# A question on Forces

1. Apr 13, 2010

### ShayanJ

A few days ago I was thinking about the time duration that a force changes the velocity of the object its applied to and I reached the conclusion that the effect of force is at the moment of its applying.I mean if a shot to the ball gives it a maximum velocity V.then the ball's velocity at the moment of applying the force from your foot is V.
You all know the formula $$F=m \frac{\Delta v}{ \Delta t}$$.if we want to apply the last paragraph to this formula,we should tend $$\Delta t$$ to zero which gives us infinity and means that just very large forces show their effect at the moment of their applying which is not what we said in the last paragraph.
I wanna know which of the things I've said is correct?
thanks alot

Last edited by a moderator: Apr 17, 2010
2. Apr 13, 2010

### tiny-tim

Hi Shyan!

I think you're confusing force with impulse …

force is rate of change of momentum …

impulse is total change of momentum …

force has to be applied over a time period, or nothing will happen.

(and impulse can be assumed to be instantaneous)

3. Apr 17, 2010

### ShayanJ

I'm still confused.Could you explain it in an example?
and one other question.As you know we tell $$K=1/2 mv^2$$ which means object has Kinetic energy till it has motion.Then we say if we have friction,energy is reduced and when it reaches zer,object stops.So we say object has motion till it has energy.An infinity cycle.Is'nt sth wrong?

Last edited by a moderator: Apr 17, 2010
4. Apr 17, 2010

### Staff: Mentor

When you have a ratio, just because the denominator tends to zero does not mean that the ratio goes to infinity. The numerator may be going to zero as well.

Consider a straight line. The slope is Δy/Δx. As you make Δx very small, does the slope change?

When something is moving, we say it has kinetic energy. When it stops moving, it no longer has kinetic energy. (The energy was transformed to another form, such as "heat".)

5. Apr 18, 2010

### ShayanJ

I mean which of them is depended on the other?(motion and kinetic energy)
And Could someone explain what happens when for example you shoot a ball?I don't know how to tell.I hope you understant what I mean!
thanks

6. Apr 18, 2010

### Staff: Mentor

If something is moving it has kinetic energy.
What do you mean?

7. Apr 20, 2010

### ShayanJ

I mean what happens every moment when for example we shoot a ball and what is the acceleration and velocity at any moment?I don't know how else can I say it?!