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

Falling object collision question

  1. Jun 2, 2013 #1
    Hello,
    I've been trying to figure out the physica behind a collision of two objects due to the force of gravity. Here's the situation as I'm picturing it:

    A massive object A is falling due to gravity which impacts a stationary object B being held up by a structure. In this particular case, the structure is unable to support the impact force of object A and fails, causing the now combined object AB to continue falling.

    My difficulty is in picturing how the forces interact during the collision. Object A is falling with a force of F=MaG, which is then applied to Object B. At this instant, object B begins its acceleration... Would a force F be applied, during the collision, to Object A in the opposite direction due to Newtons 3rd law?
     
  2. jcsd
  3. Jun 2, 2013 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hello Stellar1! :smile:
    That isn't how it works.

    Mag is the force on A (from the Earth).

    To find the force on A from B, you find the acceleration of A during the collision, subtract that from g, and multiply by Ma.

    The force from A on B is of course the same (Newton's third law).

    (that's if you analyse it instantaneously …

    the more sensible way would be to use impulse, rather than instantaneous force)
     
  4. Jun 2, 2013 #3
    If object A is on object B, the it is applying the force MaG onto it, no?
     
  5. Jun 2, 2013 #4
    The only way I can reconcile this in my head is if I take compressibility into account. That allows for object B to accelerate and object A to decelerate until a common speed is achieved and gravity accelerates the combined object.
     
  6. Jun 2, 2013 #5

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    no :confused:

    the force from A on B (or vice versa) depends on a lot of things
     
  7. Jun 2, 2013 #6
    Let me rewrite this now that I'm not doing 3 things at once and able to think about this with minimal distractions:

    I know that A will be falling with a force of Fa=mag onto object B, which can only support a force of, say, Fs. The length of time object A is under this acceleration will allow us to determine the velocity of object B at the instant just prior to collision, which we will call vi

    Now, assuming object B is incompressible and, since we assume Fa>Fs, its perfectly consistent to assume that the force during the collision will break the supports and object B will now accelerate from rest under the collision force of object A, as well as gravity.

    Since we assume that object B is not very compressible, wouldn't this mean that object A has to essentially come to rest as well, so that object B (now joined to A) will both accelerate from rest (since B is starting from an initially stationary position)? This would then imply that the velocity at the instant just after collision is 0, correct? The force exerted by A on B (and by B on A) would then equal F=mavi/t + mag, where t is the duration of the collision.

    Is there any way to determine how long this collision would take?
     
  8. Jun 4, 2013 #7

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    No, that's completely wrong.

    Before A hits B, it has an acceleration of g downward, and there is a force on A (from the Earth) of Mag.

    A does not exert a force of Mag on anything (except of course the Earth).

    When A hits B, A will slow down a lot, and will also change shape slightly. B will also change shape slightly, and will start accelerating.

    You find the force by finding the acceleration first (on either A or B), and multiplying that by the mass.

    You find the acceleration by using Newton's second law, which is about momentum, and also using your knowledge of the elasticity of A and B.
     
  9. Jun 4, 2013 #8
    Ahh, yeah. That's what I meant. Falling onto =>Falling towards with a force of F=mag which accelerates it to a certain velocity prior to colliding with B.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Falling object collision question
  1. A falling object (Replies: 12)

Loading...