# Children Are Source Of Embarrassment

1. Aug 29, 2005

### samalkhaiat

This morning my eight years old daughter asked me: "why is it so difficult to balance on my bicycle when it is not moving?". It was very embarrassing moment, I simply coud not give her a convincing answer.Then I thought of other things which "I" can not explain without using mathematics. So I decided to ask you people for help.
As well as "wy daughter's bicycle" there is the "spinning top", the issue here "I believe" is how to explain the relation between stability & motion without math?
The other "more difficult" subject, I thought about, is explaining global & local gauge principles for people with no knowledge of group theory.
So if you ,like me, can not explain the above, can you list other similar subjects?

thanks

sam

2. Aug 29, 2005

### LeonhardEuler

I think I can explain the bicycle without math, but I don't know if it would be comprehensible to an eight year old. When you lean the bike without the wheels spinning, you have to cause the little bits of rubber on the tire to start moving. This isn't that big a deal because it is just a matter of speeding them up a little. But if the wheels are spinning, then for the bike to tip over would require that you change the direction of the speeding pieces of rubber. The faster they are going, the harder this is to do, just like its harder to turn a speeding car. The result is that the bike is harder to tip over.

This was an interesting example. It made me think about this stuff, and I think I understand angular momentum better than I did ten minutes ago.

Last edited: Aug 29, 2005
3. Aug 29, 2005

### GOD__AM

The reason a bike is hard to balance has to do with the center of gravity and the weight distribution. Since the contact area of the tires is pretty small (and the profile of the tires is rounded) the weight distribution from left to right has to be nearly perfect for the bike to stand upright. Imagine a vertical line exactly in the center of the bike (looking at the bike from front to rear). As long as the weight from one side of this line is equal to the other the bike is balanced. When the bike leans to one side gravity pulls harder on that side. If the bike had tires more like a car (flat profile) it would be much more stable and could withstand greater differences in weight from side to side.

No confusing math, and possibly understandable by a child.

4. Aug 29, 2005

### LeonhardEuler

But that doesn't answer the implicit question of why it is not difficult to stay balanced while the bike is moving.

5. Aug 29, 2005

### Pengwuino

dang kids!!!

6. Aug 29, 2005

### bomba923

Ahh kids...they can learn.
(But will they really?)

(<runs and hides away>)

7. Aug 29, 2005

The reason the bike does not tip over when moving is because of a gyroscopic property called precession. When the bike is not moving, any lateral force on the bike tire creates a torque about the centre. This torque makes the bike tip. Once the bike is moving, this torque is still there, but since the tire is spinning, the torque applies towarts the whole radius since the tire is rotating. It is NOT because of the threads or centre of mass like stated above.

Regards,

8. Aug 29, 2005

### Claude Bile

The thing that flips me out is how those Superbikes stay upright when going around corners!

A spinning wheel is very hard to tip over. This is a neat experiment to do yourself provided you have a spare bike wheel lying around (or are willing to remove one from a bicycle). Spin the wheel then try to tip it over. You will find that when you try to tip it over, the wheel turns about a vertical axis and not a horizontal one. This is why leaning in a particular direction will cause the bike to turn that way (motorbike riders will know what I am talking about ).

When I was learning physics in high school, angular momentum was the stuff that spun me out the most, it is not intuitive at all to someone who has not done much physics (unless they ride motorbikes).

Claude.

9. Aug 29, 2005

### GOD__AM

Well I didn't see that that question was asked, but the reason is a little more complicated. When the bike is moving forward it is said to have momentum in the direction of travel. The faster you move forward on the bike the slower it will fall to one side when it is unbalanced. This slower fall gives us more time to make steering corrections that change the center of gravity of the bike. Many more things come into play like rake, trail, and wheelbase, but thats probably beyond the understanding of most children, and has been discussed here many times in many threads.

BTW not to be rude, but the post you made read like a bunch of gobbly gook to me. If you got some new understanding from it great, but I don't think it really explained anything.

10. Aug 29, 2005

### LeonhardEuler

Thank you for your honesty, I appreaciate it. Let me try to explain better. Look at the wheel as being made up of particles of rubber that are connected together. When the wheels are not moving and the bike is not moving, these particles are not moving. When the bike starts to tip without the wheel spinning, these particles have to start moving. This means they have to speed up, but not by much. If the wheel is spinning, they are already moving. Now if you tip the bike over you will not only have to make the particles tip over with the bike, but also change their direction. This is more difficult to do, just like it requires a lot of force to change the direction of a speeding car. (of course the reason is that the change in momentum is greater, but I am avoiding mathematical concepts). Can you understand what I'm trying to say? I don't take it as a personal offense if you don't, I always appreciate honest criticism.

11. Aug 29, 2005

### LeonhardEuler

But you don't explain why. That is basically what you are trying to show.

12. Aug 29, 2005

### OnTheCuttingEdge2005

I think the best method of teaching gyroscopic forces is to"

1. Get one bicycle wheel (Front wheel not the rear, Safer.).
2. Get two of the screw on foot rests post that attach to bicycle axles.
3. screw on the foot rests on both sides of the wheels axle.

The foot rests will act as handles to hold on too.
Lightly spin the wheel while someone is holding it, Then ask them to move the spinning wheel in any direction to see the differences in Gyroscopic forces.
When you have somebody holding a spinning wheel and ask them to wobble the wheel side to side they will feel the forces at work.

Gerald L. Blakley

Last edited: Aug 30, 2005
13. Aug 29, 2005

### LeonhardEuler

I think that is a good demonstration of what exactly gyroscopic forces are and I remember it being helpful in high school. Riding a bike and observing the increased stability is another demonstration. But neither gives an intuitive explaination of why the force should exist.

14. Aug 29, 2005

### GOD__AM

An object in motion tends to stay in motion unless acted on by another force. This is a basic law of physics, and I'm sure I could no more explain why than I could explain why the speed of light is constant.

Imagine I have 2 balls, and I'm holding them 3 feet from the ground. If I let one go it will fall to the earth rather quickly. Now if I throw the other into the air the same force of gravity will be pulling the ball down but it will take longer for it to hit the earth.

15. Aug 29, 2005

### GOD__AM

Gyroscopic forces have very little to do with a bikes stability as has been demonstrated using counter rotating wheels that cancel out this force. These bikes are perfectly ridable. I believe the gyroscopic force translates into forward momentum, but any other effects are negligible.

16. Aug 29, 2005

### LeonhardEuler

Yes, the bikes inertia will tend to slow its tip, but why more so when the bike is moving foward? It has nothing to do with the bike's linear momentum, since all inertial frames are equivalent and it has zero linear momentum in its own frame. The answer has to do with the rotation of the wheels. By the way, did my second explaination get across my point, or was it still incomprehensible? I would like to know because people ask me questions sometimes and I would like to be able give clear explainations.

17. Aug 29, 2005

### GOD__AM

The bike experiences acceleration in the direction of travel so it can not claim to be motionless. And no your post still doesn't make sense to me, sorry. It is in fact much easier to turn the wheel of a car that is moving than one sitting still. Anyone without power steering can attest to that. Given adequate traction a car could make full lock steering turns at maximum velocity.

18. Aug 29, 2005

### LeonhardEuler

Once the bike is moving at constant velocity it can claim to be motionless.

I didn't mean its harder to turn the wheel when the car is moving, I meant it requires a larger external force to turn it. That is a fact. When a particle travels in a circle without changing speed, the magnitude of the force is $m\frac{v^2}{r}$.

Can you see what I was getting at now? When the speed of the particles of rubber is higher, it requires a greater force to change their direction. Does this make sense?

Last edited: Aug 29, 2005
19. Aug 29, 2005

### GOD__AM

Constant velocity compared to what? As soon as the bike leans to one side slightly (which is a constant occurence for a bike) it has acceleration in that direction.

Yes I understand, to reiterate my point; an object in motion tends to stay in motion unless acted on by another force.

Last edited: Aug 29, 2005
20. Aug 29, 2005

### LeonhardEuler

The linear momentum that results from wobbling is quite small. In fact it exists when the bike is at rest with respect to the ground, so it can't answer the question. If the bike moves at 15 mph in a straight line with a wobble superimposed on the path, it is essentialy at rest in a frame moving at 15 mph with respect to the ground in the direction of the bike with no wobble.