2. PH 105 Dr. Cecilia Vogel Lecture 1

3. OUTLINE  Intro & units  What is Sound?  Mechanics  speed  acceleration  force

4. Units  In this class, we use SI or mks units  distance measured in meters (m)  mass measured in kilogram (kg)  time measured in seconds (s)  Get an intuition about size of m and kg  meter stick, kg weight  my mass: about 60 kg  my height: about 1.6 m

5. Units Conversion  Units Conversions  conversion factors in appendix A2  NEVER multiply by anything but 1  For example, App A2 says 1 m is the same as 39.37 in, so

6. Units Conversion Example  Take my height of 1.6 m  multiply by 1  in such a way that the meters unit will cancel, and inches will be left

7. Units  Units and variables:  variable names will be in italics  ex: ½ mv 2  m is mass variable, v is speed variable  units will not be in italics  ex: 9.8 m/s 2  m is meters unit, s is seconds unit

8. Sound  Sound in air  is a disturbance of air molecules  which propagates (moves)  Anything that disturbs the air suddenly makes a sound  anything that pushes air at rest  like guitar string, drum head  anything that disturbs the flow of moving air  like bassoon reed, branches in the wind (or other material)

9. Mechanics  To study sound, we need the language of mechanics  mechanics is the study of motion  Be careful, many physics terms have different meanings in everyday use.  speed, acceleration, force, work, energy, pressure

10. Speed  Speed: how fast an object is moving  variable name, v  the distance it travels  divided by the time it takes to travel

11. Constant Motion  If the speed and direction of motion of object doesn’t change, then  d = vt  “distance equals rate times time”  Only works if there is a single, constant value of speed, v, to plug in!

12. Examples  A car travels 50 mph for one hour. How far does it go?  d = vt  d = (50 mi/hr)(1 hr) = 50 mi  A sound wave travels 343 m/s for one ms  d = vt  d = (343 m/s)(1 X 10 -3 s) = 0.343 m  A car speeds up from rest to 30 mph in 10 s. How far does it go?  I don’t know!

13. Acceleration  If an object’s motion is not constant  the object is accelerating  do not use d = vt  acceleration in physics includes:  speeding up  slowing down (also called deceleration)  changing direction  If the police officer is a physicist, won’t mind if you say, “I accelerated as I approaches the stop sign.”

14. Constant Acceleration  If an object’s motion is not constant,  but the acceleration is constant  speed changing at a constant rate  like free-fall  then the change in v (delta- v )

15. Force  Any acceleration,  i.e. any change in motion,  requires a force (push or pull).  Demo: Object at rest tends to stay at rest. How do you get it moving?  Object moving tends to keep moving. How do you speed it up? slow it down? stop it? change its direction?

16. Equilibrium  Equilibrium means there is no force.  If there’s no force, there’s no acceleration,  there is no change in motion.  Demo: Pendulum at bottom feels no force.  What happens to it if it is at rest there?  What happens to if it is moving there?

17. Mass  How much force does it take?  depends on the size of acceleration.  depends on object’s mass  F = ma  Demo: Spring compressed more exerts a larger force, so it will cause a greater acceleration (masses same).  What effect does that have on speed?  Springs exerting same force on different masses.  Which will be speeded up more?

18. Example  What is the acceleration of a 3-kg cart, if it speeds up from 1 m/s to 5 m/s in 2 seconds?  What force is required to do this? Unit of force: 1 N = 1 kgm/s 2

19. Summary  Sound is propagating disturbance of molecules  Speed is how fast object moves  Acceleration is any change in motion  Acceleration requires a force  no force  no change in motion  greater force provides greater acceleration  greater force required for larger mass  Homework due Friday, Dec 2, before class  Chapter 1  R1, 4, 6*, 9, 10*, 11, 14, Q3*, 4, E1, 3*, 4, 5*, D1, 3, 4, 6