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Confusion about force

  1. Nov 23, 2013 #1
    I have a confusion:
    If we push an object of 10 kg with force of 60N once, does it accelerate constantly with 6m/s/s forever neglecting any resistance ?

    So, does this mean if this moving object collides with something the maximum force it applies is 60N?
     
  2. jcsd
  3. Nov 23, 2013 #2
    I probably shouldn't be answering stuff because I am new too. But I'll have a crack at it anyway.

    P=mv where p is momentum and momentum is conserved. However you said it was only pushed once with 60N which makes me wonder if it would have a constant acceleration of 6m/s/s. I would have thought it would require a force acting on it to achieve constant acceleration. Otherwise it may have uniform motion of 6 m/s meaning 10kg multiplied by 6 which equals 60N.

    In that case yes it would hit something with 60N of force. But I am assuming this is a object in space with no resistance or opposing forces acting on it.
     
  4. Nov 23, 2013 #3

    Nugatory

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    Staff: Mentor

    No, it accelerates at that rate for as long as the force is applied, and when the force is no longer applied it stops accelerating and coasts along at whatever speed it reached while the force was being applied. In your example, the acceleration is ##\frac{6m}{sec^2}##, so if the force is applied for five seconds, the object will be moving at 30 m/sec.

    No. The force applied in the collision depends on the deceleration. If our object collides with something very hard and immobile, it might only compress the target by .1 mm before being brought to rest. If our object is moving at 20 meters/sec when it hits and it's stopped cold after covering .1 mm, its average speed during the collision is 10 meters/sec (average of 20 and 0), it takes .00001 seconds to cover that .0001 meters, the deceleration is 2000000 ##\frac{m}{sec^2}##, and the force (which isn't applied for very long - think hammer blow) is 12000000 N.

    Repeat this calculation with something soft and squishy, so that it compresses by 10 centimeters, and you'll get a much smaller force.
     
  5. Nov 24, 2013 #4
    For your first question, the answer is yes. It will constantly increase its speed but the rate of change of the speed is constant (acceleration is constant).
     
  6. Nov 24, 2013 #5
    You should read the post immediately before yours.
     
  7. Nov 24, 2013 #6
    Yes you are right. I've just read it again. It's a bad thing from me not to read the word "once". In this case, Nugatory's answer is correct.
     
  8. Nov 25, 2013 #7
    @Nugatory , isn't the force applied is 20000000 and not 12000000 because the mass is 10kg not 6 kg......!
     
  9. Nov 25, 2013 #8

    Nugatory

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    Staff: Mentor

    thanks - good catch.
     
  10. Nov 26, 2013 #9

    adjacent

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    Gold Member

    Collision force is given by the formula

    ##F=\frac{mv-mu}{t}##

    -Where u=Initial velocity ,v= final velocity

    For example,Object was traveling at a constant velocity and has the same.
    It gets in contact with a wall or something,the force exerted is determined by the time taken for it to stop.
     
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