Physics MYP 4

Physical Quantities

Physical Quantity:
A physical quantity is a property of a phenomenon, body, or substance that can be measured and expressed as a number and a unit.

System, Environment, and Control

System: The part of the world we choose to study.

Environment: Everything outside the system that interacts with it.

Control: We try to keep environmental factors constant.

Scalar Quantities

A quantity with only a magnitude.

Distance
Speed
Temperature

Vector Quantities

A quantity with both magnitude and direction.

Displacement
Velocity
Force

Scalar vs Vector:

Scalar → Speed = 30 km/h
Vector → Velocity = 30 km/h northeast

Adding and Subtracting Vectors

When vectors are in the same direction, add them.
When vectors are in opposite directions, subtract them.
When vectors are perpendicular, find the hypotenuse.
When finding the resultant of perpendicular vectors, do not place the arrow tails together.

SI Units

Quantity	SI Unit	Unit Symbol
Length	Metre	m
Mass	Kilogram	kg
Time	Second	s
Electric Current	Ampere	A
Temperature	Kelvin	K
Amount of Substance	Mole	mol
Luminous Intensity	Candela	cd

Derived Units

Derived units are formed by an algebraic combination of one or more SI units.

Scientific Prefixes in Measurements

Prefix	Symbol	Factor (Power of 10)
Pico	p	10^-12
Nano	n	10^-9
Micro	µ	10^-6
Milli	m	10^-3
Centi	c	10^-2
Deci	d	10^-1
(Base)	–	10⁰ = 1
Deca	da	10¹
Hecto	h	10²
Kilo	k	10³
Mega	M	10⁶
Giga	G	10⁹
Tera	T	10¹²

Scientific Notation

Any number can be written as:
a × 10ⁿ

1 ≤ a < 10
n is an integer

Accuracy

Accuracy is how close a measured value is to the true value.

Example:
True value = 10.0 cm
Measured value = 10.1 cm → Accurate

Precision

Precision is how close repeated measurements of the same object are to each other, regardless of accuracy.

Example:
8.2, 8.2, 8.3 → Precise

Significant Figures

Significant figures are the digits in a measurement that carry meaning. More significant figures indicate higher precision.

Rules of Significant Figures

All non-zero digits are significant.
Zeroes between non-zero digits are significant.
Zeroes before the first non-zero digit are not significant.
Trailing zeroes after a decimal point are significant.
Trailing zeroes without a decimal point may or may not be significant.

Rounding Off

If the number is < 5 → drop it.
If the number is > 5 → drop it and increase the previous digit by 1.
If the number = 5:
- If the previous digit is even → drop the 5.
- If the previous digit is odd → drop the 5 and increase the previous digit by 1.

Calculations with Significant Figures

Multiplication & Division:
The answer must have the same number of significant figures as the value with the fewest significant figures.

Addition & Subtraction:
The answer must have the same number of decimal places as the value with the fewest decimal places.

Proportionality

Directly Proportional: x ∝ y → y = kx
Linear Relation: y = mx + c
Inversely Proportional: y ∝ 1/x → y = k/x
Inverse Square: y ∝ 1/x²
Quadratic Relation: y ∝ x²
Cubic Relation: y ∝ x³

Slope

Slope (m) = (y₂ − y₁) / (x₂ − x₁)

Area Under a Graph

The area under a graph represents the total quantity. Divide the area into simple shapes (rectangles, triangles, trapeziums), calculate each area, and add them.

Density

Density = Mass / Volume

Motion

Motion is the movement of an object.

Types of Motion

Linear: Motion in a straight line
Rotational: Circular motion around an axis
Vibration: To-and-fro particle motion
Oscillation: Vibration on a large scale

Dimensions

1D → Line (x)
2D → Plane (x, y)
3D → Space (x, y, z)

Types of Speed and Velocity

Average Speed = Total distance / Total time
Instantaneous Speed → Speed at a specific moment
Average Velocity = Total displacement / Total time
Instantaneous Velocity → Velocity at a specific moment

Average speed can be calculated but not measured.
Instantaneous speed can be measured but not calculated.

SUVAT Equations

S → Distance / Displacement
U → Initial velocity
V → Final velocity
A → Acceleration
T → Time

V = U + AT
S = UT + ½AT²
V² = U² + 2AS

Velocity–Time Graph

Slope → Acceleration
Area → Distance / Displacement
Straight line → Constant speed
Slanted line → Constant acceleration

Acceleration–Time Graph

Slope → Jerk
Area → Velocity
Straight line → Constant acceleration
Slanted line → Changing acceleration

Stopping Distance

Reaction distance = Initial velocity × Reaction time
Braking distance = v² / (2a)
Stopping distance = Reaction distance + Braking distance

Contact Forces

Normal (Reaction) Force: Acts perpendicular to surfaces
Friction: Opposes motion between surfaces in contact
Tension: Acts in strings or ropes
Spring Force: Restoring force, F = −kx
Buoyant Force: Upward force exerted by fluids
Fluid Resistance: Increases with speed and shape

Non-Contact Forces

Gravitational Force: Always attractive, inverse-square law
Magnetic Force: Like poles repel, unlike poles attract
Electrostatic Force: Attractive or repulsive
Nuclear Force: Holds protons and neutrons together

Free Body Diagrams

Diagrams showing all external forces acting on an object.

Momentum and Impulse

Momentum (p) = mv

Impulse is the change in momentum.

Law of Conservation of Momentum:
Total momentum before = Total momentum after

Moment (Torque)

Moment = Force × Distance (M = Fd)

Work, Energy, and Power

Work

W = Fd cosθ

Energy

Kinetic Energy: KE = ½mv²
Potential Energy: GPE = mgh

Power

P = W / t

Efficiency

Efficiency = (Useful energy / Total energy) × 100%

Pressure

Pressure = Force / Area

Bernoulli’s Principle

As the speed of a fluid increases, its pressure decreases.

Forces in Flight

Lift
Weight
Thrust
Drag

UNIT 4: Pressure & Fluid Dynamics

1. Pressure

Pressure = Force ÷ Area
P = F / A

Pressure tells us how strongly a force pushes on a surface.

Unit: Pascal (Pa)

Examples:
A sharp knife cuts better than a blunt one → smaller area = more pressure.
High-heel shoes sink into the ground more than flat shoes.

2. Fluid Pressure

Fluids include liquids and gases.

Fluid pressure increases with depth.

Reason:
More fluid above → more weight → more pressure.

Examples:
Your ears hurt when you dive deep in a pool.
Dams are thicker at the bottom.

3. Atmospheric Pressure

Air has weight, so it pushes on everything.

Examples:
Drinking through a straw.
Suction cups sticking to walls.

At higher altitude:

Less air above you
Lower atmospheric pressure

4. Manometer

A manometer is used to measure gas pressure.

Uses a liquid (usually mercury or water)
If the liquid level moves → pressure has changed

5. Barometer

A barometer measures atmospheric pressure.

Used in weather prediction
Used to measure altitude

Low pressure → rainy weather
High pressure → clear weather

6. Buoyant Force

The buoyant force is the upward force exerted by a fluid on an object.

Why objects float:
Buoyant force ≥ weight of the object

Examples:
Ships float on water.
You feel lighter when you are in water.

7. Archimedes’ Principle

An object immersed in a fluid experiences an upward force equal to the weight of the fluid displaced.

Simple meaning:
If you push water away, the water pushes you back up.

8. Forces Involved in Flight

There are four main forces acting on an airplane:

Lift: pushes the plane upward
Weight: gravitational force pulling downward
Thrust: engines push the plane forward
Drag: air resistance opposing motion

For flight:

Lift > Weight
Thrust > Drag

9. Bernoulli’s Principle

When the speed of a fluid increases, its pressure decreases.

Fast flow → Low pressure
Slow flow → High pressure

Applications:
Airplane wings generate lift.
Blowing between two hanging papers makes them move together.

UNIT 5: Thermal Physics

1. Heat

Heat is energy transferred because of a temperature difference.

Heat flows from a hotter object to a colder object.

Unit: Joule (J)

2. Temperature

Temperature measures how hot or cold an object is.

Thermometer:
Measures temperature using:

Expansion of a liquid
Electronic sensors

3. Temperature Scales

Celsius (°C)
Kelvin (K)

Conversion: K = °C + 273

Example:
25°C = 298 K

4. Heat Transfer

a) Conduction

Heat transfer through solids
Particles vibrate and pass energy
Example: Metal spoon in hot tea

b) Convection

Heat transfer in liquids and gases
Hot fluid rises, cold fluid sinks
Example: Boiling water

c) Radiation

Heat transfer without a medium
Travels as electromagnetic waves
Example: Heat from the Sun

5. Thermal Equilibrium

Thermal equilibrium occurs when two objects reach the same temperature.

No heat flows between them anymore.

6. Specific Heat Capacity (c)

The energy required to raise the temperature of 1 kg of a substance by 1°C.

High c → heats up slowly (e.g. water)
Low c → heats up quickly (e.g. metals)

7. Specific Latent Heat

The energy required to change the state of a substance without changing its temperature.

Ice melting
Water boiling

8. Gas Laws

Boyle’s Law

Pressure ∝ 1 / Volume (constant temperature)
Example: Syringe

Charles’ Law

Volume ∝ Temperature (constant pressure)
Example: Heated balloon expands

Gay-Lussac’s Law

Pressure ∝ Temperature (constant volume)
Example: Heated gas cylinder

Avogadro’s Law

More gas particles → larger volume

UNIT 6: Waves

1. Types of Waves

Mechanical Waves

Require a medium
Example: Sound waves

Electromagnetic (EM) Waves

Do not require a medium
Example: Light, X-rays

2. Mechanical Waves

Transverse Waves

Vibration ⟂ direction of travel
Example: Water waves, light

Longitudinal Waves

Vibration ∥ direction of travel
Example: Sound waves

3. Wave Properties

Wavelength (λ)
Frequency (f)
Amplitude
Wave speed

Higher frequency → higher energy

4. Echo

An echo is the reflection of sound.

Used to measure distance
Used to detect obstacles

5. Applications of Sound

Ultrasound

High-frequency sound
Used in medical imaging

SONAR

Used in ships
Detects underwater objects

6. Electromagnetic Spectrum

Order (low → high energy):

Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma

7. X-Rays

Penetrate soft tissue
Used in medical imaging

8. Gamma Rays

Very high energy
Used in nuclear imaging

9. Radiotherapy

Uses radiation to kill cancer cells.

10. Ionising Radiation – Safety

Can damage cells
High doses increase cancer risk
Exposure should be limited
Protective shielding is used
Medical use only when necessary

Exam Tip (MYP 4):
Use scientific reasoning, draw simple but accurate diagrams, and always include formulas in explanations.

Physics Formula Sheet (IB / MYP)

Work, Energy & Power

Work Done:
W = F × d

Energy:
E = W

Power:
P = E / t
P = W / t

Motion

Speed:
v = d / t

Velocity:
v = displacement / time

Acceleration:
a = (v − u) / t

SUVAT Equations

v = u + at
s = ut + ½at²
v² = u² + 2as
s = ½(u + v)t
s = vt − ½at²

Forces & Elasticity

Hooke’s Law:
F = kx

Pressure & Fluids

Pressure:
P = F / A

Liquid (Water) Pressure:
P = ρgh

Density, Mass & Volume

Density:
ρ = m / V

Mass:
m = ρV

Volume:
V = m / ρ

Thermal Physics

Specific Heat Capacity:
Q = mcΔT

Latent Heat:
Q = mL

Heat Capacity:
C = Q / ΔT

Graphs

Area under a speed–time graph = distance travelled
Area under a velocity–time graph = displacement

Laws

Law of Conservation of Energy:
Energy cannot be created or destroyed, only transferred or transformed.

Law of Conservation of Mass:
Total mass of reactants = total mass of products.

Symbols & Units

Symbol	Meaning	Unit
W	Work	J
E	Energy	J
P	Power	W
F	Force	N
d / s	Distance / Displacement	m
t	Time	s
v	Velocity	m s⁻¹
u	Initial Velocity	m s⁻¹
a	Acceleration	m s⁻²
ρ	Density	kg m⁻³
g	Gravitational Field Strength	9.8 m s⁻²
m	Mass	kg
V	Volume	m³
c	Specific Heat Capacity	J kg⁻¹ °C⁻¹
L	Specific Latent Heat	J kg⁻¹

Past Paper Practice (MYP Physics)

Section 1

Q1.1 Classification of Physical Quantities

Fundamental Quantities	Derived Quantities
Force	Acceleration
Mass	Pressure
Length	Temperature

Fundamental quantities are basic and cannot be expressed in terms of other quantities.
Derived quantities are formed using fundamental quantities.

Q1.2 SI Units

Quantity	SI Unit
Mass	kilogram (kg)
Speed	m s⁻¹
Temperature	kelvin (K)
Density	kg m⁻³
Time	second (s)

Q1.3 Significant Figures

Measurement	Significant Figures
2.00 kg	2
0.004 m	1
9900 g	2
101.001 km	6
0.500 ml	3

Q1.4 Density of a Crown (Gold Test)

Density formula:
ρ = m / V

Mass of crown = 500 g (±1 g)
Initial water level = 80.0 cm³ (±0.5 cm³)
Final water level = 105.0 cm³ (±0.5 cm³)

Volume of crown = 105.0 − 80.0 = 25.0 cm³

Density of crown = 500 ÷ 25.0 = 20.0 g cm⁻³

Accepted density of gold = 19.3 g cm⁻³
Experimental range ≈ 19.2 – 20.8 g cm⁻³
Since the accepted value lies within this range, the crown could be pure gold.

Repeating the experiment reduces random errors and improves precision.
Using more precise instruments reduces uncertainty and improves accuracy.

Section 2

Cyclist Speed Calculation

Distance = 600 m
Time = 2 minutes = 120 s
Speed = distance ÷ time = 600 ÷ 120 = 5 m s⁻¹

5 m s⁻¹ ≈ 18 km h⁻¹
This is a realistic and safe cycling speed in urban environments where traffic and signals limit movement.

Average Speed over Two Hours

First hour distance = 60 km
Second hour distance = 40 km
Total distance = 100 km
Total time = 2 h

Average speed = 100 ÷ 2 = 50 km h⁻¹

The average speed lies between the two instantaneous speeds (60 km h⁻¹ and 40 km h⁻¹).
It represents overall journey performance, not speed at a specific moment.

Q2.3 Evaluation Questions (D)

Scientific Notation & Significant Figures

Advantage:
Scientific notation simplifies very large or small numbers and reduces error. Significant figures clearly show measurement precision.

Disadvantage:
Incorrect rounding or misunderstanding precision can cause serious errors, such as unit or measurement failures.

Technological Considerations:
Digital instruments display many digits, but not all are meaningful. Proper calibration and understanding are essential.

Economic & Educational Factors:
Errors in precision can cause financial loss, safety risks, and waste. Teaching sig figs early prevents costly real-world mistakes.

Conclusion:
Significant figures and scientific notation are essential for accurate scientific communication. In school labs, rules should be applied progressively, focusing first on understanding rather than rigid enforcement.