View Full Version : momentum vs. inertia


On Radioactive Waves
02-26-03, 12:40 AM
I'm retaking mechanics. Last time, I found these concepts a little confusing.

Could some explain to me the difference between momentum and inertia?

I recall a problem of 2 balls of mass a and b with velocity a and b respectivly. The two had the same momentum , but different inertia. The small ball had more inertia. Is this due to the relationship of V squared?

lethe
02-26-03, 12:50 AM
inertia is the measure of how much resistance matter has to acceleration. the more inertia something has, the less it wants to respond to forces and accelerate. this statement is mathematically stated a = F/m. newtons s second law. m is the measure of inertia, called (inertial) mass. the more force, the more acceleration. the more inertia, the less acceleration.

momentum is the product of inerta (m) and velocity (v). p=ma. it turns out that momentum is always conserved in any closed system. momentum is related to inertia, as you can see. the more inertia something has, the more momentum it has, when in motion.

you can think of momentum as the ability to exert a force for a time. that product, Ft is called impulse. the amount of impulse a body is able to exert is exactly the same as the amount of momentum it has.

that v square thing you mentioned has to do with kinetic energy. it is closely related to momentum (and in fact, in SR they are the same), but it is subtly different.

whereas momentum is mv, kinetic energy is 1/2mv<sup>2</sup>. whereas momentum is the ability to exert a force for a time (impulse), energy is the ability to exert a force for a distance, Fd. this is called work.

energy is also conserved.

Edit: oops. newton s second law. not first

Persol
02-26-03, 12:57 AM
Originally posted by On Radioactive Waves
I'm retaking mechanics. Last time, I found these concepts a little confusing.Could some explain to me the difference between momentum and inertia?I recall a problem of 2 balls of mass a and b with velocity a and b respectivly. The two had the same momentum , but different inertia. The small ball had more inertia. Is this due to the relationship of V squared?
Momentum is the m*v equation.
KE will be the (mv^2)/2

Are you taking Dynamics now... or do they wrap it all into one class?

Persol
02-26-03, 01:11 AM
Woops :eek:
Deleting my mistake... shhh... don't tell anyone;)

On Radioactive Waves
02-26-03, 01:27 AM
dynamics? not sure exactly what that is. if you could clarify please.


speaking of Newton's laws, I vowed to attempt to make my astronomy professor derive any equation he brings up in class since he derived the radial velocity from doppler shift equation. today he somewhat offered to derive Newtons third law, of course I demanded he do so. Took him the remaining hour to do it, and he didn't finish :( but I guess I saved a few people who didnt care from taking notes today.

Persol
02-26-03, 01:30 AM
Originally posted by On Radioactive Waves
dynamics? not sure exactly what that is. if you could clarify please. It's the engineering class where all 'dynamic' problems (non-steady state) are handled. What are you studying for?

On Radioactive Waves
02-26-03, 01:30 AM
That was a beautiful explanation. The explanation my book gave left me somewhat confused, although that was a year ago. When we were covering those topics is about the time I decided to withdraw from the class because I was getting in over my head. Well this time I'm coming back with mre math skills, so look out.

On Radioactive Waves
02-26-03, 01:35 AM
persol:

I started out wanting to study chemistry. I have been through g-chem, and as of now I dont know if Ill take o chem before transferring to a university. I want math and physics under my belt.


What I really wanted to do, and I thought of this aout 6 months before I saw it start happening. I wanted to use holograms to engineer chemicals. This technique started being used about a year ago to make nanobots via the interference pattern.

I'm also interested in herbal medicine ( not just :m: kind).

So I guess my dream is to be a Nuclear-physical-biochemical-engineer doctor!:D

edit: no, i dont think its dynamics. just a bottom of the barrel calculus based physics class. I dont consider the elementry (non calculus based) physics class as such.

Persol
02-26-03, 01:37 AM
Originally posted by On Radioactive Waves
I wanted to use holograms to engineer chemicals. This technique started being used about a year ago to make nanobots via the interference pattern.
That sounds pretty cool... never heard of it before

So I guess my dream is to be a Nuclear-physical-biochemical-engineer doctor!:D
I hear the job market is kinda tight for them :D

On Radioactive Waves
02-26-03, 01:40 AM
my plan was to be the maket... before some else.


once i saw this concept I thought of happening, I quickly realised I needed to seriously hit the books before too many people beat me to it.

James R
02-26-03, 01:40 AM
He <i>derived</i> Newton's 3rd law?

I thought Newton's third law was an assumption, supported by experiment. Or, more properly, its accuracy is guaranteed by the way words like "force" are used.

How on Earth did he derive it? Where would you start?

Persol
02-26-03, 01:46 AM
Originally posted by James R
How on Earth did he derive it? Where would you start? He could have derived it from KE, but that should've only taken a minute... but then I can't think of a way to derive KE besides using the 3rd law

GundamWing
02-26-03, 01:49 AM
Newton's Third Law (refresher):
>> http://www.aloha.com/~isaac/3laws/3mid.htm

Oh what the hell (might as well put this one up, its good)
>> http://www.mty.itesm.mx/decic/deptos/f/fisica/Dibujos/Newton-4law.pdf

While we're at it...
>> http://www-istp.gsfc.nasa.gov/stargaze/Sintro.htm

On Radioactive Waves
02-26-03, 01:57 AM
Well, assumptions were made, but here is the jist of it.

he started with a picture of 2 bodies, R being the distance between them.

r1+r2=R , and the point inbetween them being the bary center.

then he went on to anylize how the forces in and out on m1 and m2 must be equal for the condition of stability.


from rate=distance/time , t=d/r P=period


P=(2 pi r)/v


F*out=F*in
F*out=(( MV )^2)/r

(m1*v1^2)/r1 = (m2*v2^2)/r2

r1*m1=r2*m2 for stability

then he started squaring equations to use to substitute into other equations. he never finished, and my notes are somewhat confusing because he was all over the board changing equations and substituting them back into previous equations. but basicly everything he did came from simple observations of the system ( and knowing where to substitute which equations)

so, sorry if this is confusing. he said for his last class he did it on a sheet of paper and handed it out, and that he would find it and give it to us. He was doing it off the top of his head so it was sorta real confusing, plus i was on some cold medicine and was not following to the best of my ability. but if you'd like, I'll do it clearly once i get the paper and see just how its done.

On Radioactive Waves
02-26-03, 02:00 AM
ummm, like i said, i was cloudy headed this morning. Maybe it was newton law of gravitation he was deriving

or maybe it was one of kepplers laws?

I know theres something i'm missong here. That will be clarified soon though, next time i see him.

RDT2
02-26-03, 05:34 AM
--------------------------------------------------------------------------------
Originally posted by On Radioactive Waves
dynamics? not sure exactly what that is. if you could clarify please.
--------------------------------------------------------------------------------

It's the engineering class where all 'dynamic' problems (non-steady state) are handled. What are you studying for?

Not quite. Basically, mechanics can be divided into two branches - statics and dynamics (it can be divided other ways, but this is a common split). 'Statics' is about things that don't move, 'dynamics' is about things that do (whether steady state or not). In statics, mass isn't important (other than as a dead load) - in dynamics it is, and inertial terms (m*a) appear in the equations.

Cheers,

Ron.

On Radioactive Waves
02-26-03, 06:38 AM
well in that case I'm most definatly taking dynamics. Silly me, yes this course is to prepare for engineering.

RDT2
02-26-03, 08:01 AM
Originally posted by On Radioactive Waves
...this course is to prepare for engineering.

Don't do it! You'll end up poor, unloved and misunderstood my most of society. You'll also be blamed for every disaster that happens. Do history instead.

Cheers,

Ron (a Mech Engineer).

Persol
02-26-03, 08:05 AM
Originally posted by RDT2
'Statics' is about things that don't move, 'dynamics' is about things that do (whether steady state or not). We broke it down slightly differently... steady-state bodies were included in statics for us.

On Radioactive Waves
02-26-03, 08:07 AM
hmmm... i'll have to think about that one. I want to be my own special type of engineer ( refer to one of my above posts)

Persol
02-26-03, 08:07 AM
Originally posted by RDT2
You'll end up poor, unloved and misunderstood my most of society. You'll also be blamed for every disaster that happens. Do history instead.
If you are going to be poor you might as well do something which has application outside of academia :p


John (also a Mech Engineer). :)

GundamWing
02-26-03, 08:23 AM
Originally posted by On Radioactive Waves
hmmm... i'll have to think about that one. I want to be my own special type of engineer ( refer to one of my above posts)

After continuous involvement in this forum, you will have all the skills necessary to build impossible machines, and will be a very special engineer indeed. ;)

Persol
02-26-03, 08:25 AM
Originally posted by GundamWing
and will be a very special engineer indeed. The cool thing is that they send a short bus to pick you up every day...

(non-americans might not get that one)

On Radioactive Waves
02-26-03, 03:13 PM
I'm assuming it comes with its own "special" stop also?