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Homework Help: Help Please Physics

  1. Feb 8, 2005 #1
    O.K . What is 1*10(6power)J mean. Do I further need to break this down in order to work the problem.
    Problem: A car travleling 25 km/hour has Kinetic energy of 1*10(6power)J.
    What is the mass of the car?
    I've converted k/h to m/s
  2. jcsd
  3. Feb 8, 2005 #2
    Do you know what mass is in terms of velocity and energy?

  4. Feb 8, 2005 #3
    If I'm looking for mass am I suppose to know?
  5. Feb 8, 2005 #4
    No I do not know what mass is in terms of velocity and energy
  6. Feb 8, 2005 #5
    [tex]KE=\frac{1}{2}mv^2[/tex]. So, if [tex]KE = 10^6[/tex], and [tex]v=25km/h[/tex] or [tex]~6.9m/s......[/tex]
    10000000/.5(6.9)^2=416666kg????? Thats almost 300X the average mass of a car

    Edited: Oops on Calculations, Thanks Mathechyst
    Last edited: Feb 8, 2005
  7. Feb 8, 2005 #6
    It doesn't look like you accounted for the fact that the velocity is given in [itex]km/hr[/itex].
  8. Feb 8, 2005 #7
    Yeah, Ive been editing it lol.. But now look, I checked it a couple of times. Is it meant to come out with non-sensical numbers?
  9. Feb 8, 2005 #8
    Since we're giving out the answer, we should show how it is solved:
    m=\frac{2K}{v^2}=\frac{2\cdot1\cdot10^6 kg\cdot{m^2}}{s^2}\cdot\frac{hr^2}{25^2 km^2}\cdot\frac{3600^2 s^2}{hr^2}\cdot\frac{km^2}{1000^2 m^2}=\frac{2\cdot3600^2}{25^2}kg=41472kg
    :surprised Hmm. A 45.6 ton car. Still, it looks right to me. Let's do a back of the envelope calculation to see what we come up with. Suppose we have a 1000 kg (2200 lb) car going 25 km/hr.

    \frac{mv^2}{2}=\frac{1000kg}{1}\cdot\frac{25^2 km^2}{hr^2}\cdot\frac{1000^2 m^2}{km^2}\cdot\frac{hr^2}{3600^2 s^2}=\frac{1000\cdot25^2\cdot1000^2 kg\cdot{m^2}}{s^2}=48225.3J

    Yes, I would say a million joules at 25km/hr is a bit much.

    Last edited: Feb 8, 2005
  10. Feb 8, 2005 #9
    O.K Mathechyst I got completed lost. I didn't see were you converted k/m into m/s. I'm sorry but i guess i'm missing the whole concept
  11. Feb 8, 2005 #10
    I find it easier to include the units conversion along with the rest of the solution. It's just a matter of multiplying by 1. For example, to convert km/hr to km/s you multiply the km/hr by the number of hours in a second, namely 1/3600.

    \frac{25 km}{hr}\cdot\frac{hr}{3600 s}=\frac{25 km}{3600 s}=\frac{1 km}{144 s}

    Notice how the [itex]hr[/itex] units cancel? That's what you want to do. Cancel out the units you don't want and replace them with the units you do want. Now to convert km/s to m/s you just multiply km/s by the number of meters in a kilometer, so:

    \frac{1 km}{144 s}\cdot\frac{1000 m}{km}=\frac{1000 m}{144 s}=6.94\frac{m}{s}

    See how the [itex]km[/tex] units cancelled?

  12. Feb 8, 2005 #11
    Thanks for the help. I have completed the activity. I appreciate you.
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