Physics Definitions

All definitions from chapter 1-17

Physical Quanity

A quantity that can be measured. Consists of a numerical magnitude and unit.

Period (of a pendulum)

Time taken for one complete oscillation.

Scalar Quantity

Physical quanities with only magnitude.

Vector Quantities

Physical quantities that have both magnitude and direction.

Velocity

The rate of change of displacement.

Speed

Distance moved per unit time.

Instananeous speed

The speed of an object at a particular moment in time; how fast it is moving at that instant.

Uniform Speed

When the change in distance travelled by an object for every unit of time is the same.

Distance

The total length traveled by an object, regardless of direction of motion.

• Magnitude only

Displacement

The shortest distance from the initial to the final position of an object, including direction.

• Magnitude and direction

Acceleration

Rate of change of velocity.

• Speed/Direction changes

  • Vector qty

Uniform Acceleration

Change of velocity for every unit of time is the same. (constant rate of change of velocity)

Non-uniform Acceleration

Change of velocity for every unit of time is not the same.

Types of contact forces

Friction, Air resistance, Normal force, Tension

Normal force

• Push exerted by surface on an object pressing on it

• Always perpendicular to surface

Types of non-contact forces

Magnetic force, Electrostatic force, Gravitational force

Mass

Measure of amount of matter in a body.

Cannot be changed by location, shape, speed.

Scalar qty

(kg)

• Beam balance/Electronic balance

Weight

Gravitational force acting on an object that has mass.

Vector qty

(N)

• Sprint balance

Gravitational field

Region which mass experience a force due to gravitational attraction.

Gravitational Field Strength

Gravitational force per unit mass placed at that point.

Relation between Mass and Weight

Weight directly proportional to mass.

If W x2, M x2.

Density

Mass per unit volume.

Pressure

Force acting per unit area

(Pa, N/m²)

Effects of Force on a moving object

1. Body at rest moves.

2. Moving body increases/decreases in speed.

3. Moving body changes direction.

Resultant Force

When forces are balanced.

F = 0N, A = 0, Constant speed, At rest

Vector qty

(N)

Newton’s first law of motion

Every object will continue in its state of rest or uniform motion in a straight line unless a F_R acts on it.

Newton’s second law of motion

When a resultant force acts on an object of constant mass, the object will accelerate in the direction of the F_R.

_{F = ma}

Newton’s third law of motion

If a body A exerts force on another body, body B will exert an equal and opposite force on body A.

Friction

Opposes motion between surface in contact.

Negative & Positive effects of friction

• Walk without slipping

• Cars less effecient by 20% (suffer wear and tear)

How to reduce negative effects of frictions?

• Ball bearings (machines)

• Lubricants

• Polishing surface (surface irregularities)

Torque

Moment of a force

The turning effect of a force about a pivot. Product of F x Perpendicular D from pivot to line of action of force.

(Nm)

Principles of Moment

When a body is in equilibrum, sum of CW moments = sum of ACW moments about the same pivot.

Conditions for equilibrum

Stationary, resultant M = 0, resultant F = 0

Centre of Gravity

Imaginary point where entire weight of object acts on. (Below pivot)

Law of conservation of energy

Energy cannot be created nor destroyed. Energy can be transferred from one store to another. Total energy in an isolated system is constant.

Work done

The amount of energy transferred when a force applied to an object causes it to move in the direction of the force.

1 J is work done by 1 N force.

(J)

Power

Work done/energy transferred per unit time.

(W)

How can energy be transferred?

• Mechanically

• Electrically (current)

• Heating (temp diff)

• Propagation of waves (electromagnetic & mechanical sound waves)

Internal Energy

Energy store made up of total KE from random motion of particles & total PE between particles.

Temperature (and energy)

Increases with avg KE of particles.

Conduction

Energy transfer where energy is transferred from passing on of vibrational motion from one particle to another.

Convection

Energy transfer by means of convection currents of liquid/gas due to a diff in density.

Radiation

Process of energy transfer by electromagnetic waves. Does not require a medium.

A wave

Disturbance that propagates through space, transferring energy with it.

Transverse wave

Direction of vibration/motion is perpendicular to direction of wave travel.

Longitudinal wave

Direction of vibration/motion is parallel to direction of wave travel.

Displacement of a wave

From rest position to the point on the wave.

Amplitude

Maximum magnitude of displacement from its rest position.

Loudness

In phase (wave)

Two points on a wave that always have the same direction of motion/oscillation. All crests & troughs are in phase.

Wavelength

Shortest distance between 2 sucessive crests/troughs.

Period (of a wave)

Time taken by each point on the wave to complete one oscillation.

Frequency (of a wave)

Number of oscillations each point on the wave completes per second.

Pitch

Wave Speed

Distance travelled by the wave per unit time.

• Determined by medium

Speed of sound in diff media

Solid < Liquid < Gas

Higher frequency ≠ Travel faster

What is a human’s audible range?

20 Hz to 20 000 Hz

Use of Radio waves

f = 10⁸

Communication, Radio/TV broadcasting

Use of Microwaves

f = 10¹⁰

Cooking, GPS

Use of Infrared waves

f = 10¹²

Cooking, keeping warm

Use of visible light

f = 5 × 10¹⁴

Photography

Use of Ultraviolet light

f = 3 × 10¹⁶

Disninfection

Use of X-Rays

f = 3 × 10¹⁸

X-ray imaging

Use of Gamma rays

f = 3 × 10²⁰

Kill cancer cells in radiotherapy

Law of reflection

Angle of incidence = angle of reflection

Incident ray, reflected ray and normal at the point of incidence all lie on the same plane.

Refraction

Bending of light as it passes from one optical medium to another.

Refractive index

Ratio of speed of light in a vacuum to the speed of light in the medium.

Electric current

Rate of flow of electric charge

Electromotive force

Work done by electrical source in driving a unit charge around a complete circuit.

E = W/Q

Potential Difference (p.d)

Work done per unit charge in driving charges through the component.

V = W/Q

Resistance

V = IR

Scalar qty.

Resistor

A conductor/insulator that has high resistance.

Fixed & Variable resistor.

Closed circuit

When there is closed loop path connecting the components together.

Open circuit

Large or infinite resistance in the path (air gap).

Short circuit (path)

A low resistance path.

Live wire

Brown. High voltage & current.

Earth wire

Yellow and green. Safety wire, earth appliances with a metal case.

Neutral wire

Blue.

0V (connection to earth).

Completes path for current to flow through appliances.

Nuclear decay

Random process by which an unstable atomic nucleus loses its energy by emission of electromagnetic radiation/particles.

Ionising radiation

Radiation with high energies that can knock off electrons from atoms to form ions.

Background radiation

Nuclear radiation in an environment where no radioactive source is deliberate;y introduced.

Rocks, Medical X-rays, Waste product from nuclear power stations.