I am saying that if we use an idealised model where we take the limit of zero collision time the impulse remains a well defined quantity when the force does not. If I got it right, you are saying that we must consider it impulse when t=0?, else it is force. 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. In this case we get the unphysical situation where the force is infinite but acts for zero time, but we don't care because we recognise it as the limiting case of increasing force for decreasing duraction and we know the impulse remains constant as we take this limit. undergraduates) are calculating how the balls recoil we generally simplify the system and assume that the collision takes zero time. With the soft balls we get a low force for a long time, with the hard balls we get a high force for a short time, but in both cases (assuming the collision is elastic) the impulse (and change of momentum) is the same. If we use extremely hard balls the collision will take a much shorter time because the balls don't deform as much. If we use soft squidgy balls then the collision will take a relatively long time as the balls touch, then compress each other, then separate again. Where I've used an integral because the force is generally not be constant during the collision. We know that the change of momentum is just the impulse, and we know that the impulse is given by: When the balls collide they change momentum. However I think it's useful to consider a collision, perhaps between two billiard balls. It's hard to think of a physical system involving a force that acted for zero time. 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. In other words, a force can cause an object with mass toĬhange its velocity (which includes to begin moving from a state of 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.įorce is not defined over a billion years, but:Ī force is any interaction which tends to change the motion of an.If I got it right, you are saying that we must consider it impulse when $t=0$?, else it is force. Gravitational force deliverting a non-zero impulse in zero time. I'm not sure it's helpful to think about the gravitational force,īecause I can't see a similar physical system where we can imagine the Isn't always a force actually an impulse?.Why if the same (short) time happens in a collision we call it an impulse?.Can you explain why do not refer to the fall of the book as the impulse of gravity?.Is there/can there be a force that doesn't act for a time?.8m) the force acts for a (short) time t = 0.4 sec. 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.īut whenever we consider gravity we must consider the time, if a book falls from the table to the ground (h =. What I find difficult to understand is how a force can exist that doesn't act for a time. Impulse is defined as the product of a force $F$ acting for a (short) time $t$, $J = F*t$, and that is very clear.
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