts perpendicularly to the surface.

Likith D
Drakkith said:
Certainly. The force on the ball during the impact (considered instantaneous in this case since we are dealing with perfectly rigid bodies) is the normal force. The word "normal" just means that the force acts perpendicularly to the surface.
Which way do you think the ball will move after the impact... with FLAT wall
I mean the only contact with the wall and the ball is a point... a POINT !
So, I can as well forget about the rest of the wall without the POINT ( Forget the wall but keep the point that the ball is in contact with )
I a Ball with a velocity hit this point, the point would have hit it against the way the ball is moving and the ball will move away from the point but in the same path that it took to approach the point ( in simple words, the motion is in 1D )
so... no matter how the wall was,... the force may or may not have been perpendicular and the ball WILL HAVE traveled the same path in reverse\
Thats my intuition .. but do correct me if I am wrong

Likith D said:
Which way do you think the ball will move after the impact... with FLAT wall

Well, if the force acts perpendicularly to the wall, and the ball is striking it at an angle of 90 degrees, which way do you think it will move?

Edit: The above post was edited subsequent to my reply. I will respond to it in a new post.

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Likith D
Likith D said:
Which way do you think the ball will move after the impact... with FLAT wall
I mean the only contact with the wall and the ball is a point... a POINT !
So, I can as well forget about the rest of the wall without the POINT ( Forget the wall but keep the point that the ball is in contact with )
I a Ball with a velocity hit this point, the point would have hit it against the way the ball is moving and the ball will move away from the point but in the same path that it took to approach the point ( in simple words, the motion is in 1D )
so... no matter how the wall was,... the force may or may not have been perpendicular and the ball WILL HAVE traveled the same path in reverse\
Thats my intuition .. but do correct me if I am wrong
And.. here i forgot about the rest of of the part of the wall, because i assumed everything to be rigid

Likith D said:
so... no matter how the wall was,... the force may or may not have been perpendicular and the ball WILL HAVE traveled the same path in reverse\

I'm afraid not. If the force points in a different direction, either because the ball is striking the wall at a different angle, or because of some esoteric mechanism, the ball will not bounce back along its original path.

I mean the only contact with the wall and the ball is a point... a POINT !
So, I can as well forget about the rest of the wall except the POINT ( Forget the wall but keep the point that the ball is in contact with )
I threw the Ball with a velocity and it hit this point. The point would have hit it against the way the ball is moving and the ball will move away from the point but in the same path that it took to approach the point ( in simple words, the motion is in 1D collision )
so... no matter how the wall was aligned as long as the ball makes a point contact,... the force may or may not have been perpendicular and the ball WILL HAVE traveled the same path in reverse
Thats my intuition .. but do correct me if I am wrong
Drakkith said:
I'm afraid not. If the force points in a different direction, either because the ball is striking the wall at a different angle, or because of some esoteric mechanism, the ball will not bounce back along its original path.
Why not...
The ball contacts the wall at a point
forget the wall but keep in mind of the point
the ball goes and hits the point
returns like a 1D collision should happen
Even though the situation is given in 3D, i reduced it to 1D

I forgot about the rest of the wall as their presence wouldn't have made much difference to the motion of the ball as they did not even have contact with the ball
I think you must remember i said that the wall and the ball are both perfectly rigid and that the ball is perfectly spherical and the wall is perfectly flat
Is my intuition correct
if i am correct, what about the normal force in this situation

Well. maybe the normal force requires an area of contact to exist...
i'm just guessing... am not sure if it's right
so, what IS happening in this case??

Likith D said:
Why not...
The ball contacts the wall at a point
forget the wall but keep in mind of the point
the ball goes and hits the point
returns like a 1D collision should happen
Even though the situation is given in 3D, i reduced it to 1D

The fact that the point of contact is, well, a point, is irrelevant. If the ball strikes the wall at an angle then the direction of the normal force is not directly opposite to the motion of the ball. Since the force is not pointing back along the ball's original path the ball will not be accelerated straight backwards.

Don't get stuck on points and perfectly rigid objects. They don't exist in the real world.

Drakkith said:
The fact that the point of contact is, well, a point, is irrelevant. If the ball strikes the wall at an angle then the direction of the normal force is not directly opposite to the motion of the ball. Since the force is not pointing back along the ball's original path the ball will not be accelerated straight backwards.

Don't get stuck on points and perfectly rigid objects. They don't exist in the real world.
So the ball will act as if the force acted perpendicular to the plane?
So, if the normal force was normal,... the ball would have gone in 2D (or in some case 1D)
but, the ball has linear velocity and contacts a point in space (not the wall as a whole...only it's point sized thingy) Which give no reason for the ball to move in 2D
Given that the Ball and the point are the only things involved...
But still you say that the ball moves in 2D.. why is that

Likith D said:
but, the ball has linear velocity and contacts a point in space (not the wall as a whole...only it's point sized thingy) Which give no reason for the ball to move in 2D

Of course there was a reason. The reason is that the force points at an angle. Draw a force diagram and you can see this yourself. Again, the fact that the point of contact is a single point makes no difference. The rules work out to be the same.

Drakkith said:
Of course there was a reason. The reason is that the force points at an angle. Draw a force diagram and you can see this yourself. Again, the fact that the point of contact is a single point makes no difference. The rules work out to be the same.
So,.. you mean that.. NORMAL FORCE WILL BE NORMAL TO THE PLANE... NO MATTER WHAT>>>PERIOD!
Okay, i will work out the diagrams and calculations on this...( will take time...lots! )
Thank you!
hmm! wonder what went wrong with the way I explained it...

Likith D said:
So,.. you mean that.. NORMAL FORCE WILL BE NORMAL TO THE PLANE... NO MATTER WHAT>>>PERIOD!

As far as I know, yes.

Likith D
Drakkith said:
As far as I know, yes.
Thank you!

Likith D said:
NORMAL FORCE WILL BE NORMAL TO THE PLANE... NO MATTER WHAT>>>PERIOD!
The definition of normal force is that it is the component of the contact force that is perpendicular to the surface that is contacted. In this context, "normal" is a synonym for "perpendicular".

Contact forces can have a component parallel to the surface. For instance, surface friction acts parallel to the surface.

If a spherically symmetric object approaches a surface dead-on, and the surface is symmetric about the point of collision then symmetry demands that the contact force during the collision can have no parallel component (there being no preferred direction for a parallel component to point). The collision force must therefore be completely perpendicular. That is, it must be a "normal" force.

CWatters
This is a rather odd question. It is like asking "If vector A is perpendicular to vector B, does A have to be perpendicular to B?"

As jbriggs has stated, the word "normal" means perpendicular to that surface. It is its definition. If it is not, we won't call it the "normal force".

There are other cases where the word "normal" is used, such as "normal incidence", "normal angle", etc... and they all mean perpendicular to something. So you might as well get used to this definition.

Zz.

jbriggs444 said:
The definition of normal force is that it is the component of the contact force that is perpendicular to the surface that is contacted. In this context, "normal" is a synonym for "perpendicular".

Contact forces can have a component parallel to the surface. For instance, surface friction acts parallel to the surface.

If a spherically symmetric object approaches a surface dead-on, and the surface is symmetric about the point of collision then symmetry demands that the contact force during the collision can have no parallel component (there being no preferred direction for a parallel component to point). The collision force must therefore be completely perpendicular. That is, it must be a "normal" force.
What about my prediction of the ball retracing it's own path after collision?

Likith D said:
What about my prediction of the ball retracing it's own path after collision?

I don't see the problem. It simply means that the force acts parallel to the initial direction of motion, and thus, the ball bounces back along the same path it came from.

Try this: what if the ball hits another fixed ball but off-center? It still, in the ideal case, hits the "wall" only at one point, the same as your scenario above. But what is the direction of the normal force, and consequently, what is the direction of its motion after collision? Do you think it'll trace the same path that it came from?

Zz.

ZapperZ said:
I don't see the problem. It simply means that the force acts parallel to the initial direction of motion, and thus, the ball bounces back along the same path it came from.

Try this: what if the ball hits another fixed ball but off-center? It still, in the ideal case, hits the "wall" only at one point, the same as your scenario above. But what is the direction of the normal force, and consequently, what is the direction of its motion after collision? Do you think it'll trace the same path that it came from?

Zz.
I had the same next question in mind ;-)
The ball could either rotate as well as move back the same path
or it xan just move back, anyway
that what I think
thanks for bringing this up

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Likith D said:
I had the same next question in mind ;-)
The ball could either rotate as well as move back the same path
or it xan just move back, anyway
that what I think
thanks for bringing this up

Wait, you actually think that it is possible for the ball hitting, say, a wall that is at an angle, and for it to move back along the same path that it came from? You've never played pool/billiards, or haven't seen anyone playing that before?

Throw a ball towards the ground at an angle and tell me what you see.

Zz.

ZapperZ said:
Wait, you actually think that it is possible for the ball hitting, say, a wall that is at an angle, and for it to move back along the same path that it came from? You've never played pool/billiards, or haven't seen anyone playing that before?

Throw a ball towards the ground at an angle and tell me what you see.

Zz.
they are inelastic collisions, so can't trust them... you know
Well, a rigid body is made of points
so the point of contact of the ball and the wall are in 1D motion
the force experienced by the contact point on ball is against the velocity v, it was initially doing
so, now our contact point of the ball gets a velocity in the opposite direction
which means, that point is moving faster than other points of the rigid body which COULD make the distace between the points of the rigid body change
So, there are two ways in which the rigid body can remain rigid after collision that is to start rotating or for all it's points to also have the same velocity ( reply if you have more possibilities )
as you can see, my questions are still valid
anyway, if the point of the body all have a velocity in the opposite direction, all at once the body collides, we can see that the body retraces it's path
Well, I'm not sure what the body will do if it does rotate...(need help)
thank you for bringing up the question, had the exact same thing in my mind before...

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Likith D said:
that point is moving faster than other points of the rigid body
What does "rigid body" mean to you?

Likith D said:
. the force may or may not have been perpendicular and the ball WILL HAVE traveled the same path in reverse
Thats my intuition .. but do correct me if I am wrong
So there are two things to point out here.

First, completely independent of any scenario you could ever outline the normal force is perpendicular by definition. It certainly can be that the interaction between a plane surface and an object may not be perpendicular to the surface. When that happens we break the force into two components. The perpendicular component we call the normal force and the parallel component we call friction. So the normal force is always perpendicular by definition.

Second, your understanding of the scenario seems wrong to me. The ball will not in general bounce back the way it came, unless it already was traveling exactly perpendicular. To do so would require a large impulsive friction force to reverse the momentum parallel to the surface. At a point of contact between two rigid objects the friction force will be minimal.

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CWatters and Likith D
Likith D said:
What about my prediction of the ball retracing it's own path after collision?
Those 'super balls' that you can buy will bounce back at you but you have to spin the ball and the surface must not be perfectly frictionless. This can introduce a net force on the ball that is back towards its source, as well as the normal reaction force. Definitely NOT an ideal situation for elementary analysis.

Likith D said:
What about my prediction of the ball retracing it's own path after collision?
You mean like here?

Likith D

## 1. What is normal force?

Normal force is the force that a surface exerts on an object that is in contact with it. It is always perpendicular to the surface and acts to prevent the object from passing through the surface.

## 2. Does normal force always have to be perpendicular to the plane?

Yes, by definition, normal force is always perpendicular to the surface. This is because it is a reaction force that is equal and opposite to the force that the object exerts on the surface.

## 3. Can normal force be parallel to the plane?

No, normal force is always perpendicular to the plane. If it were parallel to the plane, it would not be able to prevent the object from passing through the surface.

## 4. Are there any exceptions to normal force being perpendicular to the plane?

In most cases, normal force will always be perpendicular to the plane. However, in certain situations, such as when an object is on an inclined plane, the normal force may have a component that is parallel to the plane, but it will still be perpendicular to the surface.

## 5. How is normal force different from other types of forces?

Normal force is a contact force that only exists when an object is in contact with a surface. It is always perpendicular to the surface, while other types of forces, such as friction or gravity, can act in any direction. Normal force is also a reactionary force, meaning it is caused by another force, while other forces can be independent.

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