Mechanics Question (1 Viewer)

[TheStar]

Resisted Motion - Mechanics

Hello guys, I've just started resisted motion.

I'm having a little trouble with:
ma = -mg - kv^n

The issue I've got is - WHY the negative sign?

I just did a quick resisted motion question I made up, to investigate this negative sign.

I found the relationship between velocity, mass, resistance, and time for a maglev train.
For this I used ma = -kv (Resistance in air is directly proportional to velocity, and a maglev train _leviates_)

However, I found out that if I use the above equation, k itself becomes negative. I might as well just use ma = kv. It makes more sense to me if k is possitive, as the sign already reflects the 'direction' of the force.

As you can tell, I'm confused on this, so could anyone offer suggestions as to deriving the correct equations? (THat is, the _why_ we use ma = -mg -kv^n and ma = kv^n - mg)

Finally, I've been on the internet, and there is practically NOTHING there on this topic, other than my textbook, does anyone happen to have any good mathematics sites on _this_ topic?

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Wow, how's that for my first post

 

[ezzy85]

If the object is dropped, gravity is acting iwth it. so theres a force of +mg thats helping it. but there is also a resistance force thats notletting it go down smoothly. its in the opposite direction. so its -mkv. so the sum of the forces would be: ma = mg - mkv. ie. mx^.. = mg - mkv and you can cancel the m's.

if the objects thrown vertically up, its going against gravity. so this time mg is a resistive force, opposite in direction to where its being thrown and thus is -mg. so whenever theres a force in the opposite direction, its -ve

 

[TheStar]

Hmm, that's _much_ clearer, thanks ezzy85.
However, your saying that force due to resistance is proportional to the mass _and_ the velocity?

My understanding was that resistance was proportional to _only_ "Some power of it's velocity"? In this case, how does the mass cancel out?

(In the mean time, I might actually be able to answer the excercises now )

 

[ezzy85]

Originally posted by TheStar
Hmm, that's _much_ clearer, thanks ezzy85.
However, your saying that force due to resistance is proportional to the mass _and_ the velocity?

npz . with that mkv, thats just an example. im not sure what the value is and usually the question will specify what the resisive force is.

 

[Affinity]

hmm most are uni sites, but don't freak out.
try searching for classical mechanics

here's one, I didn't really look at the contents though

http://astrowww.phys.uvic.ca/~tatum/classmechs.html

 

[McLake]

Split this thread out ...