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Homework Help: Kinematic Equations problems!

  1. Sep 4, 2006 #1
    Im a 1st year physics student in the 11th grade. I havent had much trouble with the assignment, but for some reason i cannot get the correct solution on this problem.

    A certain car is capable of accelerating at a rate of 0.57 m/s2. How long does it take for this car to go from a speed of 53 m/h to a speed of 68 m/h? Anser in units of s.

    I would assume that the correct kinematic equation to use would be v=v.+a(t) if .57 is the acceleration/ 53 is v./ and 68 is v........., but when I do that I get the wrong answer, if you could help and tell me what im doing wrong I would appreciate it! =)

    ~kaci
     
  2. jcsd
  3. Sep 4, 2006 #2

    tony873004

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    re-write that equation using algebra to solve for t. Becareful of your units. Your vi and vf are given in meters per hour, but your acceleration is given in meters / s2. You want your answer to be in seconds.
     
  4. Sep 4, 2006 #3
    You have the right equation. What answer are you getting? Maybe you're not converting from mph to m/s?
     
  5. Sep 4, 2006 #4
    im getting 26. 31578947. so would i divide that number by 120? im not wery good at doing unit conversions.
     
  6. Sep 4, 2006 #5

    tony873004

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    I get that answer if I do (68-53) / .57

    So your problem is the units. 68 and 53 are in m/h and .57 is in m/s2

    Don't confuse meters with miles. Re-read the question. I'd find it surprising that they'd give you a problem where the car's initial and final velocities are about the speed with which an ant crawls. Should it really take the car a full hour to drive half the length of a football field?
     
  7. Sep 4, 2006 #6

    EP

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    Check your using the right unit conversions. Is the acceleration in meter per second^2? Is your velocity miles per hour or meters per hour? This would change your answer alot.
     
  8. Sep 4, 2006 #7
    It's best to convert the velocities you're given. 1 mile is 1.6 km. You should be able to get velocities in metres/s, then use v=v.+at.
     
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