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I Acceleration and force

  1. Aug 1, 2017 #1
    Consider a body of mass m at rest,then if I apply force f1 on that then it accelerates and attains a velocity of v1.then I remove that force..now the body is in uniform motion.if I apply a force f2 which is less than f1, then body will be accelerated or retardated?I know if same f1 is applied then acceleration will be the previous same value,and if I apply force greater than f1 then it will be accelerated ( but not uniform)..what about f2 less than f1
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  3. Aug 1, 2017 #2


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    Well, if you applied f2 in a direction opposite that of the body's motion, then yes it would be retarded/decelerated.
  4. Aug 1, 2017 #3


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    That depends on the direction of F2.

    Yes. If you apply F1 again the acceleration will be the same a2 = a1.

    If you apply a force F2 > F1 the object will accelerate faster so a2 > a1. However if the new force is constant the new acceleration will also be constant (=uniform).

    If F2 < F1 the new acceleration will be less than before, a2 < a1

    All you need to remember is Newton's Law...
    F = ma
    a = F/m

    The fact that the mass is moving does not change law.
  5. Aug 1, 2017 #4


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    F=ma, period. It doesn't remember previous action and doesn't know anything about speed.
  6. Aug 1, 2017 #5


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    Even more importantly ##\vec{F}=m \vec{a}##, i.e., force and acceleration are both vectors!
  7. Aug 1, 2017 #6
    Start with the velocity equation in case of constant acceleration:

    (t) = v(t = t0) + a⋅(t - t0)

    The body of mass m is moving with v1 when you, at time t2, suddenly apply a force f2 to the body. The acceleration is then a = f2/m. You get

    v(t) = v(t = t2) + f2/m⋅(t - t2) = v1 + f2/m⋅(t - t2)
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