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Homework Help: Average Acceleration and Time Taken to accelerate

  1. Jun 7, 2017 #1
    Hi, I've been asked to solve below as part of an assignment. Below is pretty much what I've submitted but it has been concluded to be incorrect. I am unsure as to why hence the question. Is it due to the fact that I have not taken the mass or forces into consideration?

    I have only taken these into consideration for questions (iii) and (iv) as shown below.

    Any is greatly appreciated.

    1. The problem statement, all variables and given/known data

    A racing car of mass 850 kg accelerates from 20 km/h to 160 km/h over a distance of 300 metres. Frictional forces and wind resistance can be assumed to be 800N.

    Determine the following:

    (i)The average acceleration

    (ii)The time taken to accelerate from 20 km/h to 160 km/h

    (iii)The tractive force produced by the car to provide this acceleration - not required for this thread

    (iv)The car finally reaches a speed of 250km/h. Friction and wind resistance are 1800 N at this speed. What power output is required to maintain this speed? - not required for this thread

    2. Relevant equations

    (i) v2=u22as

    (ii) s= 1/2(u+v)t

    3. The attempt at a solution


    1602-202 / 2 * 300 = a

    a=42 m/s2

    (ii) 300= 1/2(160+20)t

    t= 300/90

    t= 3.3s
    Last edited: Jun 7, 2017
  2. jcsd
  3. Jun 7, 2017 #2


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    You could first try solving the problem where the initial and final speeds are ##20m/s## and ##160 m/s## and see what answer you get.
  4. Jun 7, 2017 #3


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    Second time today I have come across this blunder in a question.
    There is not enough information to determine average acceleration. If you were told the acceleration is constant then you could find it from the two speeds and the distance, but if it is not then you need to know the time. For further discussion see section 3 of https://www.physicsforums.com/insights/frequently-made-errors-mechanics-forces/. (The discussion there is in respect of average force, but it apples equally to average acceleration.)
    There is a mistake in that equation. I would have thought it just a typo, but you seem to have taken it to be correct.
  5. Jun 8, 2017 #4
    Thanks. I think this was the answer I was looking for. The comments on the work said Velocities require to be in m/s. Was a simple case of converting km/h to m/s.
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