Physics 131, Chapter 1

• This class is about classical objects– Bigger than an electron– Smaller than a black hole– Objects you can see/hear/manipulate

• No quantum mechanics, no string theory• No calculus• You will need algebra every hour of every day, and

trig nearly that often.

Units

• We will use Scientific International (SI) units• The more you use these metric units, the

more you will like them.• Study the prefixes in Table 1-4. They are

taught in grade 7 in Greece.• Memorize “centimeter” (cm = 10-2 m) and

“liter” (L = 103 cm3 = 10-6 m3)

Units

• The period of a simple pendulum is defined as the time necessary for one complete oscillation. The period is given by

• T = 2 (l/g)1/2

• Where l is the length of the pendulum and g is the acceleration of gravity, in units of length divided by time squared.

• ? Show that this equation is dimensionally consistent. ?

Cost of Gas in Greece

• What’s the cost of gas in Greece?• Need – Euros per liter– Liters per gallon– Cost of Euros in dollars

Significant FiguresDoubtful Digits

• Given 2 digits, don’t answer with 5!• 1. x 10-3 one sigfig• 1.0 x 10-3 two sigfigs• 1 => 1.00000000• See national debt. Identify doubtful digits!• Odds of perfect NCAA Men’s Basketball

bracket: 1 in 9, 223, 372, 036, 854, 775, 808.• Identify any doubtful digits. Explain.

Body Mass Index (BMI), Units

• BMI is defined as wt(kg)/[ht(m)]2

• But we need BMI is units of pounds and inches• BMI is defined as A wt(pounds)/[ht(inches)]2

• Find “A”

• BMI OK < 25, over weight 26-30, obese >30

• UK “keep your waist circumference to less than half your height.”

Is your answer reasonable?

• Given: 250 x 106 packages of French Fries• ? End to end, how long?

• Estimations can be valuable in deciding if your answer is “in the ballpark.”

Sigfigs – block of gold

• Gold bar• L = 5.62 cm• W = 6.35 cm• H = 2.78 cm

• Don’t throw away any digits until the end. Let your calculator do the work. Do the whole calculation as one operation.

Too many sigfigs?

• Ear thermometer• Precise to 0.1o F• Accurate to 0.5o F

Scalars and Vectors

• A scalar has a numerical value• A vector has both a numerical value and a

direction. Think of an arrow.

• We’ll use “speed” as a scalar and “velocity” as a vector for this course.

Chapter 2

• Motion in one dimension – straight line• Distance – a scalar, has magnitude• Displacement – a vector– From its beginning to its end– Has a magnitude– Has a direction– Expressed as a vector a

Velocity

• Average speed – total distance / total time.• Average velocity – average speed in a

direction

Average velocity example

• Home to top of hill = 10.0 km• Pedaled up at average sped of 5.00 km/hr.

• Q1. average speed?• Q2. average velocity?• Q3. what speed down to average 10.0 km/hr

for the trip ?

Cross Country Car Race

Two cars travel in the same direction along a straight highway, one at a constant speed of 55 km/hr and the other at 70 km/hr. (a) Assuming that they start at the same point, how much sooner does the faster car arrive at a destination 10 km away? (b) How far must the faster car travel before it has a 15-minute lead on the slower car?

Average Acceleration

A car traveling in a straight line has a velocity of +5.0 m/s at some instant. After 4.0 s, its velocity is +8.0 m/s. What is the car’s average acceleration during the 4.0-s time interval?

Section 2.5 Constant Acceleration

!!! We only deal with constant acceleration in this course. !!!

2-7 v = vo + at2-8 x = xo + vavt2-9 vav = ½ (vo + v)2-10 x = xo + ½ (vo + v) t2-11 x = xo + vot + ½ a t2

2-12 v2 = vo2 + 2a (x – xo)

= vo2 + 2a (x)

Drag Racer

Assume: constant accelerationGoes quarter-mile (402.3 m) in 5.0 secondsWhat average acceleration?

Steam Catapult on Aircraft Carrier

Given: 175 mi/hr ( 78.22 m/s) in 2.5 sa)Find average accelerationb)Find distance traveled on deck

Parachute

Assume: before parachute opens, descending at -5 m/s

Assume: after parachute opens, descending at -1 m/s

Assume t = 1 s? Acceleration during that exciting second

Superball

• Given: 50.0 g superball• Given: thrown toward brick wall at 25.0 m/s• Given rebounds at 22.0 m/s from the wall• Ball in contact with wall for 3.50 ms

• ? Average acceleration ?

Lab 1

• Learn to use the motion sensor• Learn to use the Smart Tool• Learn to use the Slope Tool

• Lab 1 is designed as your “how to” lab

• Bookbags under tables, not in walkways!• May start at 1:00 p.m., must start by 1:15 p.m.

Lab 1 info

• The act of “printing” creates the pdf file on the computer “desktop” which you will e-mail to the instructor.

• gkeller@unca.edu

• I mark you “present” for a lab when I get your pdf file. Check with me as you leave the lab to be sure you sent it and I received it!