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Transcript
Instructor Outline: Newton’s Laws of Motion UM Physics Demo Lab 07/2013 Lab length: 80 minutes Lab objective: To demonstrate Newton’s three laws of motion Materials Dedicated Components (located in boxes): 8 Pitsco PC Sportster cars 8 sets of ½ inch x 2” steel hex bolts with nuts 8 sets of 8 ½ inch cut steel washers 8 9” diameter round balloons 8 2” pieces of 3/8” OD-1/4” ID clear vinyl tubing 8 3” pieces of 1/2” OD-3/8” ID black vinyl tubing (flags to trip photo gates) 8 balloon pumps 16 spring scales 50N/5000g yellow 8 clear plastic rulers 8 sets of 6 small rubber bands **Demonstrations: 1F20.33 Cloth from Under Dishes 1F30.u1 Frictionless Puck 1N22.20 Water Rocket 1D60.10 Relativity Cart 1N20.u1 Cannon Recoil Beer Bottle/$20 Bill (A. Tomasch) Model Rockets and Engines (A. Tomasch) Shared/Consumable Components: 1 digital gram scale 10 Pitsco 4-gram CO2 canisters Additional rubber bands and balloons Guide wire/data acquisition system for Pitsco cars: two ring stands to support guide wire, spring-loaded guide wire, Pitsco firing system with 12 V battery and firing button, two photo gates, ring stand photo gate support, aluminum roof edge guide rails, blue masking tape to secure guide rails to floor, laptop PC running Logger Pro “speed trap” application and MS Excel Introduction: 5 minutes-Lecture The concept of mass-inertia is reviewed and the table cloth is pulled out from under the dishes. Exploration stage: 10 minutes-Group lab work Newton’s first law is investigated by observing a static car on the table top and the straight-line motion of the air puck. The two states of motion are shown to be compatible with zero net force by means of a simple Galilean transformation to coordinates moving with the same speed and direction as the air puck. In the moving coordinates the puck is again at rest and equivalent to the car sitting motionless on the table. In both cases the motion will remain unchanged unless a net force acts. Exploration stage: 5 minutes-Group lab work The students next explore Newton’s Third Law by pulling on two scales connected together to deduce that the force each scale exerts on the other is of the same magnitude but in the opposite direction. Property of LS&A Physics Department Demonstration Lab Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109 Analysis stage: 5 minutes-Lecture Newton’s First and Third Laws are formally introduced and the results of the air puck and scale experiments discussed. The equivalence of motion with zero net force for a constant velocity or sitting at rest is introduced. The relativity cart demo is shown as further illustration. Exploration stage: 15 minutes-Group lab work The students now construct reaction propulsion systems for the Pitsco cars from tubing, balloons and rubber bands and demonstrate reaction propulsion by inflating the balloons with football pumps and propelling the cars on the table top. Analysis stage: 10 minutes-Lecture Momentum is defined and Newton’s Second Law is formally introduced in the general impulse-momentum form. Exploration stage: 15 minutes-Class group activity Each group is assigned a random number of washers (0, 2, 3, 4, 5, 6, 7, and 8) to load onto their car. Each car is then fired down the guide wire and its final speed measured and recorded along with its final mass. Analysis and summary stage: 15 minutes-Lecture/Class group activity The class data are entered into Excel and it is demonstrated that the product of the final speed and mass of the cars is remaining constant in accordance with the impulsemomentum formulation of Newton’s Second Law J=FΔt=Δp . The constant-mass formulation (F=ma) is then introduced. The fact that all objects fall with the same acceleration due to gravity is explained using Newton’s Second Law and the Free Body Diagram for a falling object. Einstein’s Equivalence Principle is mentioned to justify the statement that inertial and gravitational mass are equivalent. Concepts developed: 1. Mass as inertia - the resistance to being accelerated by a force. 2. Newton’s First Law of Motion - No force, no change in the state of motion. 3. Inertial reference frames and Galilean relativity: zero net force is compatible with being at rest or in motion with a constant velocity. 4. Newton’s Third Law of Motion Action-Reaction. 5. Reaction propulsion. 6. Linear Momentum: p=mv 7. Newton’s Second Law – Impulse equals change in momentum: J=FΔt=Δp 8. Newton’s Second Law for constant mass: F=ma 9. Newton’s Second Law explains why all objects fall with the same gravitational acceleration. 10. Equivalence of gravitational and inertial mass via Einstein’s Equivalence Principle. Property of LS&A Physics Department Demonstration Lab Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109