Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help with a velocity and accleration problem

  1. Oct 13, 2011 #1
    1. An 18 wheeler and a sports car in adjacent lanes at any given moment. If the car's speed is faster than the trucks speed, which of the following must be true about the car's velocity...

    a) its increasing
    b) its in the direction of the truck
    c) its zero
    d) none of the above

    I said it was d because the truck and car are side by side. Even though the car is going faster, it can have a negative velocity which means the driver is pushing the brake pedal to stay in line with the truck. So I was thinking the velocity was decreasing but Im confused if its right or not.

    2. A bullet takes 8 milliseconds to accelerate the entire length of the gun barrel. When the bullet leaves the barrel, it has a velocity of 240 m/s. Assuming that the bullet has a uniform acceleration, what is the length of the barrel?

    I was really lost with this one. I know final velocity is 240 and beyond that Im confused.
  2. jcsd
  3. Oct 13, 2011 #2
    These equations apply for objects with constant acceleration. Maybe give it a try with these and repost with work if you still can't get it.

    [tex] v = v_0 + a t [/tex]
    [tex] x = x_0 + v_0 t + (1/2) a t^2 [/tex]
  4. Oct 13, 2011 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I think you misread the problem. It does not say that the car and truck are side by side; it just says that they are in adjacent lanes, and that he car has a greater speed. Rethink the explanation of your chosen answer.
    You know the final velocity as it leaves the barrel. It's initial speed at the beginning of the barrel is how much? Use one of the kinematic equations to solve for the barrel length.
  5. Oct 13, 2011 #4
    A bullet takes 8 milliseconds to accelerate the entire length of the gun barrel.

    Thats the part im most confused about. Is that time or acceleration? Im assuming that its time causing its saying it takes .008 seconds to acclerate the entire length.
  6. Oct 13, 2011 #5
    So does that mean that the car's velocity is in the direction of the truck.

    But what if the car and truck were neck and neck but the car had a greater speed. Does that mean that the car has a negative velocity?
  7. Oct 13, 2011 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    the question says nothing initially about velocity. It makes reference to speed, which has no direction associated with it (speed is a scalar quantity).
    The car could be traveling neck and neck , ahead of, or behind, the truck, at any given moment, in the same or opposite direction. So which is the correct answer, and why?
  8. Oct 16, 2011 #7
    Oh so its zero right since it doesnt have a vector.
  9. Oct 16, 2011 #8
    oaky so I did 240 m/s = 0 m/s + a(.008s)

    a = 30,000 m/s^2


    x = 0 + 0 + (1/2) 30,000m/s^2 (.008s)^2

    x = .86m

    or about 2.8 feet

    but a guna that is 2.8 feet? I think I did something wrong. Is the initial velocity zero because thats what I assumed?
  10. Oct 16, 2011 #9
    That looks right, except you should check your answer for the length because your equation is right and your answer is slightly off. This is a bit long for a barrel, but not ridiculous for a rifle -you can't always assume the problem is well written either :). It's good that you think about if your answer makes sense though.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook