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Homework Help: Need help visualizing what an object is doing with force applied

  1. Sep 27, 2010 #1
    1. The problem statement, all variables and given/known data

    If an object is moving in a certain direction, with the net force also going in that direction, but the magnitude of the force is decreasing with time as it goes along. What is the speed of the object doing?

    2. Relevant equations

    3. The attempt at a solution
    Im inclined to think the speed of the object is going to become constant, because if an object is in motion in stays in motion, am I right? I can't quite picture this.
  2. jcsd
  3. Sep 27, 2010 #2
    >>in motion in stays in motion
    yeah if your in a vacuum.

    F = ma;

    so if your force is decreasing therefor you acceleration is decreasing, since your mass is static (conservation of mass)

    your acceleration will decrease until it hits zero, and when it does than the velocity will be constant.
  4. Sep 27, 2010 #3
    Ok I think I got you. Lets say im not in a vacuum. If I push a shopping cart then let go, does it still experience a force in the direction a pushed it after i let go, or does it slow down because the backward force is greater than the forward force?
  5. Sep 27, 2010 #4
    after you push an object (after you let go), the only force it experiences is external forces such as drag, friction, ect... which will slow it down until the velocity reaches zero.
  6. Sep 27, 2010 #5
    So, if Im only talking about horizontal forces, the forward force just diminishes over time with not back force?
  7. Sep 27, 2010 #6
    You can use momentum to describe this

    P[tex]_{f}[/tex] - P[tex]_{i}[/tex] = [tex]\int[/tex] Fdt

    if there are not external forces (vacuum for example) than the expression goes to

    P[tex]_{f}[/tex] - P[tex]_{i}[/tex] = 0

    P[tex]_{f}[/tex] = P[tex]_{i}[/tex]

    mV[tex]_{i}[/tex] = mV[tex]_{f}[/tex]

    masses cancel and your left with

    [tex]_{i}[/tex] = V[tex]_{f}[/tex] (for no external forces)

    But this is usually not the case, you usually have an outside force that will affect the final velocity

    You can consider the force that moves the object as a collision, an impulse
    Last edited: Sep 28, 2010
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