Physics

Unit 1: Measurements

• 10 to the power of 9: G (giga)
• 10 to the power of 6: M (mega)
• 10 to the power of 3: k (kilo)
• 10 to the power of -1: d (deci)
• 10 to the power of -2: c (centi)
• 10 to the power of -3: m (milli)
• 10 to the power of -6: micro
• 10 to the power of -9: n (nano)

Pendulum

SI unit of time: seconds

Definition of the period of a pendulum: Time taken for a pendulum to make 1 complete oscillation. (Oscillation: from X to Y and to X again)

• Mass of bob doesn't affect period
• The shorter the length of the pendulum, the longer the period

SI Units

length = m

time = s

mass = kg

luminous intensity = cd

amount of substance = mol

current = A

temperature = K

force = N 

Vernier Calipers

Outside jaws: Used to measure external lengths

Inside jaws: Used to measure internal lengths

Tail: Used to measure depths

Precautions:

• Wipe the jaws clean
• Check for zero error
• Hold an object between open jaws and close them until they just grip the object
• Check for parallax error

Micrometer Screw gauge

Ratchet: used for tightening the micrometer screw gauge to the correct pressure

• Make sure that it is zeroed. 
• Do not drop instrument, make sure it is clean
• Never tighten it too much
• Clean the ends of them
• Check for zero error

Forces

Effects of a force:

• change shape of an object
• change speed of an object
• change direction of a moving object
• change size of an object

Balanced force on an object:

1. Resultant force: 0
2. Remain at rest or move at constant speed

Unbalanced force on an object: 

1. Resultant force: not 0
2. Will move
3. Speed of moving object could change

Friction

Friction: A force that always opposes motion and are between two surfaces in contact. 

Positive effects of friction: Needed for walking, used in braking pads to slow down cars

Negative effects: reduces efficiency of cars up to 20%, causes wear and tear

Newton's laws

Newton's first law:  An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. 

Newton's second law: F=ma

Newton's third law: For every action, this is an equal and opposite reaction. 

Scalars and Vectors

Scalar: Physical quantity with only magnitude

(Examples: Distance, Speed, Energy, Time, Volume, Density, Mass)

Vector: Physical quantity with both magnitude and direction

(Examples: Displacement, Velocity, Force, Acceleration, Weight)

Inertia

• Reluctance of object to change its state of motion
• Greater mass, greater inertia 

Mass, Weight and Density

Mass: 

• measure of the amount of matter or substance in a body
• constant throughout
• measured by beam balance/calibrated electronic balance

Weight:

• gravitational force/gravity
• Measure of the gravitational pull acting on a body

Gravitational field: where gravity is felt

Gravitational field strength: gravitational force acting per unit mass on an object

W = mg

Density: 

• density = mass / volume
• substances that are less dense than water will float on water. 

Secondary 2

Two laws of reflection:

• The incident ray, reflected ray and the normal to the reflecting surfaces lies on the same plane. 
• Angle of incidence = angle of reflection

Characteristics of a plane mirror image:

• same size as object
• undergoes lateral inversion
• upright
• virtual
• distance of image from mirror = distance of object from mirror

How to draw ray diagrams for mirror:

1. Draw the distance from object to image.
2. Join image to eyes
3. Join object to point of incidence. 

Applications of mirrors:

Optical testing -- make illuminated letters appear further away

Blind corners -- allows shopkeepers to look out for shoplifters, helps drivers see around blind mirrors.

Instrument scales -- avoid parallax error 

Others: Help look over high wall (periscope), telescopes, overhead projectors and CD players. 

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Refraction

Definition: bending of light as it passes from one optical medium to another

Laws of refraction:

1. The incident ray, the refracted ray and the normal all lie in the same plane. 
2. For two particular media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. 

sin i / sin r = c

refractive index = speed of light in vaccum(or air) / speed of light in medium. 

The greater the value of refractive index of a medium, the smaller the angle of refraction. 

Can light be refracted and reflected at the same time?

Light is both reflected & refracted at the air-glass interface. Amount of light reflected at the interface is usually smaller than the amount of light that refracts.

One way mirrors

A sheet of glass sparsely coated with a layer of metal only a few tens of atoms thick. Person on the brightly lit side see their reflection. Person on the darker side can see through it.

Daily phenomena and applications

• 'Bent' objects. Light rays are bent but our brain tells us that the light rays travel in straight lines. 
• Misconception of depth. (refractive index: real depth/apparent depth)
• Archer fish overcomes visual distortion by positioning itself under prey. 
• News reader reads off partially reflected image. 

Lens:

Surface is curved, hence parallel light hitting the different parts of a lens surface will be refracted by different amounts. The light rays may diverge or converge.