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Help with my ?

  1. Jun 8, 2005 #1
    Help with my ???

    Hello! I am new to this site and I didn't know that their was online help for physics. I don't want anyone to do my work first off, I WOULD LOVE HELP to understand it better. I am currently enrolled in Physics and I am having problems with understanding it fully. I can get half way and then I just go blank and I don't understand why..so i just would really appreciate if someone can give me a better understanding.....for instance i have two problems that are confusing me...

    1. I am currently doing a problem on freely falling objects...my question is do i convert first before trying to solve it completely because my teacher said that if we convert first then we should not get stuck, but the fact is which one should i convert...such as if a ball is dropped out of a 120 cm window and it takes it 0.125 s to hit the bottom from the top and then it collides with a horizontal sidewalk and reappears at the bottom of the window 2.0 s after passing it..and my solution that i am looking for is how tall the building is. So far I started to convert the 120 cm maybe to m, because when it said how tall i know that it will come out to m because it is asking for the height of the building or either it can be in ft. So i got stuck afterwark because i don't know which formula i need to use after that such as, v=v0t-1/2 gt^2. i might be wrong, but i'm stuck so can someone please help me on that understanding.
     
  2. jcsd
  3. Jun 8, 2005 #2
    1. I am currently doing a problem on freely falling objects...my question is do i convert first before trying to solve it completely because my teacher said that if we convert first then we should not get stuck, but the fact is which one should i convert...such as if a ball is dropped out of a 120 cm window and it takes it 0.125 s to hit the bottom from the top and then it collides with a horizontal sidewalk and reappears at the bottom of the window 2.0 s after passing it..and my solution that i am looking for is how tall the building is. So far I started to convert the 120 cm maybe to m, because when it said how tall i know that it will come out to m because it is asking for the height of the building or either it can be in ft. So i got stuck afterwark because i don't know which formula i need to use after that such as, v=v0t-1/2 gt^2. i might be wrong, but i'm stuck so can someone please help me on that understanding.


    In general you wanna keep things metric, do you even know how to convert cm to feet? I hope not. Also, all these formulas for motion generally need everything in meters, seconds, and whenever you get to masses, kg. If you mix and match you're gonna get it wrong. Now next step, what do you know, what's given in the problem?

    I really didn't follow what it was asking from your description
     
  4. Jun 8, 2005 #3
    He won't get it wrong if he mixes units, he just will have an answer in non standard units. For example, a velocity in feet per minute is still avelocity. If you are a student though, or publishing a paper, you are almost certainly expected to report in MKS or SI units
     
  5. Jun 8, 2005 #4
    Oh, right. For all intents and purposes though, do it on an assignment in high school or college and you'll probably get it wrong
     
  6. Jun 8, 2005 #5
    Well, the height of the building is needed. The height of the window was given and the time it took to reach the bottom of the window and it took 2.0 s for the ball to return from the ground to the bottom of the window. First, I need to find the initial velocity and i used the formula vo-gt, because i have the time and g which equals 9.80m/s and i * those together to come out with the intial velocity so i can plug that into y=vot-1/2gt^2 if i am correct. I think i do need to convert the cm to feet which i did, but is that the correct formula i need to get the height of the building.
     
  7. Jun 8, 2005 #6
    You should use meters, seconds, and kilograms ofr all your units. You have hte correct equations to solve the problem.
     
  8. Jun 8, 2005 #7
    I know that I will use meters for the unit of the height of the building. My initial velocity has to be in m/s correct because that is what i did, but when i get that i do plug that into y=vot-1/2gt^2 to get the height of the building. I converted the 120 cm to feet and got 3.94 ft. I think I did something wrong. Then first before i converted i did do Vo-gt because i know g and t, but not the initial velocity. so i got 0-9.80 m/s * 0.125s=-1.23 m/s^2. On the other hand by me converting the 120 cm to feet then would my unit be ft or m/s.
     
  9. Jun 8, 2005 #8
    THAT'S where mixing and matching will screw you

    you converted the height of the window to FEET, yet used g as 9.8METERS/second
     
  10. Jun 8, 2005 #9
    i converted the height of the window to try to get the intial velocity. i got 3.94 ft so now i need to convert to m/s to get the initial velocity and that's where i get stuck because i'm confused on which formula i suppose to use.
     
  11. Jun 8, 2005 #10
    here's a little quick guide on which formula to use. I have these memorized and write them down immediately when I get to a test or something:

    Use this Equation....................When this is missing and you don't need it

    v = v0 + at............................X - X0

    v^2 = v0^2 + 2a(X - X0).........t

    X - X0 = v0t + (1/2)at^2.........v

    X - X0 = vt - (1/2)at^2...........v0

    X - X0 = (1/2)(v + v0)t............a

    It's easy to remember because if you look down the chart, the first 2 are V's and the last 3 are X's, and the last 3 have (1/2)'s in them.

    And I always put everything in SI -- Meters, Kilograms, Seconds, Radians, etc.--first (unless the problem asks for something different) because somewhere you're going to have to use a constant or unit that's a combination of those already--Newtons, Joules, Watts, g=9.8 m/s.

    The second thing I do is write down what I've got and write down what I'm looking for. This helps me use the table I made above.
     
  12. Jun 9, 2005 #11
    Celestia, #4 is a specific conditional equation, I recommend you remove it. The only 4 you need are the other 4. However, i recommend deriving each component rather than memorizing the equations.

    Original poster: You are still converting to feet. Do yourself a favor, forget the words feet, inches, yards, miles, ounces, gallons, quarts, and all other american units of measurement. Learn the MKS system, any component of measurement in kinematics will consist of either kilograms, meters, or seconds.
     
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