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Homework Help: Speed of a truck

  1. Sep 5, 2016 #1
    (mentor note) post moved here from non-homework thread hence no template.

    A truck is travelling at a v=180 km/h speed on a straight road. At some point, the driver sees a police car and hits the brakes. If his average deceleration is a=-4m/s^2, how long will it take for its speed to go down to 80 km/h?

    So, i converted the units to m/s and used the equation u=v-at to find the time, but the result was wrong. What am i missing?
    Last edited by a moderator: Sep 5, 2016
  2. jcsd
  3. Sep 5, 2016 #2


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    It is impossible for us to know since you did not bother to show any work (which is against forum rules).
  4. Sep 5, 2016 #3
    Right, sorry. Okay so v=180km/h is 33.33 m/s and u=80 km/h is 22.2 m/s. If I plug these numbers into the formula, along with a=-4m/s^2, i get t=2,775 sec. The correct answer is apparently 1.66 sec. I don't know what i am doing wrong.
    Last edited: Sep 5, 2016
  5. Sep 5, 2016 #4


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    Well, just as a quick check, it jumps out at you that 180/80 is not the same as 33.3/22.2 (or, if you prefer, 180/33.33 is not the same as 80/22.2). What does that tell you?
  6. Sep 5, 2016 #5
    Right, so apparently, i converted 180 km/h wrong. It's actually 50 m/s, not 33.3 m/s.
    I made another mistake. It's v=120 km/h, not 180.
  7. Sep 5, 2016 #6


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    Hi Perry, just for info.. Homework or homework like questions should go in that section. The moderators might move your thread there.
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