Bike Forums > Foo > Help with physics problem? THanks! :)
 Register All Albums Elite Membership Forum Rules Calendar Search Today's Posts Mark Forums Read

 Foo Off-Topic chit chat with no general subject.

 09-26-05, 01:45 AM #1 ovoleg Powered by Borscht Thread Starter     Join Date: Jul 2005 Location: SoCal Bikes: Russian Vodka Posts: 8,344 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Help with physics problem? THanks! :) IF anyone can help with this physics problem, I will owe you big time I tried this problem with someone else and they couldn't figure it out either. The answer is :.310m "A flowerpot falls off a windowsill and falls past the window below. You may ignore air resistance. It takes .420s to pass this window, which is 1.90 m high. How far is the top of the window below the windowsill from which the flowerpot fell?" I tried, x=x0+v0(t)+1/2(a)(t)^2. v=v0+at ... Its some simple problem which is a PITA. I have the answer I just can't find a way to get to it. THanks anyone
 09-26-05, 03:16 AM #2 Nicodemus Feral Member     Join Date: Apr 2004 Location: Roma, Italia Bikes: yes, I have one. Posts: 2,666 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 16 Post(s) x=x+vt+0.5at^2 should be used - but remember that the starting velocity of the journey for which the numbers are given is not 0. You need to calculate what that velocity was, and then use that to get your answer. journey1 = sill to top of window journey2 = top to bottom of window Journey 2 x2=bottom of window x1=top of window v=velocity at top of window t=time to travel across window x2=x1 + vt + 0.5at^2 => you get v=2.4658 (start velocity of journey 2 = final velocity of journey 1)... Journey 1: So now you can use this value of v to calculate the amount of time it took to get there: v=at => you get t=0.2516 This is the amount of time in journey1. Now you can subtitute directly into the old favourite... x=0+0+0.5at^2 = 0.310
 09-26-05, 03:24 AM #3 DannoXYZ  Senior Member     Join Date: Jul 2005 Location: Saratoga, CA Bikes: Posts: 11,633 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 23 Post(s) Here's how I did it, you have to set up the problem into multiple parts first. Draw a picture and it makes sense. You're trying to solve it all in one equation and that's not possible because you have to solve for multiple variables, thus multiple equations are necessary. Here's a picture: The various equations are (assuming constant acceleration): 1. average Velocity across window = dX/dT = 1.90m/0.420s = 4.524m/s 2. average Velocity across window = (V1+V2)/2 Solving for V1 we get (V1+V2)/2 = 4.524m/s V1 + V2 = 9.048m/s V1 = 9.048m/s - V2 3.Then solving for acceleration across 1.90m we get: average Acceleration across window = dV/dT = (V2-V1)/0.420s V2 = V1 + at = V1 + (9.8m/s^2 * 0.420s) = V1 + 4.116m/s V2 = V1 + 4.116m/s Combining these last two equations we get: V1 = 9.048m/s - V2 V2 = V1 + 4.116m/s --------------------- V1 = 9.048m/s - (V1+4.116m/s) V1 = 9.048m/s - V1 - 4.116m/s 2V1 = 9.048m/s - 4.116m/s = 4.932m/s V1 = 2.466m/s is velocity at top of lower window. 4. Pluging into V=at for velocity over constant acceleration we get: 2.466m/s = 9.8m/s^2 * t t = 0.252s time to fall to top of lower window 5. Pluging into d=1/2at^2 for distance over time w/constant acceleration we get: d = 0.5at^2 = 0.5 * 9.8m/s^2 * 0.252s^2 d = 0.310m (don't round off numbers until final step) Last edited by DannoXYZ; 09-26-05 at 06:33 PM.
 09-26-05, 08:16 AM #4 Cyclaholic CRIKEY!!!!!!!     Join Date: May 2005 Location: Sydney, Australia Bikes: several Posts: 4,269 Mentioned: 23 Post(s) Tagged: 0 Thread(s) Quoted: 354 Post(s) ....depends on what species of plant it is
 09-26-05, 09:34 AM #5 ovoleg Powered by Borscht Thread Starter     Join Date: Jul 2005 Location: SoCal Bikes: Russian Vodka Posts: 8,344 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) omg thanks so much guys!!! I think I put in 2hrs trying to solve this. I wasted the first hour because I misread the question, I thought the total distance traveled took .42s. Thanks guys!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 09-26-05, 02:25 PM #6 cycleprincess NEVER WALK A HILL     Join Date: Aug 2004 Location: Texas Bikes: Bianchi Volpe Specialized Dolce Vita Multi Sport Posts: 1,184 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Why did I even open this thread...it is soooo over my head. I think a flowerpot landed on my head in college...or maybe it was what I was smokin that was IN the flowerpot!
 09-26-05, 02:30 PM #7 ovoleg Powered by Borscht Thread Starter     Join Date: Jul 2005 Location: SoCal Bikes: Russian Vodka Posts: 8,344 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) After I got the right problem down, I was thinking with the same sense as Danno but I didn't set up the right equations. Thanks so much, I actually got to a point where I thought the answer was 1.03 and thought the teacher typo'ed and meant 1.03 instead of .310 because .31 is only the significant figure and to write .310 doesn't make any sense. Thanks again!
 09-26-05, 11:09 PM #8 Cyclaholic CRIKEY!!!!!!!     Join Date: May 2005 Location: Sydney, Australia Bikes: several Posts: 4,269 Mentioned: 23 Post(s) Tagged: 0 Thread(s) Quoted: 354 Post(s) This can all be avoided if you stick with ground floor appartments.