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How to find a percentages in kinetic energy

  1. May 20, 2016 #1
    • Misposted so no template
    Stefan throws a baseball with the speed of 140 km/ℎ. The ball goes obviously in goal, but it bounces back out of the net with a speed of 50 km/ℎ. What percentage of
    the kinetic energy remaining after contact with the net?
  2. jcsd
  3. May 20, 2016 #2
    For ’small’ velocities E=0,5m*v*v.
  4. May 20, 2016 #3
    Please check this if I am right.
    BOLL = △KE = ½ = mv²
    BOLL= ½(2kg)(140km/h)² - ½(2kg)(50km/h)²
    BOLL= 17,100 J
    %△ = 19600 J – 2500 J = 17100 J
    = 17100 J / 19600 J = 0.87
    = 0.80 x 100 = 87
    %△ = 87%
  5. May 20, 2016 #4
    That’s how much was wasted…
    remaining(%)=50*50/140*140*100=12,755% (some simplifications made)
    So the answer: 12,8%
  6. May 20, 2016 #5
    Thanks very Replusz.
  7. May 20, 2016 #6
    Okay :)
  8. May 20, 2016 #7
    To get energy in Joules the speeds have to be in m/s.
    Your KE values are all wrong. Or at least in the wrong units.

    The percentage does not depend on the units though.
    Still your value does not look right. If the speed is halved the KE is 1/4 of the initial value.
  9. May 20, 2016 #8
    Yes, I know that, but we only wanted to know the percentage! Thats why I didn’t care about the conversion…
    And the speed is not halfed…
  10. May 20, 2016 #9
    Can you please show me the right equation then.
  11. May 20, 2016 #10
    The answer is correct! (I think)
  12. May 20, 2016 #11
    Sorry, my bad. I read initial speed as 100 km/h.
    Yes, the answer is correct.

    However I think is important to point the wrong use of units, even if the final answer is ok.
    It is already too often that students think that throw in some numbers without any justification is enough to call a problem "solved". :)
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