# Why things do not stop immediately?

• I
Why things do not stops immediately when a retarding force is applied on a moving ball.
• I know this is a foolish question but I have tried to give the answer
• my attempt : suppose I have applied force on a moving ball(its moving on a non frictional plane) and I want to stop it immediately means at a time = 0.
• F = m.a(this is a retarding force applied on ball and 'a' is the constant deaccelaration due to force on a ball )
• a = dv/dt , adt = dv , suppose initial velocity of a ball is 0.
• ∫a dt = ∫dv , lower limit and uppar limit for velocity is 0 and v and for time 0 and t
• Therefore v = at , to get immediate stop, t =0 , in that case a = ∞ , and F = ma , ∴F = ∞ , but it is impossible to apply infinite force. Therefore it is impossible to stop things immediately.

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andrewkirk
Homework Helper
Gold Member
Your reasoning looks correct. Average acceleration is change in velocity divided by time. So to change velocity by a nonzero amount in zero time would require infinite negative acceleration, which would require infinite force.

If you watch slow-motion footage of a cricketer or baseballer catching a fast ball, you'll see they let their hand(s) move backwards a little way after making contact. That increases the deceleration time, thereby reducing the force, thereby reducing the likelihood of the ball bouncing out of their hands (as well as lessening the potential damage to their hands).

Thanks andrewkirk for fielders example.

russ_watters
Mentor
Why bother with the math? You should be able to just state Newton's First Law of motion.

Why bother with the math? You should be able to just state Newton's First Law of motion.
Yes you are right but maths makes everything more clearer

russ_watters
Mentor
Yes you are right but maths makes everything more clearer
If you say so....let me know if your teacher agrees.

If you say so....let me know if your teacher agrees.
Hee hee , what's wrong in this answer , this question was mine and I will not showiy to my teacher. ^_^ ^_^

• a = dv/dt , adt = dv , suppose initial velocity of a ball is 0.
So you want to "stop" a ball with zero initial velocity?
You can do this instantaneously.

bsheikho
nasgot itpost: 5565525 said:
So you want to "stop" a ball with zero initial velocity?
You can do this instantaneously.
Hey nasu u got it , I was thinking of editing , ball was moving with constant velocity v and limit should be from v to 0 . And equation will be at = -v , a = -(v/t) it is negative because retarding force was applied ,