Physics 102-002Announcements • 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
Chapter 3Linear 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) http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=227 Unless stated otherwise, giving something’s speed means “relative to the earth’s surface”.
Speed Measure of how far an object can go in a given amount of time 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. 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 http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/kinema/trip.html
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:
Acceleration 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: Direction of velocity and acceleration http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/kinema/avd.html Acceleration applet http://www.ngsir.netfirms.com/englishhtm/Kinematics.htm Acceleration with ramps – Interactive Figure http://wps.aw.com/wps/media/objects/2315/2370801/_wps_stylereplacable/js/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?
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/s2 . Standard practice in Physics to call this acceleration due to earth’s gravity “g” g = 9.81 m/s2 (for now, round off to 10 m/s2 ) 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. http://wps.aw.com/wps/media/objects/2315/2370801/_wps_stylereplacable/js/iFigs.html?Hewitt_IF_03_08.swf