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Homework Help: Kinetic energy of moving automobile

  1. Nov 19, 2007 #1
    suppose an atomobile has a kinetic energy of 20,000J. If it moves twice as fast, what will be its kinetic energy? three times as fast?

    i think that if the velocity is 2 times as fast, then the energy will be 2times^2, and if it is 3 times as fast, then it will be 3times^2...is that correct? and what is the physics reasoning behind it cuz i just saw it logically
  2. jcsd
  3. Nov 19, 2007 #2
    also, another question

    a shopping bag is hangg straight down from your hand as your walk across a horizontal floor. does the force that your hand exerts on the bag's handle do any work> does this force do any wokr while you are ruding up and elevator? does this force do any work while you are rding up and escalator? (assume in all three cases that the velocity is constant)

    in the first part of it,the force is upwards, and displacement is to the side, so there is a 90 degree angle and work is done: w=FScos(theata) in the second part, force is up and so is displacement, so there is no work done b/c cos(theata)=0 in that part. and in the last part, there is some angle, so work is being done. However, it said that velocity was constant, and constant velocity=0 acceleration and acording to F=ma, there would b no force...so which is it?
  4. Nov 19, 2007 #3
    and another question,

    During batting practice, a baseball player hits a baseball three different times. He makes contact with the ball at the same height and the ball leaves the bat with the
    same speed each time. The only difference is the angle with which the ball leaves the
    bat. He first hits the ball up at an angle of 30° above the horizontal, then horizontally,
    then down at a 30° angle below the horizontal. Which baseball strikes the
    ground with the fastest speed? Which strikes the ground with the slowest speed?
    (Air resistance is assumed negligible.)

    i think that the ball hit below the horizontal would hit the ground with the greatest speed. velocity is distance over time, and the ball hit above the horizontal would have the greatest distance, and so on. however, based on the conservation of energy formula, since all of the initial info is the same, then shouldnt the final velocity b the same for all o them?
  5. Nov 19, 2007 #4
    Keep in mind the equation for kinetic energy: K=1/2mv^2
  6. Nov 19, 2007 #5
    yes i know that, so is my reasoning correct?
  7. Nov 19, 2007 #6
    Your reasoning is correct. I wasn't understanding you at first.
  8. Nov 19, 2007 #7
    ok nice...what about the other problems?
  9. Nov 19, 2007 #8


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    In the future, please post separate questions under separate topics. Otherwise, the responses are going to read like the litany of saints.

    For question 2, you've got your cos thetas mixed; cos theta is 1 and cos 90 is 0. Regarding net work at constant velocity, yes, it is zero, but does this mean the work done by your hand is 0?
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