PH 105

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PH 105

• Dr. Cecilia Vogel
• Lecture 2

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OUTLINE

• Mechanics
• Force
• Pressure
• energy
• power
• Oscillation
• period
• frequency
• amplitude

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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?

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Equilibrium

• Equilibrium means there is no force.
• If theres no force, theres 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?

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Mass

• How much force does it take?
• depends on the size of acceleration.
• depends on objects 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?

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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
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Pressure

• The total force determines how an object
  accelerates,
• Pressure is
• pressure greater if
• demo
• my weight spread on flat shoes vs high heels --
  which will damage floor?

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Fluid Pressure

• The pressure of a fluid pushes on all sides of an
  object in the fluid
• The air around us is under pressure, because
• about 100,000N/m2
• Under water has additional pressure,
• The deeper,

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Floating

• What force makes an object float toward the
  surface of water?
• Under water, pressure on all sides
• the pressure on the
• because its
• more force ___ on the ______ than _____ on the
  ____
• net effect is an

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Work

• If I push an object, I can speed it up
• the farther I push,
• Define the work done by me as force times
  distance
• W Fd
• unit of energy is Nm J (Joule)

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Energy

• Work changes
• Types of energy
• Kinetic energy is
• work done object
• KE ½ mv2
• the ______ it goes, the more _____ it has
• the more ______ it has, the more ______ it has

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Potential Energy

• Sometimes you do work and the object doesnt
  speed up.
• The energy you gave it might be
• thats
• Examples
• compressing or stretching spring
• stretching or tensioning string
• lifting an object

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Energy Again

• An object has energy, if it has the ability to do
  work
• Does moving object (even air) have ability?
• wind
• Does a object lifted high have ability?
• Does a compressed spring have ability?
• Does a stretched string have ability?

moving oject
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Power

• You have to do work to speed something up or lift
  something up.
• You can do that work
• Which is harder, going up stairs (i.e. lifting
  yourself) walking or running?
• Power is
• WPt
• or EPt

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Power

• Power is work/time or energy/time
• WPt
• or EPt
• The more quickly the work is done,
• Unit of power is J/s W (Watts)
• A 50 Watt bulb left on for 2 hours uses ________
  as 100 W bulb left on for one hour.

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Oscillation

• An object oscillates if it
• examples
• vibrating string
• pendulum
• mass on a spring

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Simple Harmonic Oscillator

• Suppose a system has an equilibrium point
• where there is
• but on either side of equilibrium, there is a
  force
• force always pushes system
• If you displace this system from eqlb,
• it will

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Analyze Oscillation

• The system is initially away from eqlb,
• the force pushes it
• It will move back toward eqlb
• will it stop when it gets there?
• Now its going away from eqlb
• the force pushes
• what happens when force is opposite motion?

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Properties of Oscillations

• Amplitude is
• how far it gets from
• larger amplitude,
• Period is the time it takes to
• time before
• Frequency is how
• the _____ the period, the _____ frequently it
  repeats.
• f 1/T

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Period and Frequency

• Period and frequency of a system
• depend on properties of system
• like, ______ of spring, _____ of pendulum, ______
  in string
• does ____ depend on ______ for simple systems
• Demo
• What is the period of pendulum for small
  amplitude oscillations? For large amplitude?
• What will change the period?

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Damping

• Will the oscillation go on forever
• all oscillations lose
• Because of
• Damping is due to factors like

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Summary

• Pressure is force divided by area
• Work changes the energy of an object
• kinetic energy is motion energy
• potential energy is stored energy
• Power is how quickly energy is gained or lost
• Oscillations are repeated motion
• due to forces pulling back toward eqlb
• Properties of oscillations period, frequency,
  and amplitude