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PH 105

PH 105. Dr. Cecilia Vogel Lecture 1. OUTLINE. Intro & units What is Sound? Mechanics speed acceleration force. 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)

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PH 105

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  1. PH 105 Dr. Cecilia Vogel Lecture 1

  2. OUTLINE • Intro & units • What is Sound? • Mechanics • speed • acceleration • force

  3. 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

  4. 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

  5. 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

  6. Units • Units and variables: • variable names will be in italics • ex: ½ mv2 • m is mass variable, v is speed variable • units will not be in italics • ex: 9.8 m/s2 • m is meters unit, s is seconds unit

  7. Sound (or other material) • 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

  8. 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

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

  10. 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!

  11. 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!

  12. 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.”

  13. 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)

  14. 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?

  15. 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?

  16. 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?

  17. 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/s2

  18. 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

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