# Search results

1. ### F=ma, but Gravity is a Force with no mass?

The force will vary according to mass. So a 10-pound bowling ball will have twice the gravitational force ("F" in your equation) as a 5-pound bowling ball. But they will both have the same acceleration ("a" in your equation). Gravity follows the same laws everywhere, but the actual...
2. ### Initial Conditions in Classical Physics

IMO, Yes. But I don't quite get what you are saying. If anything determinism is a theory that cannot be proven empirically because of being unable to measure all conditions.
3. ### Why do colored freezy pops turn clear when left outside in the cold?

Why is it that a thread called "what is time?" got hundreds of replies, when my legitimate scientific inquiry gets one? :cry:
4. ### Why is the kilogram the base unit of mass, instead of the gram?

I still would think kg are used more often in speech and informal discussions. I guess there's no way to be sure though - I doubt there are any studies on this. Besides, what we really should be talking about is how people tend to use time units like this: microsecond < millisecond < second...
5. ### Why do colored freezy pops turn clear when left outside in the cold?

So this last winter, my roommates and I left a bunch of freezy pops outside to get cold fast (the temp was -20F). They were in clear plastic and may have been exposed to the sun. After about a day, we went to pick them up, and they had all turned clear. They stayed clear even when brought...
6. ### Why is the kilogram the base unit of mass, instead of the gram?

A kilogram just has more everyday uses than a gram. Most non-scientists don't talk about grams of this or grams of that.
7. ### River flow on a planet or large object.

I think what he's saying is that a larger sphere could accomodate a larger river. In a way I think we are both wrong, in that the river doesn't think anything. Its molecules are just obeying the forces at hand.
8. ### River flow on a planet or large object.

A larger sphere is more flat at the surface. If a planet were 10m in diameter, I think the river certainly would be 'concerned' about the curvature!
9. ### River flow on a planet or large object.

Ultimately the water will only need to flow from a high gravitic potential to a low one. Since I get to decide exactly what the planet is made of, my planet is similar in proportions to a RAZR phone. It is very thin, about 50m wide, and inifinitely long. The imaginary material goes from being...
10. ### Are all atoms considered to be isotopes?

I believe so.
11. ### Does mosquito fly >80km/h?

Generally speaking, anything flying in the air has a top speed relative to the air.
12. ### Time for two point masses to collide under gravity

I took DiffEq a few years ago. Maybe you could help by pointing out the first mistake I made? This is how I brush up on my skills, by posting on PF, I'd rather not retake any courses.
13. ### Time for two point masses to collide under gravity

I want to do this with integration. Where did I go wrong in setting up my integral? F=G\frac{m_1m_2}{r^2}=m_1a_1=m_2a_2 a_1=G\frac{m_2}{r^2}, a_2=G\frac{m_1}{r^2} a_{total}=G\frac{m_1+m_2}{r^2} d=\frac{1}{2}at^2, d=r, a=\frac{2r}{t^2} \frac{2r}{t^2}=G\frac{m_1+m_2}{r^2}...
14. ### Time for two point masses to collide under gravity

It's not that I "don't even bother to check units", I just happened to not do so in this case. This wasn't an academic exercise where I'm obligated to be extremely thorough. It was me playing around and brushing up on math during my lunch break over a chicken sandwich. Thank you for the...
15. ### Time for two point masses to collide under gravity

Please, explain what happened to me. Sorry I got the units wrong - no need to get snappy about it.
16. ### Time for two point masses to collide under gravity

Are you saying my original answer is correct, or are you referring to someone else here? Sorry :) And if my answer is correct, I prefer it over anything with pi in it.
17. ### Time for two point masses to collide under gravity

As a thought experiment, I have attempted to calculate the time it takes two point masses to collide solely due to the force of gravity. The final answer I got was this: t=\frac{R^\frac{5}{2}2\sqrt{2}}{5\sqrt{G}\sqrt{m_1+m_2}} Where G is the gravitational constant, R is the distance (at...
18. ### The movement of hockey pucks on an air hockey table.

It depends on the size and shape of the pucks. If they are the exact same size and shape, but one is heavier, then it will reach the end faster. The reason is because it will be more aerodynamic relative to its mass. You can think of it similar to a dropping a bowling ball and a playground...
19. ### Teen uses Fibonacci sequence to make solar energy breakthrough

Sounds like a load of crap. I guess there really is no accountability in journalism these days.
20. ### What would be the force in pounds of 25,000 lbs. moving at 40fps

Look, I'm not trying to be a jerk, but you are being extremely lazy. You could have typed the conversion into google in the time it took you to ask me about it. We aren't a personal answer service. This forum focuses on learning. http://tinyurl.com/3dku84f
21. ### What would be the force in pounds of 25,000 lbs. moving at 40fps

Good thing I'm here to type things in for you. http://www.wolframalpha.com/input/?i=%282500+pounds+in+kilograms%29*%2840+feet+per+second%29%2F%280.1+seconds%29
22. ### What would be the force in pounds of 25,000 lbs. moving at 40fps

You can get an extremely rough estimate by guessing at the time of contact, and using Force = mass * acceleration (in this case, deceleration). For concrete blocks, this could be around 0.1 seconds.
23. ### A taut rope pulled - delay on the other end?

To add to what rcgldr said (which is correct), that speed of sound is determined by the electromagnetic forces holding the atoms together, so the speed of light is a theoretical upper-limit on the speed of sound. The tensions felt at point B would simply be felt at different times. You can...
24. ### Can't Understand t 2

Maybe we should have a "Recognition" called "Thread Necromancer".
25. ### Invisible paint? is it possible?

I don't know if it's possible, but I'm pretty sure it doesn't exist. Even the most "invisible" of luminous paints can easily be seen reflecting light and creating glare.
26. ### Physics of a Syringe

This is what tiny-tim was saying above, just in different words. The air pressure from outside the syringe is too great compared to the pressure inside the syringe for it too fall out.
27. ### Physics of a Syringe

Actually, I believe it is more complicated than that. Water tends to stick to the walls of it's container - this is why you see the edges in a glass of water climbing up the sides a bit. And because of surface tension, it doesn't like to form really 'small' volumes - this is why if you spill...
28. ### What is energy

If you ask my crazy ex-girlfriend, she'll tell you it's the feeling you get when you have Jesus in your soul!
29. ### Longest hangtime for vertically-propelled rocket?

You have the right idea - shoot it at an angle. Another option is to have some sort of parachute mechanism. Beware though, that launching with compressed fluid (air, CO2, etc.) tends to be rather unpredictable.
30. ### Hi,I'm having difficulty trying to figure out the following

Just a tip: sometimes it's easier to plug in fake numbers than to work it out symbolically.