8/2/2019 Jan 18-23 Notes

1/8

Jan 18

Jan 20

Jan 23

Jan 25

Jan 27

Physical Science Notes

I. Dynamics Describes the laws that explain why and how the objects move or stay in equilibrium The state of motion or rest can be changed by interactions between systems Three laws of mechanics describe dynamics

The law of inertiaThe fundamental laws of mechanicsThe law of action and reaction

An object can be in two states; In motion & at rest The law of inertia

An object stays rest or in its motion, (same speed and same direction) if nothinginterferes with it

The fundamental laws of mechanicsThe acceleration (a) of an object is parallel and directly proportional to the net

force (F) and inversely proportional to the mass (m).

o a F, a 1/mF=ma The law of action and reaction

To every action there is always an opposite actionMutual action of two bodies upon each other are always equal

Dynamics has three fundamental propertiesChanges in motion (acceleration)Mechanical system (inertia) Interactions (force)

II. Acceleration Change in motion An object increases its speed if and only if its acceleration is positive accelerated

motion a

If acceleration is negative, the object slows down, and speed decreases deceleratedmotion a 0

8/2/2019 Jan 18-23 Notes

2/8

Jan 18

Jan 20

Jan 23

Jan 25

Jan 27

Examples:

An airplane has an initial speed (V0) = 3.5 MPH. The take-off procedure takes 15 seconds and the final

speed before liftoff is (V1) = 66.8 MPH. What is the acceleration of the air plane?

SI conversion: 3.5 MPH = 1.5 m/s 66.8 MPH = 29.8 m/s

a =

=[(29.8-1.5)m/s]15s

=1.8m/s2

III. Inertia Objects keep on doing what they are doing until something interferes with it At zero friction. All objects resist changes in their state of motion or rest. All objects have this tendency

they have inertia.

The tendency of an object to resist changes in its state of motion varies with its mass. The mass is the quantity that is solely depends upon the inertia of an object. More inertia (resistance to change) that an object has, the more mass that is has. Massive objects have a greater tendency to resist changes in their state of motion.

*The ball would roll almost forever in an effort to reach the original height.

IV. Force SI unit for force is Newton, N Mechanical motion and mechanical interaction is transmitted through forces If many forces act upon an object, the object feels just the net (total) force and not each

of them separately

If all forces act along one axis, we add the forces, otherwise we take vector sum.F1 (10N)

8/2/2019 Jan 18-23 Notes

3/8

Jan 18

Jan 20

Jan 23

Jan 25

Jan 27

Examples:

A book is at rest on a table, what are (if any) the forces involved? There are no forces involved and they

are in equilibrium

What is the net force on a box that is being pushed to the left with a force of 40N and to the right with a

force of 50N? 10N

+5040 = +10N

Using the Newtons second law and the acceleration of the plane a = 1.8 m/s2 find the thrust force of the

engine (Mass of the plane = 1000kg) (Newton, N = Kg m/s2

F=m x a

F = 1000kg x 1.8 m/s2

= 1800N

F2

F1

F3

F2 (10N)

50N40N

8/2/2019 Jan 18-23 Notes

4/8

Jan 18

Jan 20

Jan 23

Jan 25

Jan 27

A car on cruise is moving in uniform motion with a

constant speed

Four forces are acting on the car

1) Gravity downwards

2) Reaction of the road upwards

3) Wind resistance and friction back wards

4) Engine force forward

At any time the forces of 1 and 2 are opposite and equal

Net vertical force is zero

At cruise, forces of 3 and 4 are opposite and equal

Net horizontal force is zero

Force vector is zero

Car is in dynamic equilibrium

Zero force, zero acceleration43

If an object moves along a straight line path at constant speed then it must be acted on by a force.

In which example of motion there is a non-zero net force involved? Deceleration

8/2/2019 Jan 18-23 Notes

5/8

Jan 18

Jan 20

Jan 23

Jan 25

Jan 27

V. Mechanical Wave

Moving perturbation that transportso Energyo Momentum (mass x velocity)o Displacement of particles

To create a mechanical wave we needo A source of perturbationo A propagating medium

A wave extends from one location to another Sound is the propagation of vibrations through a material medium (solid, liquid, gas) If there is no medium to vibrate....NO SOUND!! Sound cannot travel through a vacuum A wave possesses

o An amplitude (A)o Wavelength ()o Frequency (f)o Speed (v)

Amplitude (A)o Maximum displacement from its equilibrium position (to the crest or to the trough)o Amount of energy a waves carries depends on its amplitude

Wavelength ()o Distance between consecutive corresponding points of the same phase (crest, troughs,

ect.)

Top of one crest to the top of the next one Frequency (Hz)

o How many full oscillations (wave lengths) per secondo How frequency a vibration occurs

8/2/2019 Jan 18-23 Notes

6/8

Jan 18

Jan 20

Jan 23

Jan 25

Jan 27

Speed of the wave (v)o Speed of wave is related to the frequency (f) and wavelength () of the wave

Speed = Wavelength X Frequency or V = x fExamples:

A loud speaker produces a frequency of 320 H. It has a speed of 340 m/s. What is the wavelength?

V = x f or

= (340 m/s)/ 320 Hz (Hz = 1 wave per second)

= 1. 06 m

VI. Technology

AM waves (kilohertz, KHz) FM waves (Megahertz, MHz) 960 KHz AM radio = 960,00 Hz (960,000 vibrations per second) 101.7 MHz GM radio = 101,700,00 Hz These radio frequencies are the frequencies at which electrons are forced to vibrate in the

antenna of a radio stations transmitting tower

Frequency of the electrons and the frequency of the wave produced in the air are the sameVII. Frequency (Hz)

Frequency (Hz)

Increasing energy

51

Electromagnetic spectrum

8/2/2019 Jan 18-23 Notes

7/8

Jan 18

Jan 20

Jan 23

Jan 25

Jan 27

VIII. Wave

Disturbance that travels from one place to another transporting energy No permanent displacement of the particles of the medium

o Transverse waveo Longitudinal wave

If you drop a stone into a clam pond, waves will travel outward in expanding circles Energy is carried by the wave The water itself goes nowhere (floating leaf goes up and down) The waves move along by energy travel of waves

IX. Transverse Wave

Vibrations that are right angles to the direction of wave travelo Stretched string of musical instrumentso Radio waveso Light waves

X. Longitudinal Wave

Direction of wave travel is along the direction in which the source vibrates Part of the Slinky is compressed and wave of compression travels along the slinky In between the compressions is a stretched region called (rarefaction) Both compressions and rarefactions travel in the same direction (along the slinky)

XI. Sensing Path

Pitch = frequency of sound Sound can be categorized by pitch, loudness and quality

o Auditory perceptual property that allows the ordering of sounds on a frequency relatedscale

High-pitched soundo High vibration frequency (tiny bell)

Low-pitched soundo Low vibration frequency (large bell, bass guitars)

Human ear can hear pitches from sound ranging from 20 Hz to 20,000 Hz Sound waves below 20 Hz infrasonic waves Sound waves above 20,000 Hz = ultrasonic waves

o Can penetrate through soft medium & their reflections reveal inner structure of themedium

o Bats for hunting

8/2/2019 Jan 18-23 Notes

8/8

Jan 18

Jan 20

Jan 23

Jan 25

Jan 27

*Humans cannot hear infrasonic or ultrasonic sound waves

XII. How can we hear sound?

Air molecules next to vibrating object themselves set into vibration Air molecules do not travel (as wind), but moves a small distance as it vibrates Those molecules, in turn, vibrate against neighboring molecules, which turn do the same, and so

on.....

A rhythmic patterns of compressed and rarefaction air emanate from the sound source. The vibrating air sets your eardrum into vibration

XIII. Constructive and Destructive Interference

When two waves are in same phase with each other constructive Two waves of equal amplitude results in twice as large amplitude What is the amplitude for a constructive wave that is made out of 100 individual waves in same

phase? 100 times larger than the amplitude of an individual wave.

When two waves are in opposite phase with each other Destructive A high-pitched sound from a quickly vibrating source sets your eardrum into....fast vibration