Homework Help: physics

Posted by physics on Friday, July 31, 2009 at 10:56pm.

so I got a question about projectile motion

take the example of shooting of a plastic air rocket into the air and asked to predict its landing point and how high it goes...

ok I know how to do this...

the thing is now were not neglecting air resistence and I don't know what to do...

I know how to prove the derivation of this equation for distance traveled in the x direction

X = Xo + Vo t + 2^-1 a t^2

ok now sense we were once negelcting air resistance I was always told to use this equation because the velocity stays the same and therefore there is no acceleration

X = Xo + Vo t

because the acceleration is zero taking out that whole set of terms

ok now lets not neglect air resistance...

I know I would have to use every component of this equation except for the Xo which we can let be zero

X = Xo + Vo t + 2^-1 a t^2

take out zero term

x = Vo t + 2^-1 a t^2

ok now what???

I obviously have to do something else...
How do I find the acceleration???

We are given the launch angle but have to derive the launch velocity :(

how does the drag force effect motion in the y direction???

I got to use the same equation I know but don't know how to find the acceleartion

also what's the range equation it's something like

x = f (v, /0)

were the "/0" is like a o with a slash mark thingy with it... what does this symbol represent and how do I derive this equation (I just need the start) and what does teh f represent

physics - physics, Friday, July 31, 2009 at 11:00pm
also I was always told to use this equation when the velocity is small through the medium

Fd = -bv

were this is just a proportion

also that other formula that is suppose to be used when there is a high velocity

Fd = 2^-1 p v^2 A Cd v

Ok what is exactly meant by high and low velocity and how do you derive the equation for a high velocity all I've been told was the equation never taught how it was derived and dont know were to start
physics - physics, Friday, July 31, 2009 at 11:03pm
I just would like to make sure of something

an object experiances a drag force when it travels through a medium like air or water...

so dosen't every thing experience a drag force that is in motion because its moving through air??

So like if a person was walking do they experiance a drag force?

I would think so... just making sure
physics - drwls, Friday, July 31, 2009 at 11:17pm
There is always a drag force when a body moves through a fluid, except in some rare exotic situations involving superfluids like liquid helium. To treat the drag force, you need to know how it varies with velocity. At very slow speeds the resistance is proportional to velocity, but at "normal" speeds it varies with the square of velocity. The drag force needs to be included in the differential equation of motion. (calculus is needed)

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