More Physics Fun

[johnjohn320]

(If anyone knows of a better place I can ask for help, please let me know, I feel guilty starting these things so much).

This should be easy, I just can't figure out what I'm missing.

An artillery shell is fired with an initial speed of 1.70 x 10^3 m/s at an angle of 55 degrees above the horizontal and returns to its original level before impact. Neglecting air resistance find (a) the time it is in motion and (b) the horizontal distance traveled.

I'm lost. Every thing I try I seem to come up with one too many unknown variables. Please help!

 

[ClueLis]

The key is to break the displacement vector into components, then take the horizontal velocity as a constant, and the vertical velocity has negative acceleration due to gravity. Find out how long it takes for gravity to pull the shell to the ground.

 

[esun]

vx = 1.7E3*cos 55 (I'll use E notation to mean 10^n)
vy = 1.7E3*sin 55 - 9.8t (v = v0 + at)

To find the time, set vy equal to zero and solve for t. Then double that result (v = 0 at the top of the path, and it takes the same amount of time to fall back down). So:

t = [(1.7E3*sin 55)/9.8]*2

Horizontal distance traveled is just: x = vx*t

So: x = 1.7E3*cos 55 * t (see above for t)

And that's it.

 

[Lithium381]

trajectories in physics were one of my favorite things!!!! pretty much the help i could have given you is here....break it into it's seperate parts, horizontal and vertical, calculate vertical over time(using gravity) to get time, and then use that with the horizontal speed!

 

[FrustratedUser]

Woah!

277km travelled.
284 sec in air.

 

[Tom]

why bother including gravity and igonoring air resistance ? I never understood this about physics. Why is ok to ignore stuff that is really there just for the sake of solving a problem that can't happen in the real world ? Why not just ignore gravity too ?

 

[DrPizza]

because if you ignore gravity, it never comes down.
It's a lower level physics course... The math is much more advanced when you include air resistance. (it's no longer a matter of plugging a few numbers into an equation). Also, for dense spherical objects, at the speeds they're talking about in the problems, air resistance isn't really that significant. If you doubt that, drop some heavy objects from a 5 story building. Gravity is the same for all of them, but the air resistance is going to vary, based on the objects size, shape, texture, density, etc. If you choose objects such as watermellons, cantelopes, honeydews, bananas, cans of soda, gallons of milk, you should realize that the air resistance will be different for each of those objects. However, if released simultaneously, they'll hit the ground at pretty close to the same instant. I videotaped this over the summer for my physics class... cool stuff to watch.

 

[Tom]

Yea, but we're talking about an artillery shell going fast, air resistance is a big deal.

 

[johnjohn320]

OK I know I'm just a moron, but I have trouble breaking it into horizontal and vertical. I'm in trig so I understand trig fucntions, why can't I do this?

 

[oog]

draw a right triangle with the initial trajectory being the along the line of the hypotenuse. the angle of the initial trajectory is one of the acute angles of the triangle. the two other sides of the triangle are the cosine and sine of that angle.

 

[DrPizza]

Originally posted by: Dead Parrot Sketch
Yea, but we're talking about an artillery shell going fast, air resistance is a big deal.

Yeah, I guess you're right... I didn't look at the starting velocity in the original problem.

 

[glugglug]

Initial velocity in the vertical direction is 1.70x10^3 m/s * sin(55°). Divide by g to get time going up, then multiply by 2 to get total time in motion.

From this figuring out distance should be easy.

 

[datalink7]

Don't forget to factor in the rotation of the earth!

 

[Yossarian]

Originally posted by: Dead Parrot Sketch
why bother including gravity and igonoring air resistance ? I never understood this about physics. Why is ok to ignore stuff that is really there just for the sake of solving a problem that can't happen in the real world ? Why not just ignore gravity too ?

because this level of physics is about understanding the basic concepts of falling bodies and moving objects. air resistance makes the problem much more complicated since the force varies along the object's path. if you're gonna include air resistance, may as well add wind, temperature, air pressure gradients, airborne particles, imperfections in the shell casing, shell rotation, etc. etc. etc.

 

[RaynorWolfcastle]

johnjohn, based on the this and the other thread you posted about physics questions, I recommend you either get yourself a tutor or RTFM

 

[Mingon]

I can give you an excel sheet if you wish some old assignment of mine

 

[RossGr]

Physics Help fourms.

 

[johnjohn320]

Originally posted by: RaynorWolfcastle
johnjohn, based on the this and the other thread you posted about physics questions, I recommend you either get yourself a tutor or RTFM

What's RTFM? You can't get a tutor for AP courses.

Anyway, I basically understand the stuff, there's just like one step I don't get. 'Preciate the advice though.

 

[Howard]

Originally posted by: datalink7
Don't forget to factor in the rotation of the earth!

Is there a formula for that?

 

[Gibson486]

If you are lost on this, just wait till you hit physics 3 (Gausses law, Amperes law). Man, Physics 3 made me cry. Anyways, if you cannot do a trajectory problem, then go to a tutor. The key is understanding vectors. You will be using vector all throughout physics, so get make sure you know how to do them.

 

[johnjohn320]

Originally posted by: Gibson486
If you are lost on this, just wait till you hit physics 3 (Gausses law, Amperes law). Man, Physics 3 made me cry. Anyways, if you cannot do a trajectory problem, then go to a tutor. The key is understanding vectors. You will be using vector all throughout physics, so get make sure you know how to do them.

I'm not going to take Physics 3. Hell, I didn't even wanna take this.