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Homework Help: Velocity after force applied for 0.01 s

  1. Apr 16, 2016 #1
    1. The problem statement, all variables and given/known data
    A ball(1kg) at rest was hit by a stick to set it in motion. Assuming Force (10N) was applied by a stick, and stick remained in contact with the ball for 0.01s. Ball moves from A to B (10m) in time t.
    a) Velocity at B ?
    b) Time t ?
    (assume frictionless surface)

    2. Relevant equations
    F = ma, V = U + aT, V2 = U2 + 2as

    3. The attempt at a solution

    a = F/m = 10N/1kg = 10ms-2, how to proceed after this ?
  2. jcsd
  3. Apr 16, 2016 #2
    i think for such short duration force acting the concept of impulsive force and its effect on the state of motion/rest must be considered.
  4. Apr 16, 2016 #3
    You know the acceleration and you know the amount of time that the force is applied. So, what is the velocity after acceleration stops?
  5. Apr 16, 2016 #4
    Velocity will be V = 10*0.01 = 0.1ms-1.
    Now, this velocity will be come initial velocity for motion A to B.

    And since surface is frictionless, so

    Vb = Ua = 0.1ms-1.

    And as velocity will be constant from A to B, so t = AB/Ua = 10m/0.1ms-1 = 100sec.
  6. Apr 16, 2016 #5
    Let's visualise what's happening
    The stick remains in contact with the ball for 0.01 seconds
    So the impulse transferred equals force exerted times the duration=0.1kgm/s
    Now before contact , as the ball was initially at rest
    The velocity of the ball when it loses contact with the bat/stick
    Equal to 0.1/1=0.1m/s
    Therefore this becomes your initial velocity of travel
    Now remember that the 10N force no longer acts and thus there is no acceleration and due to the surface being frictionless, the ball moves with a constant velocity of 0.1m/s
    When the distance is divided by this, we get the time taken
    Which equals 10/0.1=100 seconds!
    Your approach is absolutely correct!

  7. Apr 16, 2016 #6
  8. Apr 16, 2016 #7
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  9. Apr 16, 2016 #8


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    If the ball starts at A then the answer should probably state that the duration of the acceleration phase is << than the coasting phase so it's effect on the time is negligible and can be ignored. It's not always true.
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