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**Chapter 15: Oscillatory motion part 1**

Reading assignment: Chapter Homework : (due Monday, Nov 7, 2005): Problems: 32, 33, 43, 45 We’ll deal mainly with simple harmonic oscillations where the position of the object is specified by a sinusoidal (sine, cos) function.

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**Two cups are filled to the same level with**

water. One of the two cups has ice cubes floating in it. When the ice cubes melt, in which cup is the level of the water higher? 1. The cup without ice cubes. 2. The glass with ice cubes. 3. It is the same in both.

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**Two cups are filled to the same level with**

water. One of the two cups has plastic balls floating in it. If the density of the plastic balls is less than that of ice, which of the two cups weighs more? 1. The cup without plastic balls. 2. The cup with plastic balls. 3. The two weigh the same.

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**A boat carrying a large boulder is floating**

on a lake.The boulder is thrown overboard and sinks. The water level in the lake (with respect to the shore) 1. rises. 2. drops. 3. remains the same.

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**A lead weight is fastened on top of a large**

solid piece of Styrofoam that floats in a container of water. Because of the weight of the lead, the water line is flush with the top surface of the Styrofoam. If the piece of Styrofoam is turned upside down so that the weight is now suspended underneath it, 1. the arrangement sinks. 2. the water line is below the top surface of the Styrofoam. 3. the water line is still flush with the top surface of the Styrofoam.

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**A lead weight is fastened to a large solid**

piece of Styrofoam that floats in a container of water. Because of the weight of the lead, the water line is flush with the top surface of the Styrofoam. If the piece of Styrofoam is turned upside down, so that the weight is now suspended underneath it, the water level in the container 1. rises. 2. drops. 3. remains the same.

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**Consider an object floating in a container of**

water. If the container is placed in an elevator that accelerates upward, 1. more of the object is below water. 2. less of the object is below water. 3. there is no difference.

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**Consider an object that floats in water but**

sinks in oil.When the object floats in water, half of it is submerged. If we slowly pour oil on top of the water so it completely covers the object, the object 1. moves up. 2. stays in the same place. 3. moves down.

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**In the following section we assume:**

the flow of fluids is laminar (not turbulent) There are now vortices, eddies, turbulences. Water layers flow smoothly over each other. the fluid has no viscosity (no friction). (Honey has high viscosity, water has low viscosity)

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**Equation of continuity**

For fluids flowing in a “pipe”, the product of area and velocity is constant (big area small velocity). Why does the water emerging from a faucet “neck down” as it falls?

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**A circular hoop sits in a stream of water, oriented**

perpendicular to the current. If the area of the hoop is doubled, the flux (volume of water per unit time) through it 1. decreases by a factor of 4. 2. decreases by a factor of 2. 3. remains the same. 4. increases by a factor of 2. 5. increases by a factor of 4.

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Bernoulli’s equation Conservation of energy

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Black board example 15.7 Homework 15.51 Bernoulli’s law Water flows through a horizontal pipe, and then out into the atmosphere at a speed of 15 m/s. The diameters of the left and right sections of the pipe are 5.0 cm and 3.0 cm, respectively. What volume of water flows into the atmosphere during a 10 min period? What is the flow speed of the water in the left section of the pipe? What is the gauge pressure in the left section of the pipe?

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**Two hoses, one of 20-mm diameter, the other**

of 15-mm diameter are connected one behind the other to a faucet. At the open end of the hose, the flow of water measures 10 liters per minute. Through which pipe does the water flow faster? 1. the 20-mm hose 2. the 15-mm hose 3. The flow rate is the same in both cases. 4. The answer depends on which of the two hoses comes first in the flow.

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**Simple harmonic motion/oscillation**

Block attached to a spring Motion of a swing Motion of a pendulum Vibrations of a stringed musical instrument Motion of a cantilever Oscillations of houses, bridges, … Clocks

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**An object hangs motionless from a spring.**

When the object is pulled down, the sum of the elastic potential energy of the spring and the gravitational potential energy of the object and Earth. 1. increases. 2. stays the same. 3. decreases.

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**Simple harmonic motion/oscillation**

Restoring force: F = - k·x Acceleration: Acceleration and restoring force: proportional to x directed toward the equilibrium position

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**Simple harmonic motion/oscillation**

An object moves with simple harmonic motion whenever its acceleration is proportional to its displacement from some equilibrium position and is oppositely directed. w is angular frequency

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**An object hangs motionless from a spring.**

When the object is pulled down, the sum of the elastic potential energy of the spring and the gravitational potential energy of the object and Earth. 1. increases. 2. stays the same. 3. decreases.

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**Properties of simple harmonic motion**

Position of particle at time t: A…amplitude w…Angular frequency f…phase constant, phase angle T…period (wt + F)…phase

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**Properties of simple harmonic motion**

The period T of the motion is the time it takes for the particle to complete one full cycle Periodic motion The value of x at time t is equal to the value of x at time t +T

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**A person swings on a swing.When the person**

sits still, the swing oscillates back and forth at its natural frequency. If, instead, two people sit on the swing, the natural frequency of the swing is 1. greater. 2. the same. 3. smaller.

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**A person swings on a swing.When the person**

sits still, the swing oscillates back and forth at its natural frequency. If, instead, the person stands on the swing, the natural frequency of the swing is 1. greater. 2. the same. 3. smaller.

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**Properties of simple harmonic motion**

Displacement: Period T: Frequency: Angular frequency: Units: 1/s = 1 Hz Velocity: Acceleration:

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**The traces below show beats that occur**

when two different pairs of waves are added. For which of the two is the difference in frequency of the original waves greater? 1. Pair 1 2. Pair 2 3. The frequency difference was the same for both pairs of waves. 4. Need more information.

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**Properties of simple harmonic motion**

Phase of velocity differs by – p/2 or 90° from phase of displacement. Phase of acceleration differs by – p or 180° from phase of displacement.

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**Properties of simple harmonic motion**

Acceleration of particle is proportional to the displacement, but is in the opposite direction (a = - w2·x). Displacement, velocity and acceleration vary sinusoidally. The frequency and period of the motion are independent of the amplitude. (demo).

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Black board example 13.1 An mass oscillates with an amplitude of 4.00 m, a frequency of 0.5 Hz and a phase angle of p/4. What is the period T? Write an equation for the displacement of the particle. Calculate the velocity and acceleration of the object at any time t. (d) Determine the position, velocity and acceleration of the object at time t = 1.00s. (e) Calculate the maximum velocity and acceleration of the object.

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**The block-spring system**

The frequency depends only on: - the mass of the block - the force constant of the spring. The frequency does not depend on the amplitude.

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**Black board example 13.2 (Problem 13.7)**

A spring stretches by 3.90 cm when a 10.0 g mass is hung from it. A 25.0 g mass attached to this spring oscillates in simple harmonic motion. Calculate the period of the motion. Calculate frequency and the angular velocity of the motion.

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Chapter 15 Oscillations. Periodic motion Periodic (harmonic) motion – self-repeating motion Oscillation – periodic motion in certain direction Period.

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