Motion Study of Motion Peer Led Team Learning
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Peer Led Team Learning Motion PLTL ES105x CRN 21220 Chapter 1: Pages 14-31 Review Questions 3-8, 10, 22, 24, 26-28 Galileo Use experiment to test logical ideas Defined ‘inertia’: keep moving in same direction and speed without outside influences— resistance to change of motion NS 016, 4:00-5:30 PM Wed. Take blue sheet to registrar, or show up to class this evening to decide Galileo's Study of Motion Discovered speed not dependent on weight, only on amount of time for falling Noted that gravity increased speed of falling objects, decrease speed of rising objects Study of Motion Aristotle—4th century BC Student of Plato Tutor of Alexander Used logic to describe natural world: collected, classified Motion ceased when objects in their proper place Thought speed of falling objects depended on their weight Ignored friction, air resistance Influential for 2000 years Aristotle , marble portrait bust, Roman copy (2nd century BC) of a Greek original (c. 325 BC); in the Museo Nazionale Romano, Rome Galileo’s investigation of motion • Used inclined planes to slow the descent of objects, because he didn’t have a precise timer http://airandspace.si.edu/etp/discovery/disc_galileo.html Galileo’s inclined planes Balls roll down faster and faster Roll up slower and slower Weight not a factor Galileo’s inclined planes Rises to same height as it is released Height not dependent on incline Mass How much matter is there Corresponds to weight—the influence of the acceleration of gravity on the mass à Mass is universal for object à Object Object’ss weight depends upon effect of gravity They are proportional 1 Inertia vs. weight Mass Weight is the force, due to gravity— Measured in kilograms Influence of gravity gives weight à Pounds lb. à Newtons N On Earth: 1 kg = 9.8 N Measure of inertia: resistance to movement Not a measure of volume Applied forces pulling iron ball down Inertia is resistance to change of movement—ball is not moving Pull slowly, y, you y increase force and break string that is holding the ball up Rapid jerk will break string below ball, because it has large mass that is not moving—has inertia Objects not moving Force Weight is a force due to gravity Force is VECTOR QUANTITY Vectors have magnitude and direction Multiple vectors add up Objects not moving Force of weight is equal to force of string holding it up The sum of the forces is zero There is mechanical equilibrium In equilibrium Support Force Dynamic Equilibrium Can be moving At a CONSTANT SPEED in a straight line Net forces are zero Friction Force that acts to resist motion Always in opposite direction to applied force When you are pushing something, and it moves at a constant speed, the frictional force is the same as the pushing force Weight acts downward Atoms push back upward Forces equal—in equilibrium 2 Speed Study of Motion Speed—how fast Velocity—how fast and what direction Acceleration Acceleration—how how fast it is speed = Common units of speed Miles per hour distance time à Don’t use this abbreviation of the words à Use mi./h changing velocity 320km = 4h Speed of cheetah 100m 4s Car traveling Rate × time = distance Keep units with numbers, numbers so you know you have set up the problems correctly Speed and Velocity 60 km/h for 4 h 60km • 4h = 240km h 60 km/h for 10 h 60km • 10h = h 80km h Distance equation 25m = s mph à Means ‘miles per hour’ Kilometers per hour km/h Meters per second m/s Example of rt=d calculation 80km • 4h = 320km h • Notice that hours cancels because it is above and below the fraction bar Acceleration • Acceleration = Change in velocity Time interval CHANGE of speed over time time, not the RATE of speed RATE OF CHANGE 600km 3 Delta Δ Acceleration of gravity Δ is the fourth letter in the Greek alphabet Used in equations to represent change Δv = change in velocity à Find final velocity, find initial velocity, and subtract Δt = change in time, or time interval from beginning to end Units of time appear twice in denominator Acceleration of Gravity Free fall of object Time elapsed Speed increases 10 m/s for every (seconds) second of fall Speed (meters/ second) 0 0 over more time 1 10 10 m/s = 10m 2 20 3 30 4 40 à It goes faster s s2 How to Convert mi./h to m/s x mi. 1h 1 min 1610 m 0.447 m • • • =x h 60 min 60 sec mi. s Acceleration of Gravity Acceleration same for 9.81 m/s2 at sea level Round off to 10 m/s2 for ease of calculation each second of travel Free falling objects in lecture. Use more precise value for lab calculations. …or if you are trying to launch a rocket to space, etc. Acceleration of Gravity is downward Upward throw 30 m/s Gravity acts against it Slows to stop at 10 m/s2 Falls and gains speed at 10 m/s2 Neglecting air resistance Galileo’s investigation of motion Surface area changes air resistance Objects reach terminal velocity due to air resistance In vacuum, this is not a factor Acceleration a= 25mi. / h = 1mi. / h ⋅ s 25s Notice that time units appear in denominator twice Because it is an amount of time over which the change of speed occurs Does not need to be the same time units, (but it’s neater if it is: can change miles per hour into miles per second…) Acceleration Neglect air resistance for these equations a= Δv/ Δt Æ v=at • acceleration is velocity divided by time • velocity acquired is acceleration multiplied by time 4
