Announcements. Describing Motion. Teaching Assistant. Where to find the extra-credit and home-experiment turn-in box

Transcription

1 Describing Motion Announcements Homework # 1 and extra credit # 1 due on Friday at 11 PM. 5 September 2013 Teaching Assistant Bryce Johnson brycesjohnson@gmail.com Office hours: Tues Thurs Fri 2:20-6:20 PM 9 AM-Noon 10 AM-1 PM Held in the 106/123/220 Tutorial lab room, N-361 ESC Where to find the extra-credit and home-experiment turn-in box North stairs from the lobby North Elevator The turn-in slot is in the box under this window. North-East staircase 1

2 Here is a picture of the Physics 101 turn-in box. It is on the east side of the hall. Class Profile Major: Open/undecided 19 Business/Econ/Accounting 8 Bio Sciences 15 Technical (ME/CS/Phys) 5 Humanities (Eng/Lang) 7 Social Sciences 2 Fine Arts/Performance 0 Education 4 Class Profile Year in college: 1st 24 _ 2nd 8 _ 3rd 16 _ 4th or more 12 _ What is your purpose in taking this class? (Please mark all that apply.) This class fills the Physical Science requirement. 37 _ This class is required for my major. 3 _ This class is recommended but not required by my major. 3 _ I wish to prepare myself for a more advanced physics class. 16 _ I am just interested in physics. 18 _ Other. 10 _ Class Profile What is the highest level of math you have completed? Basic Algebra 10 College Algebra 10 Geometry/Trigonometry 8 Calculus or more advanced 32.How confident are you in your ability to use algebra? Completely confident (No doubts at all.) 14 Quite confident (Very few doubts) 23 Moderately confident. 18 Not very confident. (A lot of doubts) 5 Not confident at all. (Full of doubts) 0.Which science courses did you complete in high school or college? (Mark all that apply.) Biology 55 Chemistry 53 Earth Science 29 Physics 34 Other: 15 2

3 Where to find things Is physics in danger? Vectors and Scalars Check the reasonableness of your results! If a complete definition of some quantity requires a number (with units) and a direction then the quantity is a vector quantity. Examples: Position, velocity, force, acceleration, momentum 3

4 Vectors and Scalars If a complete definition of some quantity requires only a number (with units) then the quantity is a scalar quantity. Examples: time, mass, temperature, brightness of a light, area, volume Equality of Vectors Two vectors are equal if they have the same magnitude and the same direction. A B if A = B and they point along parallel lines All of the vectors shown are equal. Allows a vector to be moved to a position parallel to itself Adding Vectors Two methods: Graphically Good conceptually, good for checking answers, Not so good for accuracy. Algebraically More complicated conceptually, but accurate. We won t do this in this class (whew!) Graphical Addition Choose a scale. Draw the first vector in the right direction and length. Draw the second vector from the tip of the first. The sum is the vector from the start of the first to the tip of the second. Measure the length and direction of the result. Convert the length to real units according to the scale. 4

5 Adding more than 2 vectors Just follow the above procedure for as many vectors as you have. Vectors are commutative, it doesn t matter what order you use. Subtracting vectors Just as in normal subtraction, it is the same as adding the negative of the given vector. What is the negative of a vector? The vector that will give 0 when added to the original vector. This is one of the same length and directly opposite direction. Subtracting vectors To subtract, just add the negative of the vector you want. Definitions of terms to describe motion One of the things that makes physics difficult for many people is that sometimes we use ordinary words in very specific ways that sometimes conflict with the everyday usage. For example, in everyday usage the words Force, Energy, and Power can all mean the same thing. In physics they are all different and each means a specific thing. 5

6 Examples from Math Think of two different usages for each of these words, one if you were in a Math class and one if you were in an English class. Meter Angle Constant Domain Factor Gross Mean Negative Parallel Prime Range Rational Variable Imaginary complex Describing Motion We need to define 3 quantities to describe motion: Speed Velocity Acceleration These definitions are similar to, but slightly different from those used in ordinary speech. Change in position x Average Speed Average speed = Distance traveled time traveled s ave d t x t stands for " Change in" x x x initial Average speed is a time rate Units of speed: distance/time» Feet/second, MPH=mi/hr, m/s, km/hr, furlong/fortnight Convert 2 furlong/fortnight to m/s furlong 660 feet 1 meter 1 fortnight 1 day 1 hour m/s fortnight 1 furlong feet 14 days 24 hours 3600 seconds =3.33 mm/s 6

7 Average speed Examples 1. Estimate average speed of ball on desk d s ave t 2. Estimate average speed of my recent trip to Coral Pink Sand Dunes State Park in southern Utah: Departure time: 12:30 PM Arrival time: 5:00 PM Odometer reading: 275 miles s ave 275 mi 4.5 hr 61 mi/hr 98 km/hr Instantaneous Speed This is what the Highway Patrol cares about. It is the average speed over a very small time interval Instantaneous speed Rolling ball Make a graph of the instantaneous speed of the balling rolling along the track. Michael is going to the store 6 miles away. He rides his bike at 12 mph for the first half of the trip, then walks at 4 mph for the remainder. Michael's average speed for the trip to the store is closest to: A. 6 mph B. 8 mph C. 10 mph D. 14 mph E. impossible to determine 7

8 Instantaneous speed How small of a time interval is necessary to estimate instantaneous speed? Pre-class quiz Acceleration and velocity are vectors. Liters, grams, meters, and inches are all scalars. Velocity, acceleration and force are vectors. Mass, Volume, Temperature, Time and disk capacity are scalars Velocity In normal use speed and velocity are synonyms. In physics, velocity is a vector, it includes direction as well as magnitude. The speed is the magnitude of the velocity. Length of arrow indicates the magnitude of the vector Positive and Negative Vectors Often the sign on a vector is related to particular directions. 1. Positive is up, negative is down Example: Ball thrown up and coming back down Speed Velocity s v Speed is the same Direction is different Velocity is different time time 8

9 Positive and Negative Vectors 2. Positive is right; negative is left Example: Ball rolls across floor and bounces off a wall Position v x time Velocity time Acceleration Acceleration is the rate at which velocity changes Average acceleration = change in velocity/elapsed time a ave a ave v v t t v t initial initial v 20 m/s 0 4 m/s t 5 s 0 2 Pre-Class Quiz If an object's velocity is at a constant zero, then it has no rate of change, and therefore no acceleration. Since acceleration is the change of velocity over a certain period of time, the velocity can't stay at zero if the object is accelerating. Because when velocity is zero, acceleration is that amount per second, which is then also zero. Pre-Class Quiz If a man is running in a constant direction, and another person pushes them backwards, the runner will slow down and then start going backwards. The moment when they switch from running forwards to running backwards, they will have zero velocity and a negative acceleration. When you throw a ball up into the air, it has an upward heading velocity, but a downward acceleration due to gravity. This acceleration will remain constant, but at one point the velocity hits zero as it changes direction itself from up to down. This moment, at the peak of the ball's flight is an example of how this is possible. When a car is starting, it can accelerate without any velocity. 9

10 Positive Acceleration a ave v v t t v t initial initial Negative Acceleration a ave v v t t v t initial initial If the change in velocity is positive, the acceleration is positive If the change in velocity is negative, the acceleration is negative In each of the diagrams to the left, is the acceleration (a) positive, (b) negative or (c) zero? Assume that the car is pointed in the positive direction. (a) positive (b) negative (c) zero 10

11 Video Demo Graphs of Motion Uniform Acceleration Uniform Acceleration Uniform acceleration means that the rate of change of the velocity is constant. 11

12 Announcements Homework # 1 and extra credit # 1 due on Friday at 11 PM. 12