Whenever we consider an explosion of a projectile in air, we solve for the velocities of the exploded elements by using the Law of Conservation of Momentum (ignoring any sound or light effect here). But I just don't get it, how can we conserve momentum, when the external force gravity is acting on each and every particle considered.
The same problem came up to me while solving one more question. Let us say we have an Atwood machine in the vertical plane with a pan on one side and a block on the other side. Suddenly a particle comes up and hit the pan from the bottom and stick to it, here also we considered the impulse imparted due to the normal reaction between the pan and the particle and due to the tensions in the string. Why are we not considering any impulse due to the gravity here?