Chapter 6 Section 2

Objectives  Describe the interaction between two objects in terms of the change in momentum of each object.  Compare the total momentum of two objects before and after they interact.  State the law of conservation of momentum.  Predict the final velocities of objects after collisions, given the initial velocities, force, and time.

Momentum of conservation  One of the most powerful laws in physics is the law of momentum conservation. The law of momentum conservation can be stated as follows.momentum  For a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the momentum gained by object 2.n of conservationisolated system

Law of conservation m 1 v 1,i + m 2 v 2,i = m 1 v 1,f + m 2 v 2,f total initial momentum = total final momentum

Example #1  A 76 kg boater, initially at rest in a stationary 45 kg boat, steps out of the boat and onto the dock. If the boater moves out of the boat with a velocity of 2.5 m/s to the right,what is the final velocity of the boat?

Given: m 1 = 76 kgm 2 = 45 kg v 1,i = 0 v 2,i = 0 v 1,f = 2.5 m/s to the right Unknown: v 2,f = ? Continue v 2,f = 4.2 m/s to the left

Video  Let’s look at the video physics 602 conservation of momentum

STUDENT GUIDED PRACTICE  LETS DO WORKSHEET

Newton’s third law collision is an interaction between two objects that have made contact (usually) with each other. As in any interaction, a collision results in a force being applied to the two colliding objects. Newton's laws of motion govern such collisions. In the fourth unit, Newton's third law of motion was introduced and discussed. It was said that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the force on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

Newton’s third law  Newton's third law of motion is naturally applied to collisions between two objects. In a collision between two objects, both objects experience forces that are equal in magnitude and opposite in direction. Such forces often cause one object to speed up (gain momentum) and the other object to slow down (lose momentum). According to Newton's third law, the forces on the two objects are equal in magnitude. While the forces are equal in magnitude and opposite in direction, the accelerations of the objects are not necessarily equal in magnitude. In accord with Newton's second law of motion, the acceleration of an object is dependent upon both force and mass. Thus, if the colliding objects have unequal mass, they will have unequal accelerations as a result of the contact force that results during the collision. Newton's second law of motion

Newton’s third law  The total momentum is the same before and after the collision.

Homework!  Finish worksheet

Closure  Today we learned about the law of conservation and collisions.  Next we are going to continue with chapter 6 section3.