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LAB# 11 MOMENTUM - ELASTIC & INELASTIC COLLISIONS Figure 1: Equipment for the "Elastic & Inelastic Collisions" experiment showing the air track, two air gliders, elastic and inelastic bumpers, two photocells and the glider flags. Introduction: Momentum is one of the important physical variables used in the quantitative description of dynamics. What makes it important is its usefulness, and what makes it useful is that in an isolated system it is conserved. The conditions for momentum conservation are: If the net external force acting on an object, or system of objects, is zero the total momentum of that object, or system of objects, is constant. The linear momentum of a physical body is defined as the product of its mass and its velocity, . v m P (1) It is a vector quantity like velocity, acceleration, and force, and every moving physical body possesses momentum. When two freely moving bodies collide, m omentum is “conserved” . That is, the momentum of the system before the collision is equal to the momentum of the system after collision. This Conservation of Momentum Law applies to elastic and inelastic collisions, regardless of the nature of the interaction force between the objects 1 . Thus, using conservation of momentum even though the details of the forces of collision are complicated and unknown, you can find the motion of the bodies after a collision if you know their motion before the collision. In the first part of this experiment you will analyze only elastic collisions in one dimension. These are collisions in which the objects rebound from the collision in such a way that kinetic energy is conserved. 1 Unlike conservation of energy, conservation of momentum is not violated by the actions of internal forces. For energy, energy is conserved only if the internal forces are conservative.
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In the second part of this experiment we will investigate inelastic collisions. These are collisions where the objects stick together and/or deform so that kinetic energy in not conserved. Specifically, we will investigate perfectly inelastic collisions. This is a collision where the objects stick together and move off as one after the collision with the same final velocity. Equipment : Pasco® air track, the Lab# 11 Collisions” program, two gliders, air track parts kit which includes the elastic and inelastic bumpers, along with two flags and two photocells. See Figure 1. Figure 2 Bumper Plate. One possible choice for bumper in elastic collisions. (Hint: Fill in the open circles in front of the procedure steps to help you perform the experiment) Procedure: o For all parts of this experiment you will use two gliders of equal mass (m a = m b ). Attach counterweights so that the masses of the gliders are equal. o Find the mass of the gliders after you have attached the bumpers and counterweights. As shown in Figure 1, attach the rubber band bumper to one glider and a bumper plate to the other. Note: Some groups find that using two rubber band bumpers, as shown in Figure 1, and turning one so that they are at 90
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