Very Basic Science That You Still Don't Understand

I suggest that Whack-a-Mole and Muttrox go to Australia together!

That’s what I thought as well, untill I actually went to the southern hemisphere and was surprised. I had to sit down and think for a few minutes, but then it came to me.
The moon is (of course) the same, and shows the same surface to every point on earth (to a very high degree at least). BUT when you say that the crescent of the moon leans (looks like a tilted “)”), you unconsiously use the locally flat surface of the earth as a reference (you happen to be standing on it). And THIS will make the moon look different at the antipodes!

If you are on the northern hemisphere the new moon looks like a “,” (comma - it’s coming :)), but at the equator it would be a shallow “u” or a bowl (or upside down if the moon is setting).

Precession only partly explains countersteering. I’ve read an account of someone who wanted to build a near-uncontrollable bicycle by mounting counterrotating weights next to the wheels, so that any gyroscopic effects would be exactly cancelled. To his surprise, the bike handled just fine.

I use countersteering on my bicycle, and it can be explained without precession. If I want to make a hard right turn, I momentarily turn the handlebars to the left, which makes the wheels move to the left of my center of gravity, in other words, lean to the right. When I’ve achieved the proper amount of lean, I turn the handlebars to the right to maintain that amount of lean. I can make very sharp, fairly high-speed turns as long as I focus on countersteering.

Precession, however, does play a part, especially at the higher speeds that a motorcycle travels.

Now, on to my question. I made it out of college with a EE degree, without ever really learning how a gyroscope works. I don’t mean a bunch of formulas with vector cross products, I mean an intuitive understanding of it. Surely this can be accomplished - it’s just Newtonian physics, the stuff we observe every day. Can anyone help? I’ve asked this question before, and still am in the dark.

Another question. Why is it that the same side of the moon always faces the earth?
In order for that to happen, the rotation of the moon would coincide exactly with it’s orbit, so that, for every orbit around the earth, it would also have exactly one rotation.

Is this caused by some property of physics and spinning bodies or a just an amazing coincidence?

Thanks for the thread Jarbabyj, I’ve always wanted to ask this.

Underlining mine

Well, angular momentum is DEFINED by a vector cross product, which always produces a resultant vector that is perpendicularto the two vectors you are taking a product of. In fact,one way to think about a cross product is that the magnitude is equal to the area of the parallelogram formed by theoriginal vectors, and the direction is perpendicular to that parallelogram - the cross product describes that particularparallelogram (its area and orientation at least). But that does not exactly explain what it has to do with rotation of a gyroscope…

Actually, if we lived in any dimension otherthan 3, angular momentum would not be a vector at all, but would be an antisymmetric second-rank tensor - if that helps!

So, to some extent it is a little coincidental, and you have to try and understand the math behind it, or else just accept it on faith that this vector along the axis of rotation has these particular conservation properties that follow from conservation of momentum. Once you accept that (or understand it) then the properties of the gyroscope are not that mysterious - it is only changing this vector when some kind of force is applied just like with regular momentum, except that the “angular force” (or torque) is also obtained by this cross product rule, and so is perpendicular to the axis and the vertical direction (in the case of gravity) and with the torque always perpendicular to the angular momentum you get a kind of circular motion…

MC Turning:

I think part of the process is being missed. I believe the turn is actually accomplished because when you counter steer you bring the edge or curved surface of the tire in contact with the road. The same principle applies to turning on downhill skis. You create a bow in the ski and turn in the direction of the EDGE touching the ground.

Because its period of axial rotation is identical to its period of revoluation around the earth. As explained on this site, it has to do with gravitational forces, and the conversion of rotational energy to mechanical friction.

What the hell was I thinking? Obviously I wasn’t. Aargh!

Don’t feel bad. I thought that was true until I read this thread.

“Precession” describes the way a motorcycle behaves??? Come on…that myth was debunked years ago. Not saying that precession doesn’t produce the forces you are saying, but those forces are neglible compared to the what really controls the bike.

Leaning does not cause the bike to turn, leaning is necessary to keep your center of gravity in line with the forces acting on the bike (keeps you from tipping over).

The motorcycle leans to the left because pushing the left handlebar moves the front wheel to right side causing the bike to tip. As the bike leans over, the rake on the forks causes the front wheel to want to turn way back to the left so you need to continue pushing with the left hand to keep the bike stable. But during the turn, look at the wheel, if you are turning left, the wheel is also turned to the left.

In every case, at any speed, during the turn, the front wheel is turned in the direction of the turn. The method for getting the wheel in that position, however, changes depending on the speed of the bike. At speed, the lean is initiated with a small turn of the wheel to the right. You need this lean “before” you can “start” the turn. To start your turn, you allow the handlebars to come back, past straight, to the left. If you don’t immediately turn back to the left to stabilize the bike, you are going down if a real hurry. It is this very “GET THE BIKE LEANING, THEN GET IT TURNING” action that you learn in gradeschool. Once you learn it, you don’t even realize that you make those two separate motions to turn the bike.

At very slow speeds, you can just hang your butt off the side of the bike and face the wheel where you want to go without leaning the bike much at all.
I’ve been street riding for years on purpose. Went off-road riding once on accident.

How do they know photons exist? I mean you just can’t catch one and study it under a microscope.

Err… when I say photon I mean a particle of light.

One more thing about the Moon. You may be wondering (as I often did) why, if the Sun, Moon, and Earth are in roughly the same plane, we don’t get a Lunar eclipse every month, or at least more often than every couple of years? For me, the explanation that covers this best is seeing a scale model of the Earth, the Moon, and the distance between them. Compared to their sizes, the distance is enormous. It’s pretty clear looking at it that if the moon were just a little bit off, it would miss the Earth’s shadow. And that’s exactly what it does.

As for photons, I think one of the earliest experiments that showed the existence of photons was the photoelectric effect. Think of this analogy: You’re at a county fair game booth, and the object is to knock down milk bottles. You’re given a garden hose with which to do this. You spray the hose back and forth across the row of milk bottles, and knock maybe a couple down. You then turn up the intensity, and knock down a few more. The higher you turn up the intensity, the more you knock down. Now imagine instead you’re given ping-pong balls to throw. It doesn’t matter how many of these suckers you throw at the bottles, they aren’t falling. But if you had baseballs instead, you could knock some down. See, in the case of a continuous stream (the hose), the amount of damage you do depends on its intensity, but in the case of individual particles (the balls), it depends on the size of the balls in question more than how many there are.

Okay. Long analogy, but whatever. The garden hose and the balls both represent light, but only one is correct - either light is a continuous stream, or a lot of individual particles. The milk bottles are electrons in a metal that it takes a certain amount of energy to knock out. So, you shine light on the metal, and see if any electrons get knocked out. Lo and behold, you find that whether electrons are liberated does not depend on the intensity (brightness) of the light beam, but rather on the “size” (color, or wavelength) of the light. So, you know light must be made up of individual particles. In classic physicist style, you name them after the Greek word for light, ie, photons.

I’m still having trouble with string theory, but that’s not basic science so I’ll pass on asking about that one. Same with relativity.

But here’s a basic one I don’t get. When you throw a ball up into the air, what is its speed/velocity (if any) at the apex of the arc? That is, at the moment it stops climbing up and starts falling down? My fricking physics teacher was a biology teacher who took a summer of physics to get her prepared for teaching us (science teacher shortage) and she gave us two answers and for the life of me I can’t remember which was true or if I should even believe her.

Zero. If you throw a ball up its upwards velocity decreases at roughly 9.8 metres per second until it reaches zero at the apex of the arc.

Hiya Cranky. My thought, and I may be totally wrong, would be that the velocity would be exactly zero. After all, there has to be a point at which it’s neither moving up OR down. It may be for only the tiniest fraction of a second or so but it would still be there. I find it difficult to believe that something could change directions instantaneously.

damn simulposts…

grumble, grumble

No! Don’t apologize! You are my hero. Your explanation is the same one I’ve been giving to my three-year-old grandson. I had no idea I was wrong. And, in my circle of population, I’m considered knowledgable.

So, bless you. I will be getting out my baseball, basketball, and flashlight – the next time my college-kids come home.

What other misconceptions have we in common?

=Another

Achernar
Wow. Bless you. What a swell explanation. You’ve made my week.
(No, I’m not being sarcastic.)

Highlighting mine

Maybe I’m missing the question. If the ball goes straight up then reverses direction and comes straight back down then the ball does indeed stop (zero speed) for a moment. Just jump up and down where you are and you can experience this (or use a trampoline to magnify the effect…for a moment you are weightless).

However, to me an arc is not straight up and down but describes…well…an arc (or parabola). Think of a baseball player throwing to home from the outfield. Ball goes up and down and also travels from A to B. In that case I think you get into vectors where there is a horizontal and vertical component. The ball will keep moving in the horizontal axis but the vertical axis will show a zero speed for a moment as the ball transitions from going up to going down. Movement on each axis is separate and unaffected by movement on the other axis.

Good point - I was thinking about the case where you throw the ball straight up, in which case the ball obviously doesn’t describe an arc in space (though you can get an arc if you graph height versus time :slight_smile: ).