1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Basic Kinematics Questions - What's wrong with my answers?

  1. Sep 21, 2009 #1
    1. The problem statement, all variables and given/known data


    (b) Is it possible to be accelerating if you are traveling at constant speed? Explain your answer.

    (d) Explain why, in American football, a field goal is kicked from the ground while a punt is drop kicked.

    (f) Reaching out from a balcony, you throw rock A vertically upward. Then you throw rock B vertically downward from the same release point and with the same initial speed as rock A. If you neglect air resistance, which rock has the higher speed just before it hits the ground?

    2. Relevant equations

    None, I believe.

    3. The attempt at a solution

    (b) No, traveling at constant speed is a straight diagonal line on a position graph. That means that there's no change in spped. No change in spped means no change in velocity (which is the slope of speed). With velocity then having a 0 slope, there is no acceleration (slope of velocity). -1

    (d) This is done to change the angle of where the football is projected to go. Until 90 degrees, items are projected to go farther if the angle is higher. -2

    (f) Throwing a rock downward vertically will cause its speed to be higher than if it was thrown upward. -1

    I had points taken off for my answers, and I'm not sure why. Could someone explain to me what's wrong with my answers?

    Thank you.
  2. jcsd
  3. Sep 21, 2009 #2
    b)Its possible.There are 2 types of acceleration.(acceleration is a vector quantity)
    d)Sorry i don't know what a punt is.
    f)Conservation of energy.(terminal velocity)
  4. Sep 21, 2009 #3


    User Avatar
    Homework Helper

    b) In a uniform circular motion, the speed remains constant but the velocity continuously changes the direction, hence has an acceleration towards the center.
    f) A rock thrown up with a certain velocity, return back to the point of projection with the same velocity. So the initial velocities of both the rocks towards the ground is the same. Hence...?
  5. Sep 21, 2009 #4
    Oh, okay. I think I understand (b) and (f) now.

    Acceleration is a vector, which means direction must be considered.

    And, both rocks will have the same speed before they hit the ground, right? Or, will the rock thrown upward have had more distance to have a higher acceleration (and speed)?
  6. Sep 21, 2009 #5


    User Avatar
    Homework Helper

    Here you heve to consider the displacement, not the total distance traveled.
  7. Sep 21, 2009 #6
    That makes sense. Thanks.

    Does anyone know why a field goal is kicked from the ground while a punt is drop kicked.

    From personal experience, I know accuracy may have something else to do with it, but I have little clue how it relates to kinematics specifically.
  8. Sep 21, 2009 #7


    User Avatar
    Science Advisor
    Gold Member

    The goal in a punt is to get the ball to hang to give time for the players to get down field.

    The goal of the field-goal is to a.) be accurate and b.) get the ball over the defenders quickly.

    How would ball placement (higher for punts lower for ground kicks) affect these variables?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook