COL - COL CollisionsConservation of Momentum revised July...

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1 Collisions Learning Objectives: During this lab, you will 1. communicate scientific results in writing. 2. estimate the uncertainty in a quantity that is calculated from quantities that are uncertain. 3. test a physical law experimentally. A. Introduction The law of conservation of momentum is one of the great conservation laws in physics. Unlike energy, which is a scalar quantity, momentum is a vector. Also unlike energy, momentum can have only one form, the mass of an object multiplied by its veloc- ity, or Pm v = G G . Energy, on the other hand can take many forms: kinetic, potential, thermal, etc. Thus, there is rarely any ambi- guity about momentum conservation; for an isolated system, the vector sum of the momenta of the individual particles before an interaction must be equal to the vector sum of the momenta after the interaction. This principle applies even to those reactions in which new particles are created, initial parti- cles were destroyed ( i.e., mass converted into energy) , or mechanical energy was lost to friction or deformation. You will consider collisions between two carts on a low-friction track. Under ideal circumstances, i.e ., when no external forces are exerted on either cart, the net sum of the momenta of the two carts must be unchanged, so that the momentum gained by one cart is equal and opposite to the momentum lost by the other, 21 PP ∆= GG . In an elastic collision, the total mechanical energy is unchanged as well. An inelastic collision is one in which mechanical energy is either lost or created in the collision. You will investigate elastic collisions as well as both types of inelastic collisions in this experiment. You must write a report for this lab worth 60 points. B. Apparatus You will use a PASCO track with two low-friction carts, mass bars, a spirit level and a computer with Logger Pro software. To measure the motion of the carts, you will use a grating and two photogates. The grat- ing is a set of parallel black lines on a trans- parent holder. A photogate is simply a light- emitting diode aimed at a light sensor. It detects motion by sensing the alternate transmission and blocking of light as a grat- ing, mounted on a cart, slides by. Knowing the thickness of the lines on the grating and the time in each light cycle lets the computer calculate the speed of the cart. A schematic diagram of the experiment is at Figure 1. C. Theory C.1. Elastic Scattering When two objects ( 1 and 2 ) collide and scatter elastically, both the total mechanical energy and the total momentum are conserved in the interaction. Consider a COL Collisions–Conservation of Momentum revised July 11, 2005 m 1 m 2 (b) v' 1 v' 2 Figure 1: Schematic of experiment (a) before collision and (b) after collision. m 1 m 2 Photogate 1 Photogate 2 (a) v 1 Grating
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Collisions 2 system of two particles constrained to move in one dimension. A projectile particle ( 1 ) has some initial velocity towards a target particle ( 2 ), which is at rest before the colli- sion ( so the target’s initial momentum and energy are zero ). Our conservation equations
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This note was uploaded on 04/07/2008 for the course CHEM 111 taught by Professor Kenney during the Spring '08 term at Case Western.

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COL - COL CollisionsConservation of Momentum revised July...

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