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Homework Help: Having some trouble on these physics questions!

  1. Sep 9, 2006 #1
    Hey everyone! i'm new to the forums:biggrin:

    I just started AP Physics a few days ago and i'm having some trouble on the following problems:

    1) A truck on a straight road starts from rest accelerating at 2.0 m/s^2 until it reaches a speed of 20 m/s. Then the truck travels for 20 s at a constant speed until the brakes are applied, stopping the truck in a uniform manner in an additional 5.0 s. (A)How long is the truck in motion? (B)What is the average velocity of the truck for the motion described?

    For (A) I found the correct time to be 35 s.

    For (B), I used the equation:

    Average velocity = (Vo + V)/2

    I set it up like:

    V = [(0 m/s) + (20 m/s)] / 2

    i came up with 10 m/s as my velocity which was wrong. I know this is kind of a multifaceted problem and i'm having trouble adjusting the equation accordingly. Can someone please point out my error and how i can solve this?

    2) A cesna aircraft has a lift-off speed of 120 km/h. (A) What minimum constant acceleration does this require if the aircraft is to be airborne after a takeoff run of 240 m? (B) How long does it take the aircraft to become airborne?

    For this problem i used the equation:

    V^2 = Vo^2 + 2ax

    I set it up like:

    (120 km/h)^2 = (0 km/h)^2 + 2(a)(.240 km)

    i came up with 30,000 km/h^2 which doesn't sound right at all:confused:

    Can someone please point out my error and how i am able to better approach this problem?

    Help would be very much appreciated thanks!:biggrin:

    edit: sorry for the vague title, i'll be sure to make it more specific next time.
    Last edited: Sep 9, 2006
  2. jcsd
  3. Sep 10, 2006 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    No. Average velocity is total distance/total time.

    Convert km/hr to m/sec.

    It is easier to determine the time first. If it has constant acceleration and ends up traveling at 120 km/hr (33.3 m/sec), what is the average speed? What is the distance in terms of average speed and time?

    Once you have the time, you can easily find the acceleration.

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