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!

Fas-Moving projectiles-Satellites

  1. Jul 4, 2011 #1
    Fas-Moving projectiles--Satellites


    In my book it states that an object thrown will fall 4.9 meters one second later. First of all, why does it fall in that precise amount--does the mass of an object change that amount? Secondly, does it fall that much in every second it travels? Also, does all of this only apply to objects thrown horizontally?

    Thank you
  2. jcsd
  3. Jul 4, 2011 #2


    User Avatar
    Gold Member

    Re: Fas-Moving projectiles--Satellites

    It's actually an average, the ball isn't always falling at that amount:

    You're in a gravity field that's always pulling down at 9.8 m/s per second. If you drop a ball, it starts at 0 m/s velocity and speeds up to 9.8 m/s in one second.

    If we assume it's a linear trend, than the average of those two velocities should give us the average velocity over the whole trip: (0 + 9.8)/2 = 4.9

    The mass of an object has no effect on this directly, though in the real world, on Earth, large objects may catch more wind and air resistance (which is being ignored here). But then, large doesn't always mean more massive.

    This average applies to an object that starts out with 0 vertical direction. You can throw it horizontally or just drop it to achieve this.

    If you threw it up or down, you'd have to figure out the vertical component of the starting velocity when it left your hand and then average that instead. The horizontal velocity doesn't factor in (horizontal and vertical dimensions are independent).
  4. Jul 5, 2011 #3
    Re: Fas-Moving projectiles--Satellites

    So, if I were to through an object horizontally two seconds later it would have fallen vertically 9.8 meters from its horizontal path?
  5. Jul 5, 2011 #4


    User Avatar

    Re: Fas-Moving projectiles--Satellites

    No, actually. It would have fallen 19.6 meters. In the first second, it will fall 4.9 meters, in the second second, it will fall 14.7 meters. The object is accelerating, so each second, it will fall farther than the second before.
  6. Jul 5, 2011 #5
    Re: Fas-Moving projectiles--Satellites

    Oh okay, I understand. Thank you.
  7. Jul 5, 2011 #6
    Re: Fas-Moving projectiles--Satellites

    Mybusters performed a fantastic experiment that demonstarted this. (gravities effect on an object traveling horizontally)

    They fired a bullet from a gun and at the exact same time dropped a bullet at the calculated distance/location where the gun fired bullet would land.

    after a number of attempts they got the timming right and both hit the ground at the exact same time.
    Last edited: Jul 5, 2011
  8. Jul 5, 2011 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Fas-Moving projectiles--Satellites

    Apollo 15 also did an experiment where they dropped a hammer and a feather at the same time on the moon.
    Look here:

    The feather, since it is on the moon, has no air resistance to impede its drop like it does here on earth. Both the hammer and feather both hit the ground at the same time.

    The mass of the objects don't matter, an object will always be accelerated by the same amount compared to another object of different mass. Hence the feather and hammer hitting the ground at the same time. Note that this is when you keep the mass of the larger object the same and switch out smaller objects. On the moon, as the video shows, the acceleration is much less because the moon has so much less gravity than the earth.
    Last edited by a moderator: Sep 25, 2014
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Similar Threads for Moving projectiles Satellites
B Runing in a moving train
I What causes pressure and is it isotropic in a moving fluid?