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Motion at Constant Acceleration Question 3

  1. Jan 14, 2010 #1
    1. The problem statement, all variables and given/known data
    A car slows down uniformly from a speed of 21.0 m/s to rest in 6.00s. How far did it travel in that time?


    2. Relevant equations

    I am not really sure what one to use, which is the problem.

    3. The attempt at a solution
     
  2. jcsd
  3. Jan 14, 2010 #2

    Matterwave

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    First, if it slows from 21 to 0 uniformly, what's the average speed at which the car travels within those 6 seconds? Then, what is the distance the car travels in 6 seconds at that average speed?
     
  4. Jan 14, 2010 #3
    the average speed is 3.5 m/s it travels 21 m in 6 seconds right???
     
  5. Jan 14, 2010 #4

    Matterwave

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    Is the average speed 3.5m/s?

    Why would 21m/s divided by 6 give you the average speed?
     
  6. Jan 14, 2010 #5
    Im not really sure I'm really confused...... I thought that 21 was the distance and 6 was the time but I know that is wrong so isn't 21 m/s the average speed?
     
  7. Jan 14, 2010 #6

    Matterwave

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    If the initial speed is 21 and the final speed is 0, and the deceleration is uniform, what do you think the average speed is? Just from intuition, the answer should be obvious, though the proof requires calculus.

    For finding the average speed, disregard the fact that it took 6 seconds. How long it took to decelerate is unimportant. What is important is that the deceleration was constant.
     
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