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Homework Help: Know all forces

  1. Jun 2, 2008 #1
    If you know all of the forces acting on a moving object, can you tell the direction the object is moving? If yes, explain how. If no, provide a counter example.

    At first i thought the answer was yes, that if you knew the net force on the object the object would be moving in the direction of the net force but that only works for a nonzero net force, and also doesn't tell you the direction of motion only the direction of the accelleration which would could be opposing the motion but the object still moving in the same direction (slowing down). If the net force is 0 then the object keeps moving in the direction it was set in originally. So i figure the correct answer is no.

    Is my logic right? and as a result my conclusion?
  2. jcsd
  3. Jun 2, 2008 #2
    no one will use the term "nonzero net force" cuz net force itself is non zero so simply by saying there is a net force or no net force is enough.yes there is nothing wrong with ur logic,it is according to newton 1nd law,a body will continue its state of "uniform" motion or rest if no external force(net force)acts upon it.to know the direction of a body the initial state of motion is needed to determine the direction of the body if it acts by a net force,because the newton 1st law applies to 2 cases--->body initially at rest and body initially in motion.for a body at rest if the net force is act towards the right on the body then the body will continue its motion towards the right direction with acceleration since F=ma if there is a net force then there will be acceleration.if the motion is initially moving with uniform speed towards the right then if the force acts to the left its direction still remains towards the right but with a deceleration and eventually the body might move to the left with acceleration.if the net force is cause by the friction then it simply stop at its final position(why?anyone can explain this?)if the body in motion towards the right is act by a force towards the same direction as the motion then the motion will move towards the right with acceleration.
  4. Jun 2, 2008 #3
    that would be a pretty cool thing to model in MATLAB or something actually.

    first of all i agree with yc90, but i can probably add to it a bit. you cannot simply determine a direction in which the object travelling because it shall depend on the time the force is acting. my solution would be resolve all the forces on the object into one resultant force so you are left with moving object with force applied to it. To determine the direction it takes it becomes an iterative problem. Obviously after an infinite time, the object will eventually move in the direction of the force because the force will either overcome the objects initial momentum or add to it depending on the direction of the applied force.

    I would iterate the problem using newton's laws of motion and plot a graph. if you were that keen, and know how to do it, set up variables you can enter such as forces, directions, mass of object etc and have your program create a video using your timesteps.
  5. Jun 2, 2008 #4


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    Hi forty! :smile:

    No, it doesn't even work for a non-zero net force … the direction of velocity of a fast-moving object experiencing a given sideways net force will depend on its speed, won't it? :wink:
  6. Jun 2, 2008 #5
    To add to the discussion, you should always look for counter examples first. Don't underestimate them, as they are quite powerful. You don't even need to think about the theory if you have a counter example.

    For this question, there are many good counter examples. Just image you have a object moving at constant velocity south with a force acting west. Can you tell from the force that it is moving west? If yes, what if the same object (still moving west) had the force acting on it north? I'll let you finish it... :)
  7. Jun 2, 2008 #6
    if an object is moving at a constant velocity it doesnt have a force in that direction. Fnet=ma. Force=mass*acceleration, not velocity. thats momentum. a net force of 0 just means there is no acceleration. a net force shows an objects acceleration. i would say the easiest way to look at this is draw a force diagram.
  8. Jun 2, 2008 #7
    I guess I worded it wrong. I should have said "initial moving at a constant velocity, then a force..."
  9. Jun 2, 2008 #8


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    A good, and quite familiar example, is that of an object being thrown straight up in the air.

    The force (gravity) acts downward for the entire time the object is in the air.

    Is the velocity always in the same direction as the downward force, the entire time the object is in the air?
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