Section 9 3 Collisions Impulse Lets Briefly consider

Section 9 -3: Collisions & Impulse

• Lets Briefly consider the details of a collision – Assume that the collision lasts a very small time t • During the collision, the net force on the object is, from Newton’s 2 nd Law: ∑F = p/ t Or: p = (∑F)/ t (the momentum change of the object considered) Definiton The Impulse p J that the collision gives the object (The impulse received is the change in momentum for the object!) • We usually replace the net force by the average collision force: ∑F= Fc so that the Impulse received is J = p = Fc t

Equivalently, during a collision, objects can be deformed due to the large forces involved. Since we write Integrating gives This integral is defined as the Impulse J: The impulse one mass receives from the other in a collision is equal to the change in momentum of that mass:

Since the time of the collision is often very short, we often can use the a average force, Favg, which would produce the same impulse over the a same time interval. The maximum F is usually very large! ∆t is often very small & F is time dependent. The impulse in the collision is the area under the F vs. t curve. Replacing F with Favg is equivalent to replacing the area under the curve with the area of the rectangle. The collision time ∆t is usually very small!