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Pure Physics - Coggle Diagram
Pure Physics
Chapter 1: Physical Quantities, Units and Measurements
1.2: What are physical quantities?
Physical quantity
is a quantity that can be measured. It consists of a numerical magnitude and a unit
Base units and their symbols
Length
SI unit: metre (m)
Mass
SI unit: kilogram (kg)
Time
SI unit: second (s)
Electric Current
SI unit: Ampere (A)
Thermodynamic Temperature
SI unit: Kelvin (K)
Amount of substance
SI unit: mole (mol)
Prefixes
Multiples (Positive)
Kilo- (k) (10^3)
Mega- (M) (10^6)
Giga- (G) (10^9)
Tera- (T) (10^12)
Sub-multiples (negative)
Deci- (d) (10^-1)
Centi- (c) (10^-2)
Milli- (m) (10^-3)
Micro- (µ) (10^-6)
Nano- (n) (10^-9)
Standard form
A way of writing numbers, in which a number between 1 to 10 is multiplied by an approriate power of 10
Examples
5.67x10^-3 = 0.00567
1.68x10^4 = 16800
1x10^3 = 1km
3x10^6 = 3MJ
1x10^-3 = 1mA
1.3: How do we measure physical quantities?
Measurement of length
SI unit: metre (m)
Commonly used instrument
Metre rule and measuring tape
Metre rule can measure lengths of up to one metre.
Steel measuring tape is suitable for measuring straight distances longer than a metre.
Cloth measuring tape is suitable for measuring the length along a curved surface
Measuring range
Metre rule: zero to one metre
Measuring tape: zero to several metres
Smallest division
Metre rule: 1mm
Measuring tape: 1mm
Example of usage
Metre rule: Height of a table
Measuring tape: A person's waist
Digital Calipers
Used to measure the internal and external diameters of an object accurately
Measuring range
zero to 15 centimetre
Smallest division
0.01mm
Example of usage
Diameter of a test tube
Digital micrometer screw gauge
Used to measure objects that are too small to be measured using the digital calipers
Measuring range
zero to 2.5 centimetres
Smallest division
0.001mm
Example of usage
Diameter of a wire
Precision of an instrument
Smallest unit an instrument can measure is known as its
precision
.
Avoiding errors of measurement
random error
varies unpredictably from one measurement to another
Unavoidable, but cluster around the true value
systematic error
Can often be avoided by calibrating equipment, but if left uncorrected, can lead to measurements far from the true value.
Constant and not random, having the same value for every measurement
Measurement of time
SI unit:
Seconds (S)
Other units for measuring time
Year
Month
Day
Hour
Minute
Pendulum
Consists of a heavy object called a bob (e.g. a metal ball), that is attached to one end of a string
When swing freely, it will move back and forth at regular intervals
Each complete to-and-fro motion is one
oscillation
Period
is time taken for one complete
oscillation
It can be callibrated to measure time accurately by adjusting the length of the pendulum
Human Reaction time
Most stopwatches can measure time to a precision of 0.01s
We usually take readings to the nearest one decimal place, because human reaction time is about 0.3-0.5s for most people
1.4: What are scalars and vectors?
Scalar quantitites
are physical quantities that have only
magnitude
Examples
Distance
The total length covered by a moving object regardless of the direction of motion
Scalar quantity (i.e has
magnitude
only)
SI unit: metre (m)
Speed
Distance moved per unit time
Scalar quantity (i.e. has
magnitude
only)
SI unit: metre per second (m/s)
Speed = Distance / Time taken
Refers to how fast something moves
Average speed
Assumes each object running at the same speed throughout the entire distance.
Average speed = Total distance / Total time taken
Uniform Speed
The change in distance travelled by an object for every unit of time is the same.
Mass
Energy
Time
Vector quantities
are physical quantities that have both
magnitude
and
direction
Examples
Displacement
The distance measured in a straight line in a specified direction
A vector quantity (i.e. has both
magnitude
and
direction
)
SI unit: Metre (m)
Velocity
Rate of change of displacement with respect to time
Vector quantity (i.e. has both
magnitude
and
direction
)
SI unit: metre per squared second (m/s^2)
Velocity = Displacement / Time taken
Average Velocity
Average velocity = Total displacement / Total time taken
Acceleration
Force
Weight
Ways to add up two vectors
Head to Tail method
Parallelogram method
Chapter 2: Kinematics
2.1: What are speed, velocity and acceleration?
Acceleration
An object undergoes acceleration when
Speed or direction changes
Both direction and speed changes
Refers to the rate of change of velocity
Velocity = Change in velocity / Time taken
Vector Quantity
SI unit: Metre per Second square (m/s^2)
Uniform acceleration
When the changes (increase/decrease) in the velocity of an object for every unit of time is the
same
, the object undergoes constant or uniform acceleration
Constant rate of change of velocity
a= v-u/tv-tu
v stands for final velocity (m/s)
u stands for initial velocity (m/s)
tv stands for the time which an object is at final velocity (s)
tu stands for the time which an object is at intial velocity (s)
Non-uniform acceleration
An object undergoes non-uniform acceleration if the the change in its velocity for every unit of time is
not the same
.
2.2: How do we analyse motion graphically?
Displacement-time graphs
It gives us some information about the motion of the object
Gradient of a displacement-time graph is velocity
Velocity-time graphs
It can be used to show uniform and non-uniform acceleration of a car that is travelling along a straight line in one direction
Gradient of a velocity time graph is acceleration of the object
Comparison between both of them
Velocity-time graphs show us the velocity of the object (speed with respect to distance)
Displacement-time graphs show us the scalar quantity: speed
Velocity-time graphs show us the vector quantity: velocity
Displacement-time graph of an object gives us some information about the motion of the object.
Area under Velocity-time graphs
The area under the velocity-time graphs gives the displacement of the object.
Comparison with speed-time graphs
For speed-time graph: Total distance travelled = Total area under the speed-time graph
For velocity-time graph: Total displacement = Total area under the velocity-time graphs
2.3: What is acceleration of free fall?
Acceleration due to gravity, g, is a constant.
For objects close to Earth’s surface, the value of g is generally taken to be 9.8m/s^2.
For simplicity in calculations, we take this value to be 10m/s^2 throughout the whole Sec 3 and Sec 4.
Regardless of mass and weight, all object will experience constant acceleration during free fall.
Chapter 4: Dynamics II: Forces
4.1: What are Newton's Laws of Motion?
Newton's First Law: Inertia
Definition
every object will continue in its state of rest or uniform motion in a straight line unless a resultant force acts on it.
inertia of an object refers to the reluctance of the object to change its state of rest or motion, due to its mass.
Newton's Second Law
Definition
when a resultant force acts on an object of a constant mass, the object will accelerate in the direction of the resultant force.
Equation
F = ma
F stands for resultant force (N)
m for mass of the objects (kg)
a for acceleration of the object (m/s^2)
Newton's Third Law
Definition
For every action, there is an equal and opposite reaction.
Characteristics of action-reaction pair
Forces always occur in pairs. Each pair is made up of an action and a reaction forces.
Action and reaction forces are equal in magnitude
Action and reaction forces act in opposite directions
Action and reaction forces act on different bodies.
4.2: What are free-body and vector diagrams?
Free-body diagrams
using arrows to represent the forces acting on individual objects
helps us to identify and visualise the forces and their
effects on the object.
Vector diagrams
Definition
drawn to scale using the graphical method to find the resultant
force
Ways to draw
Parallelogram method
Head-to-tail method
Useful notes
If arrows representing the forces result in a closed triangle, we say that the forces are in equilibrium.
If the arrows do not result in a closed triangle, there is a resultant force acting on the object. It is represented by the double-headed arrow from the tail of the first arrow to the head of the last arrow.
4.3: What are some effects of resistive forces on motion?
Examples of resistive forces
Friction
Definition
contact force that opposes or tends to oppose motion between surfaces in contact.
Effects of friction
Negative
Examples
Cars are less efficient by up to 20%
Moving parts in engines, motors and machines suffer wear and tear
Ways to reduce
Using wheels: Being circular in shape, wheels greatly reduce the friction between the object and the floor. A smaller force can be applied to move the object around.
Using ball bearings: Being spherical in shape, ball bearings are used to reduce friction between moving parts of machines, cars and in-line-skates. The ball bearings are placed between moving parts so that the ball bearings can roll around. This prevents the moving parts from rubbing against each other.
Using lubricants and polished surfaces: Applying a layer of lubricant, such as oil or grease, between surfaces in contact can greatly reduce friction. Lubricants are frequently used between the moving parts of an engine to reduce wear and tear. This helps prolong the life of engine. Polishing a surface removes surface irregularities. This can also reduce friction between surfaces in contact.
Positive
Examples
We can walk without slipping
Moving objects are able to slow down when needed
Ways to enhance
Using treads: Friction is important to the motion of vehicles. Without friction, a vehicle cannot move as its tyres will just spin at the same spot. Friction enables the tyres to grip the road surface and roll without slipping. On a rainy day, a moving vehicle may
skid on wet roads. Its tyres need to have more grip on the road to prevent skidding. Thus, tyres are designed with threads — grooves that quickly channel water out from underneath the tyres. This improves the grip of the tyres on wet roads, thus preventing skidding.
Using parachute: Air resistance is a type of friction in air. A skydiver in midair varies air resistance to change his speed.
How can it be used
3 more items...
Air resistance
Characteristics
It always oppose the motion of moving objects.
It increases with the speed of the objects
It increases with the surface area (or size) of the objects
It increases with the density of air
Chapter 3: Dynamics I: Mass and Weight
3.1: What are the types of forces?
Definition
A push or pull
due to interaction between objects to explain changes in motion.
Effects
Slow down or stop an object
Speed up or start an object from moving
Change the direction of a moving object
Different types of forces
Contact force
Definition
force that acts on an object only when it is
physically touching
another object.
Examples
Friction: a contact force that resists the motion of one surface sliding over another.
Air resistance: type of frictional force that acts against the motion of objects moving through air.
Normal force: the support force exerted by a surface that is perpendicular (at a right angle) to the object resting on it.
Tension: a contact force that is transmitted through a string, rope, cable, or wire when it is pulled tight by forces acting from opposite ends.
Non-contact forces
Definition
forces that act on an object
without
any
physical contact
between the objects involved. Instead, they act over a distance through fields
Examples
Gravitational force: attractive force that pulls any two objects with mass toward each other.
Magnetic force: force of attraction or repulsion that acts between certain materials, like iron, due to their magnetic fields.
Electrostatic force: force between two charged objects — it can be either attractive or repulsive depending on the types of charges (positive or negative).
Balanced force
When the resultant force acting on an object is zero, the forces acting on the object are balanced.
Unbalanced force
If the resultant force acting on an object is not zero, the forces acting on the object are unbalanced.
3.2: Is mass the same as weight?
Mass
Definition
a measure of the
amount of matter
in a body.
does not change with its location or shape.
depends on the number and composition of atoms and molecules that make up the body.
Quantity/Unit
SI unit: kilogram (kg).
scalar quantity
Instruments used to measure
Beam balance
Weight
Definition
gravitational force acting on an object that has mass.
Quantity/Unit
SI unit: Newton (N)
vector quantity
Gravity
Gravitational Field
Definition
a region in which a mass experiences a force due to gravitational attraction.
Gravitational field strength g is defined as the gravitational force per unit mass placed at that point.
Equation: W=mg
g for gravitational field strength (N/kg)
W for weight (N)
m for mass of the object (kg)
Instruments used to measure
Spring Balance
Bathroom Scale
Electronic Balance
Chapter 5: Turning Effects of Forces
5.1: When does a force cause something to turn?
Moment
Definition
The moment of a force, M, or torque, about a pivot is the product of the Force, F, and the perpendicular distance, d, from the pivot to the line of a action of the force.
Formula
M = Fd
SI unit
Newton metre (Nm)
Principles of Moment
when a body is in equilibrium, the sum of clockwise moments (SCM)about a pivot is equal to the sum of anticlockwise moments (SACM) about the same pivot.
Conditions for Equilibrium
Resultant moment on the body = 0
Resultant force on the body = 0
5.2: How can we prevent objects from toppling?
Centre of Gravity
Definition
an imaginary point where the entire weight of the object seems to act.
Stability
Definition
a measure of its ability to return to its original position.
Conditions
Positions of the Centre of Gravity: The lower the centre of gravity, the more stable it is
Area of the base of objects: The larger the base area of an object, the more stable it is.