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: Finding final velocity

  1. Oct 26, 2014 #1
    I am not good at typing out formulas and equations on the internet, so I took pictures of what I did instead. I hope that's not too informal.

    1. The problem statement, all variables and given/known data

    A rock is tossed straight up with a velocity of 19 m/s. When it returns it fell into a 10 m deep hole.

    Q: What is the rock's velocity as it hits the bottom of the hole?

    2. Relevant equations

    3. The attempt at a solution

    So in the end I figure out that I am suppose to plug 19 m/s as my initial velocity, but I don't get why.
    If the rock is tossed straight up should it not means that the initial velocity = 0 because initially the rock is at rest?
  2. jcsd
  3. Oct 26, 2014 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    According to the OP, "A rock is tossed straight up with a velocity of 19 m/s."

    You can't write a plainer statement than that. IDK how you could assume that the rock is at rest, which implies no movement of any kind.
  4. Oct 26, 2014 #3


    User Avatar
    Gold Member

    Perhaps you are concerned because going from rest to 19m/s instantly would imply infinite acceleration. This is an issue that is always ignored in such problems. Just assume that the rock had some acceleration that got it up to 19m/s at the starting point and go from there.

    If you pay careful attention to the problem you can see that there is another thing being ignored, and that is the height of the person throwing the rock. Even though the statement implies that it is being thrown by a person, who would be more than zero meters tall, you are supposed to assume that it leaves the ground going 19m/s. Again, this is just something that is usually ignored in this kind of problem.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted