1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    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.

    AM
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Having some trouble on these physics questions!
Loading...