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: Vector description of motion

  1. Oct 26, 2014 #1
    1. The problem statement, all variables and given/known data
    What are the average velocity and average acceleration of the tip of the 2.4-cm-long hour hand of a clock in the interval from 12p.m. to 6p.m.

    I'm given the displacement which is 4.8 cm.
    I know the motion takes six hours
    I set my x axis at three-o-clock and my y-axis at 12

    2. Relevant equations

    Average velocity = displacement / change in time
    Average acceleration = change in velocity / change in time

    3. The attempt at a solution

    The answer I get for velocity agrees with the back of the book:

    Avg Velocity = -0.8 j cm/hr

    But I have no clue how they get an average acceleration of -0.42 i cm/hr2.
    I don't understand why there would be an average acceleration if the hand moves at a constant speed. I understand that a change in direction changes acceleration, but I'm missing something.
  2. jcsd
  3. Oct 27, 2014 #2


    User Avatar

    Staff: Mentor

    Hi ianperez. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Acceleration = (change in velocity of a point on the tip) / elapsed time

    So you first need to determine magnitude and direction of the tip's velocity at the two times given.

    If the tip experienced no acceleration, it would not follow a curved path.
    Last edited by a moderator: May 7, 2017
  4. Oct 28, 2014 #3
    I got it! Thank you!!!

    The circumference is 4.8π so the velocity is 4.8π/12hrs=1.26cm/he
    Vi= 1.26i + 0j
    Vf = -1.26i + 0j
    Acceleration = (change in velocity of a point on the tip) / elapsed time
    A= -1.26-1.26 / 6hrs = -0.42cm/hr
  5. Oct 28, 2014 #4


    User Avatar

    Staff: Mentor

    looks good
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted