Physics Announcements Website is up!!! Clickers WebAssign – must have Account Codes in 2 weeks (from yesterday) –You can use without a code till then –Homework opens on the day of class, closes 1 week later before class. Exam #1 next Wednesday –Covers Chapters 1-4

Class Schedule 1/17Chapter 1Introduction, About Science 1/22Chapter 2Newton’s First Law 1/24Chapter 3Linear Motion 1/29Chapter 4Newton’s Second Law 1/31Midterm Exam 1

Chapter 3 Linear Motion Relative Motion Speed –Instantaneous Speed –Average Speed Velocity –Constant Velocity –Changing Velocity Acceleration Free Fall

Relative Motion - Speed is always measured with reference to another “fixed” object. (usually the ground) - The speed of a car is measured relative to the road’s surface. - Anything moving must be moving with respect to a fixed frame of reference. This is “Newtonian Relativity” (compared to Einstein’s Theory of Relativity). - Speed of car A relative to the ground = 40 m/s - Speed of car B relative to the ground = 30 m/s - Speed of car A relative to car B = 10 m/s (car A is moving AWAY from car B) - Speed of car B relative to car A = -10 m/s (car B is moving TOWARD car A) Motion of a boat on a river (more complicated) Unless stated otherwise, giving something’s speed means “relative to the earth’s surface”.

Speed Instantaneous Speed – Measure of an object’s speed at a particular instant. A car’s speedometer gives you the instantaneous speed. A police radar ALSO measures instantaneous speed. Measure of how far an object can go in a given amount of time Average Speed – Total distance traveled divided by the time or travel. If you drive 490 miles and it takes you 7 hours to get there: Average vs. Instantaneous Speed applet

Velocity A velocity tells you both the speed and the direction of motion. Speed: 60km/hr Velocity: 60km/hr to the north Velocity is what’s called a vector quantity in mathematics. Vectors: Include information on both magnitude and direction. Other vectors … wind velocity, acceleration, ocean currents … Scalar quantities (like mass, time, temperature, etc.) don’t have a direction. Constant Velocity – unchanging speed AND direction. Changing Velocity – changing speed OR direction OR both. A car traveling around a curve at constant speed is accelerating.

Question 1 The thing that distinguishes speed from velocity is: a.magnitude. b.time. c.time squared. d. direction.

Acceleration Direction of velocity and acceleration Acceleration applet Measures how fast velocity is changing, either by changing speed or direction. Can be positive or negative. Speeding up = positive acceleration Slowing down = negative acceleration (or de-celeration) Example: You speed up on your motorcycle (a 1996 Harley Road King) from 30 km/hr to 35 km/hr in 1 sec. What’s your acceleration: Acceleration with ramps – Interactive Figure s/iFigs.html?Hewitt_IF_03_06.swf

Question 2 A car goes from 90 km/hr to rest in 10 seconds. What’s the car’s acceleration? a.9 km/h·s b.900 km/h·s c.- 9 km/h·s d. 45 parsecs/h·s

Free Fall When an object is dropped near Earth’s surface, it falls at a velocity that increases about 10 m/s every second. Really closer to 9.8m/s 2. Standard practice in Physics to call this acceleration due to earth’s gravity “g” g = 9.81 m/s 2 (for now, round off to 10 m/s 2 ) The motion of object in free fall is determined by the acceleration of gravity (neglecting air resistance). The acceleration due to gravity (g) is so predictable, you can predict its speed and how far the object will fall in a given amount of time. Not only that, if you throw an object up, it will eventually stop and turn around …. Arriving back at your hand traveling the same speed as you originally threw it up at. cable/js/iFigs.html?Hewitt_IF_03_08.swf