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Homework Help: How to calculate watt

  1. Apr 13, 2010 #1
    ive always been interested in calculations, so i wanna try to calculate watt but im stuck

    bear in mind this is a pure hobby for me and i am far from a math genious

    in this example i use a car

    here is how far i have come

    it accelerates from 22.2m/s to 33.3 m/s in 7 seconds
    to calculate acceleration you do like this

    highest speed-lowest/time 33.3-22.2=11.1 11.1/7 is almost 1.58

    giving the car acceleration of 1.58m/s^2 so far so good

    next is calculating work
    work= mass*acceleration^2

    so if the car is 1200kg(newton)

    then we take 1200* (1.58*1.58) = 2996 joules (this is correct i hope)

    and this is where i get stuck because the formulas i find state that watt=work/time

    2996joules/7sec is obviously wrong.....

    can anybody help me?

    Last edited: Apr 13, 2010
  2. jcsd
  3. Apr 13, 2010 #2

    Doc Al

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    Staff: Mentor

    OK, that's the average acceleration. The units should be m/s², not m/s. (But you don't really need the acceleration to find the work done.)
    This is incorrect. (Note that that quantity does not have the correct units for work/energy.)

    To calculate the work done, find the change in kinetic energy. Kinetic energy = ½mv².
  4. Apr 13, 2010 #3


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    Staff: Mentor

    A couple issues. First the acceleration would be 1.58m/s^2. Velocity has units of m/s and acceleration has units of m/s^2.

    Next, I do not think this is correct: "work= mass*acceleration^2" Where did you get that?

    Force = mass*acceleration (F=ma), and work = Force*distance.

    Does that help?
  5. Apr 13, 2010 #4

    sorry my mistake, like i stated above im only doing this as a hobby, i will alter the acceleration immediatley

    thanks for the fast reply, im sorry but i have no idea how to calculate the kinetic energy can you help me?
  6. Apr 13, 2010 #5
    ahh now i see one of the problems in my calculation

    it stated work=N*m/s^2

    so force should be 1200*1.58??? (1896watt?)

    is it possible to calculate distance from the starting speed of 22.2m/s and finish speed of 33.3m/s over 7sec

    sounds over-complicated.....

    thanks for the fast reply
  7. Apr 13, 2010 #6

    Doc Al

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    Staff: Mentor

    You have the mass (m) and the speed (v). Calculate the initial and final kinetic energy, then find the change.
  8. Apr 13, 2010 #7
    ahhh :)

    starting kinetic 0,5*1200*(22.2*22.2)= 295704 joules

    ending kinetic 0.5*1200*(33.3*33.3)= 665334 joules

    369630 joules

    (just thinking loud now)
    369.6/7=53kw sounds very reasonable as a medium power

    is it really that simple???

  9. Apr 14, 2010 #8

    Doc Al

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    Staff: Mentor

    Yes, it's that simple.
  10. Apr 14, 2010 #9
    thanks a lot :) this forum is great!!!
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