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• kkkk234 於 25/12/2013 評論 NO. 1

  回樓上~~~ 請review一下中學物理.........
  
  依個situation 唔係force既問題....係momentum既問題 momentum depends on velocity.......
  
  如果你同我講velocity 係maximum in the beginning......咁我地就冇討論落去既必要.........."因為你應該係文科既 "

• eddylm 於 25/12/2013 評論 NO. 2

  re: kkkk234Change in momentum(mv-mu) / impact time(t) = force (F) How come it is not related to force ? Since the ball is at rest in the beginning, u = 0ms-1 Thus, Ft = mv When the ball leave his leg , the external force of the ball= air resistance , in which it's against it's motion , and thus , it has a negative acceleration. For negative acceleration , the velocity would just decrease instead of increasing . The longer the ball in the air , the smaller the velocity , and hence the lower the force impacted with the man ((mv-mu)/t= F) Why isn't the velocity maximum in the beginning with a negative acceleration later on :o) 我覺得好有討論嘅必要喎:) Btw, please revise ur physics dude :)

• kkkk234 於 26/12/2013 評論 NO. 3

  try to do a experiment and see if the velocity would "just decrease instead of increasing".....
  
  basic on what you said I guess you mean the velocity would just decrease and you also said that in the beginning the velocity is zero...... if you think this makes sense.....you win.....
  
  In the beginning, velocity is zero and acceleration is maximum. Basic on Newton second law we can write that ma = -(air resistance constant) time the velocity. However acceleration is maximum in the beginning due to the player applied a force on it. Acceleration will decreasing until the acceleration goes to zero. During this time, the velocity of the ball is increasing not decreasing and this is the reason the ball will move forward. After the zero points, the acceleration will be a negative value and the velocity of the ball will decrease but the velocity will never be a negative value. This is because the air resistance force depends on velocity. As the velocity decrease, the air resistance force decrease. (Here, we do not consider wind.)
  
  Now go back to the equation you give me "momentum(mv-mu) / impact time(t) = force (F)". You just help me answer why the focus of this problem is the momentum not force because force depends on momentum....and momentum depends on the "velocity" and I just shows that velocity is not the maximum in the beginning.
  
  I always review my physics before I answer question but I do not need high school physics to answer you that it is not possible for a ball moving forward but has a zero velocity the beginning and it only decreases......
  
  Thank you.
  
  
  

• eddylm 於 26/12/2013 評論 NO. 4

  Yup the velocity of the ball (u)is zero in the beginning before the player touched the ball , but after the player has contacted the ball and at the instance that the ball leave the player's leg , the ball has a velocity (v) , as illustrated by the equation ( (mv-mu)/t(impact time of the ball and the player's leg) =F(force applied by the player )) . Yup , by newton second law , f= ma (net force = mass x acceleration ) , however ,how come there is a forward force exist when the ball is in the air. In fact, it has a backward force enacted by the air resistance only . Thus , the ball has a negative acceleration immediately right after the ball has left the player's leg , instead of acceleration decrease to zero step by step. With a negative acceleration , the velocity of the ball will just decrease at any instance in the air instead of increasing. It is the velocity which will eventually goes to zero , but not the acceleration which will decrease to zero :) Please figue out the difference between the acceleration and velocity :) Yup , force depends on momentum . but it is the force and the impact time which serve as the main factors that kills a player or not . But in fact , you said this is not the problem about force , which does shock me for a while :) If u revise ur high school physics , I bet you will know what I mean about Ft=mv , and that v (forward)is the velocity of the ball at the instance the ball leave the player's leg :) Thank you .

• kkkk234 於 26/12/2013 評論 NO. 5

  Thank you for reminding me this basic concept. I agree with you right now!