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Homework Help: Kinematics - Acceleration and deceleration

  1. Sep 13, 2004 #1
    1. A cab driver picks up a customer and delivers her 2.00km away, on a straight route. The driver accelerates to the speed limit and, on reaching it, begins to decelerate at once. The magnitude of the decelerates is three times the magnitude of the acceleration. Find the lengths of the acceleration and deceleration phases. *** this one i have no idea what they are talking about***

    2.A speedboat starts from rest and accelerates at 2.01m/s^2 for 7.00 sec. At the end of this time, the boat conts. for an additional 6.00sec. with an acceleration of 0.518m/s^2 . Following this, the boat accelerates at - 1.49m/s^2 for 8.00 sec. (a) what is the velocity of the boat at t=21.0sec. (b) find the total displacement of the boat
    ** i don't know where to start cuz there so many A and V.

    3. A Boeing 747 " Jumbo Jet" has a length of 59.7 m. The runway on which the plane lands intersects another runway. The width of the intersection is 25.0 m. The plane decelerates through the intersection at a rate of 5.70m/s62 and clears it with a final speed of 45.0m/s. How much time is needed for the plane to clear the intersection??

    out of 20 probs..that's theonly 3 i don't understand so anyone can help i'll greatly appreciated.. thanks in advanced
  2. jcsd
  3. Sep 13, 2004 #2


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    (1) You can relate the maximum speed during each segment to the corresponding acceleration and distance traveled. Note the maximum speeds are for the two segments are the same!

    You also know the total distance traveled so you can directly relate the length of one segment to the other. Finally, you know the relation between the two rates of acceleration.

    This establishes enough information to fully determine the lengths of each segment.
  4. Sep 14, 2004 #3
    Picturing all these as graphs can be a big help. Sorry I can't be a big help at the moment. I'll post later.
  5. Sep 14, 2004 #4
    2) How about just starting from the beginning? You have known times and accelerations, so you can calculate the velocities at the end of every part. Current velocity is the sum of the starting velocity and acceleration times time. You just need to do the same thing three times.

    3) You need to find out what the starting velocity is. You can't calculate the numerical value at the beginning, but you can insert the equation for it inside the distance equation so you'll only have one unknown.
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