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Homework Help: Time assumption

  1. Apr 24, 2007 #1
    ok so perhaps not a calculation queery of sorts but a coursework question none the less.

    in my investigation i'm using a newton meter to fire projectiles and i need to know the initial velocity, to work it out i need the time it takes to travel up the runway as it were.
    now I have the newton meter infront of me and it isnt as simple as timing it because it is VERY fast.

    so perhaps i should assume the time, (its relatively constant at different parts of the newton meter because as you increase the distance it travels you also increase the average force on it)

    so what should i assume it to be? 0.1s? 0.01s? although i cant tell the difference between 0.1s and 0.01s it changes my initial velocity value 10fold
  2. jcsd
  3. Apr 24, 2007 #2
    You mean you are using a spring and a Newton meter to fire projectiles? Perhaps a more descriptive account of your apparatus would be helpful.

    Whatever you are doing, you should not estimate the time by any sort of visual cue or other human ability because we are really bad at that kind of thing.
  4. Apr 25, 2007 #3
    the apparatus is a newton meter which has a spring in it so when you put a weight on it the spring stretches to tell you the force being applied.
    however i have a thing attatched to to the pointer which would tell you the force applied, this holds projectiles, i secure the newton meter at an angle and pull it back 5N or so then let go, then the projectile is launched.

    is there anthing that can measure it then?
  5. Apr 25, 2007 #4


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    Staff: Mentor

    I'm only familiar with the term "Newton meter" in the context of units. A Newton (N) is a unit of force, and a meter (m) is a unit of distance. Thus, the Nm is a unit of work, since work is force multiplied by the distance that an object is moved by the force.

    As Mindscrape is asking, is the apparatus really a spring launcher? Why are you calling it a Newton meter? Is that just the whimsical name that was put on the spring launcher by some other student? If it is a spring launcher, and you know the spring constant k and the mass of your projectile, then you can calculate the projectile velocity in several different ways, and you can also calculate how long the spring is in contact with the mass as it is being launched.
  6. Apr 26, 2007 #5
    its a newton metER though not a newton metRE, although spelling may be a UK thing.
    yes a newton metre Nm is an SI unit, but a newton meter (or at least as far as I'm aware is a meter that measures force/weight, it is a spring basically yeah. and as force is applied by pulling the spring, a pointer points the the number of newtons of force.

    apologies for the language difference.

    How can i calculate the k of the spring.
    then what're these 'several different ways' to calculate the velocity.

    I was going to get 2 light gates to measure the time an object took to go through the two.

    any thoughts?
    Last edited: Apr 26, 2007
  7. Apr 26, 2007 #6
    You can figure out the spring constant by measuring its length with a mass on the end, and its length when it is stretched by an additional masses. If Fsp = kx, then the sum of forces for the first part are kx_1 = m_1g, and the second part shows that kx_2 = (m_1+m_2)g, where x_1 and x_2 are amount stretched beyond the rest length. Then you can continue to add masses and record lengths to get the best calibration (average the spring constant, or a least a squares fit of gravitational force against displacement would be better if you know how).
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