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Resultant force when a body is accelerating

  1. Jun 8, 2014 #1
    1. The problem statement, all variables and given/known data

    There is a body of mass 500kg accelerating at 2.5ms^-2.
    It experiences a frictional force of 1000N.

    2. Relevant equations

    Resultant Force = ma

    3. The attempt at a solution

    Resultant Force = (500)(2.5)N = 1250N
    The body is moving forward with a net force of 1250N
    A frictional force of 1000N is acting on the opposite direction.
    The body must be moving with this forward force:

    Forward Force - Frictional Force = Resultant Force
    Forward Force = (1250 + 1000)N = 2250N

    But what does all this mean?

    The body is accelerating. Shouldn't the forward force as well as the frictional force be continuously increasing?
  2. jcsd
  3. Jun 8, 2014 #2

    Doc Al

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    Staff: Mentor

    If the acceleration is constant, the net force must be constant. No reason to think that the frictional force is changing.

    What an acceleration means is that the velocity is continuously changing, not the force.
  4. Jun 8, 2014 #3


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    Homework Helper

    yeah, in most real-world situations the friction force will increase in this scenario. But the question specifically states that the friction force is constant. So as always, you should do as the question says.
  5. Jun 8, 2014 #4
    Here is the scenario:

    At t=0s,
    The body has a forward force of 2250N and a frictional force of 1000N
    F res = 1250N

    At t=1s or any time in the future,
    The body will still have a forward force of 2250N and a frictional force of 1000N
    F res = 1250N

    Will it have a constant velocity or acceleration? How?

    From what you said, if there is constant acceleration, there will be a resultant force. If there is constant speed, there would be no resultant force? How? Is there any equation?
  6. Jun 8, 2014 #5


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    Gold Member

    ##\vec{F}=m\vec{a}## :smile:
    If there is a constant speed, acceleration is zero,therefore net force is zero
  7. Jun 8, 2014 #6


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    Gold Member

    The same equation :##F=ma##. If the force and mass is constant, acceleration is constant and speed is changing(As acceleration is defined as the change of speed with respect to time. ##\frac{dv}{dt}##
  8. Jun 8, 2014 #7

    Doc Al

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    Staff: Mentor

    Right. (Unless you are told otherwise, assume the givens stay the same.)

    As long as the forces do not change, neither will the acceleration.

    The equation is simply Newton's 2nd law:
    ∑F = ma

    When the velocity is constant the acceleration will be zero. Thus the net force will be zero.
  9. Jun 8, 2014 #8
    Ok. Thanks!
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