Welcome to Physics 151 ©James Walker, Physics, 3 rd Ed. Prentice Hall

Walker, Chapter 12 Preliminary list of topics Walker, Chapters 1-18 Description of motion: Kinematics –Position (in 1-, 2-, 3- dimensions), Velocity & Acceleration Forces and Motion: Net Force equals mass times acceleration Examples of forces: –Contact forces (friction, “normal” force) –Gravity –Elastic forces: Springs and waves Kinetic Energy, Potential Energy, Work Momentum, Collisions Torque and Statics Oscillations & Waves Fluids & Thermodynamics

Walker, Chapter 13 Introduction Physics and Laws of Nature Units of Length, Mass, and Time Dimensional Analysis Significant Figures Converting Units Precision and Significant Figures Problem Solving in Physics

Walker, Chapter 14 What is Physics? In science in general, and physics in particular, we seek to develop and evaluate theories/concepts/ideas/perceptions.... The goal is to gain a deeper understanding of the world in which we live. Physics is the study of the fundamental laws of nature, which are the laws that underlie all physical phenomena in the universe. These laws can be expressed in terms of mathematical equations. We can make quantitative comparisons between the predictions of theory and the observations of experiment.

Walker, Chapter 15 Why Physics? 19 th century: economy was dominated by harnessing the power of steam. 20 th century economy was dominated by the internal combustion engine and the micro- electronics revolution. 21 st century economy may be revolutionized by the manipulation of individual atoms.

Walker, Chapter 16 Distance It is impossible to talk about distance or motion in any terms other than quantitative. –How far is it from CNU to Colonial Williamsburg? –How far is it from the Earth to the nearest large galaxy? It is impossible to speak quantitatively without defining a unit of measure. –It is meaningless to say that the distance to the Colonial Williamsburg is 20. Physical quantities have both numerical value and units. It is 20 miles from CNU to Colonial Williamsburg. 20 miles equals 32 kilometers. 20 miles also equals 105,600 ft = 32,208 m.

Walker, Chapter 17 Typical Distances Distance from the Earth to the nearest large galaxy (the Andromeda Galaxy, M31) 2 x m Diameter of our galaxy (the Milky Way)8 x m Distance from the Earth to the nearest star (other than the Sun)4 x m One light year9.46 x m Average radius of Pluto’s orbit6 x m Distance from Earth to the Sun1.5 x m Radius of Earth6.37 x 10 6 m Length of football field10 2 m Height of a person2 m Diameter of a CD0.12 m Diameter of the aorta0.018 m Diameter of the period in a sentence5 x 10 –4 m Diameter of a red blood cell8 x 10 –6 m Diameter of the hydrogen atom10 –10 m Diameter of a proton2 x 10 –15 m

Walker, Chapter 18 Time The second is defined as the duration of 9,192,631,770 oscillations of a particular atomic transition in Cesium-133 (defined by the spectral color of the light). –It is not an accident that the typical human heartbeat is 1 sec. Age of the universe5 x s Age of the Earth1.3 x s Existence of human species6 x s Human lifetime2 x 10 9 s One year3 x 10 7 s One day8.6 x 10 4 s

Walker, Chapter 19 The Equals Sign At the heart of mathematical notation is the equals sign "=". In algebraic notation, "=" means "has the value of" –v = 60 mi/hr, means “my speed on the highway, represented by the symbol ‘v’ has a value of 60 mi/hr” –F = m a, means “The net force acting on an object is numerically equal to the mass ‘m’ of the object times its acceleration ‘a’. In this example the left and right hand sides of the equal refer to totally different concepts, that Newton tells us nonetheless have the same numerical values. Notice that the symbol (e.g. ‘V’) includes both magnitude and units. –We do not write V mi/hr, nor V = 60, –but V = (60 mi/hr)

Walker, Chapter 110 Mass In SI units, mass is measured in Kg. We don’t define the Kg to be the weight but rather the ‘amount’ of substance contained in the object, which is an intrinsic and unchanging property. Weight, in contrast, is the measure of the gravitational force acting on an object. W = mg Galaxy (Milky Way) Sun 4 x kg 2 x kg Earth5.95 x kg Elephant5400 kg Human70 kg Honey bee1.5 x kg Bacterium kg

Walker, Chapter 111 Converting Units How do we convert from inches to cm, or miles to km (without crashing into Mars as NASA recently did in 1999!).crashing into Mars as NASA recently did in 1999!). By international convention 1 in = 2.54 cm Convert 4.75 in to cm Notice that units divide just like numbers (in/in) =1

Walker, Chapter 112 Experimental Uncertainties Physics is quantitative, not exact. Every physical quantity has an associated uncertainty, either in its measurement or in its prediction. –Note that ‘error’ is used sometimes to mean uncertainty and sometimes to mean mistake. –Carpenters rulers are marked off in 1/8 inch increments. Given the flexibility of a wooden house it is unnecessary and useless to try to measure dimensions in home construction to better than 1/8 inch –Estimating the uncertainty is often the most difficult part of a measurement. In your physics lab, you must make judgments about the precision of the distances, times, weights, etc. that you measure.

Walker, Chapter 113 Precision and Significant Figures If we say the acceleration due to gravity is 9.80 m/s/s, (three significant figures) we imply an uncertainty of  0.01  Thus: 9.80 m/s/s = (9.80 ±0.01) m/s/s  9.80 m/s/s is not the same as 9.8 m/s/s In the homework and tests, we will assume 3 significant figures unless specified otherwise. –To avoid round-off errors, you must carry through all intermediate calculations to at least 4 significant figures. –Use of excessive significant figures will result in point deductions on tests. Even though your calculator gives answers to 10 significant figures, it is wrong to write, e.g. “the ball stayed in the air for sec” if all the input data are not given with that precision.