Science questions not worth a thread I: I'm a moron!

[ParadigmShifter]

I thought static friction was dependent only on the normal force?

Yeah, but it is the factor by which the velocity is reduced. It's the dot product between the velocity and the normal scaled (by the coefficient of friction) multiplied by the velocity vector.

EDIT: Nope, got that wrong Because that would mean moving perpendicular to the normal would imply no friction. Maybe I mean perpendicular to the direction of the velocity instead...

 

[uppi]

Air resistance. I want to construct a realistic, approximation-less equation for the force applied on a pendulum.

0.5 * air density * velocity^2 * area * drag coefficient

The drag coefficient is the part where it gets tricky, as there is no formula for it and in principle it has to be measured. There are some ways to simulate it, but I have no idea how and this has only limited accuracy. There is a reason why people still use wind tunnels.

 

[Xanikk999]

Does every object that has mass have its own gravitational field? Or does an object on earth have its gravity negated because the earth as a whole supercedes it?

Maybe I dont understand gravity that well. I'm guessing it is weighed as a force by the collective mass of objects in one area, so that means one object on earth would count as part of earths gravity. Am i right?

Would there be anyway for human's to test this in space?

 

[madviking]

F_g = G*m₁*m₂ / r²

Every mass in the universe exerts a force on every other mass in the universe.

 

[Xanikk999]

F_g = G*m₁*m₂ / r²

Every mass in the universe exerts a force on every other mass in the universe.

I always sucked at math can't you give me an explanation that I can picture in my head?

How big do you think something would have to be to have a noticible gravitational field?

 

[madviking]

I always sucked at math can't you give me an explanation that I can picture in my head?

How big do you think something would have to be to have a noticible gravitational field?

Pretty much anything that has mass 'attracts' everything else in the universe. F_g is the gravitational force. the m's are the masses of the two objects you're comparing. G is Newton's constant. r is the distance between the two forces.

It depends on what you mean by "noticeable".

 

[peter grimes]

A single proton induces a gravitational field. The 'gravity of Earth' is nothing more than the sum of the gravitational fields induced by all the protons and all the neutrons of all the atoms of all the molecules of all the matter that we call 'Earth'.

How big does something have to be to have a noticeable gravitational field? Well, that's a tough one. How big does a magnet have to be to have a noticeable magnetic field? I can magnetize a sewing pin in a couple of seconds, but that stuff is several orders of magnitude stronger than gravity.

Gravity is phenomenally weak. If you were in a zero G environment, I don't think you'd be able to notice the gravitational attraction between your body and that sewing pin.

My father told me of an experiment that they conducted in his physics class at RPI back in the 1960's. 2 masses (iron? lead?) were suspended from very thin wires from the loft of the bell tower. Something like 5 or 6 stories tall. The wires were positioned such that the masses were very close together, but not touching. They measured the distance between the two masses. Over the next several days they made several more measurements. He claims that after a while there was a statistically noticeable decrease in the gap between the masses. But to be honest I'm skeptical. Gravity is, fundamentally, extremely weak.

 

[madviking]

My father told me of an experiment that they conducted in his physics class at RPI back in the 1960's. 2 masses (iron? lead?) were suspended from very thin wires from the loft of the bell tower. Something like 5 or 6 stories tall. The wires were positioned such that the masses were very close together, but not touching. They measured the distance between the two masses. Over the next several days they made several more measurements. He claims that after a while there was a statistically noticeable decrease in the gap between the masses. But to be honest I'm skeptical. Gravity is, fundamentally, extremely weak.

That's a two hundred year old experiment.

 

[Xanikk999]

Does anyone know a good website that explains general relativity and spacetime in easy terms? Even with wikipedia I am confused since I am not familiar with the subject.

 

[IdiotsOpposite]

Does anyone know a good website that explains general relativity and spacetime in easy terms? Even with wikipedia I am confused since I am not familiar with the subject.

Wikipedia won't help you. I'm not sure what will, but if you asked this same question in Physics Forums, they might be able to help you.

 

[Xanikk999]

Ok sorry here is another one that always confused me. Two questions, first one is more important to me.

If light and all other electromagnetic radiation has no mass, how come it is subject to the incredible gravitational pull of a black hole and cant escape its gravity and wouldn't that make any black hole invisible?

 

[madviking]

Link to video.

 

[Xanikk999]

That was very helpful and informative for me, thanks for the video!

 

[SS-18 ICBM]

Why are most crystals of organic compounds white? What is it about carbon bonds that reflect all visible light instead of absorbing it?

 

[member66170]

That question, I guess, lies deep within quantum mechanics and is best left alone.

In short - something to do with the electrons having lots of closely packed energy level in the range of the visible spectrum. But thats more of a restatement of the question than an actual explanation

 

[Bartleby]

The colour of organic compounds is related to the absorption of visible light. To absorb visible light requires extensive conjugation, more extensive than you find in typical organic molecules.
If an organic molecule has sufficient conjugation it will be coloured in the crystalline state.

 

[Leifmk]

My father told me of an experiment that they conducted in his physics class at RPI back in the 1960's. 2 masses (iron? lead?) were suspended from very thin wires from the loft of the bell tower. Something like 5 or 6 stories tall. The wires were positioned such that the masses were very close together, but not touching. They measured the distance between the two masses. Over the next several days they made several more measurements. He claims that after a while there was a statistically noticeable decrease in the gap between the masses. But to be honest I'm skeptical. Gravity is, fundamentally, extremely weak.

This sounds like a slightly miscommunicated version of an experiment which has been done by legions and legions of undergrads (including myself, back in year 2 or thereabouts). Yeah, I've measured the gravitational attraction between objects small enough to lift in one hand. Nothing to be skeptical about. Don't need a bell tower either. What you need is something (i.e. a couple of small lead balls on a stick) hanging suspended from a wire of a material where you know the torsion needed to twist the wire a given amount, then you place something heavy (i.e. bigger lead balls) nearby. You have mounted a reflector on the wire and are bouncing a laser beam off said reflector onto the wall a few meters away; when the wire twists, the laser spot on the wall moves. The rest is basically trigonometry.

 

[peter grimes]

Yeah, I'm sure it was miscommunicated - both on his end as well as on mine. He was indeed in an undergrad program. He was studying Architecture, and this class was required for the engineering component. And now that you're all mentioning the original (i read the wiki), I remember that he said they projected focused light through the gap, and measured the spread against the back wall (magnifying it, obviously).

My was only that it's possible to see gravity in action, but it's so weak you have to jump through a bunch of hoops in order to see the signal.

 

[member66170]

It's very easy to see gravity in action. Just drop something

 

[IdiotsOpposite]

It's very easy to see gravity in action. Just drop something

I dropped a balloon. It went up.

GRAVITY IS REPULSIVE!!! (for balloons)