Impulse is defined as the product of a force $F$ acting for a (short) time $t$, $J = F*t$, and that is very clear. What I find difficult to understand is how a force can exist that doesn't act for a time.
If we consider the most common and observable force: gravity, the force of gravity is defined as $m*g$ and for a body of 1 Kg of mass is equivalent to $\approx$ 10 N.
But whenever we consider gravity we must consider the time, if a book falls from the table to the ground (h = .8m) the force acts for a (short) time t = 0.4 sec.
- Is there/can there be a force that doesn't act for a time?
- Can you explain why do not refer to the fall of the book as the impulse of gravity?
- Why if the same (short) time happens in a collision we call it an impulse?
- Isn't always a force actually an impulse?
I'm not sure it's helpful to think about the gravitational force, because I can't see a similar physical system where we can imagine the gravitational force deliverting a non-zero impulse in zero time. - John Rennie
If I got it right, you are saying that we must consider it impulse when $t=0$?, else it is force.
- But, also when the book falls to the ground because of gravity there is a change of momentum, why is that not impulse? That is the elusive difference, for me.
force is not defined over a billion years, but:
a force is any interaction which tends to change the motion of an object. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described by intuitive concepts such as a push or a pull.
therefore also in a collision there is a push on a ball, exactly the same as here: there is a push on the book that tends to change its motion. What is the difference?