physics notes o level

Chapter 1: Physical Quantities, Units, Measurement

What are physical quantities? -A physical quantity is a quantity that can be measured -It consists of a numerical value and a unit -There are 7 basic physical quantities, of which others are derived from Base quantity SI unit Symbol for SI unit length metre m mass kilogram kg time second s electric current ampere A thermodynamic temperature kelvin K amount of substance mole mol luminous intensity candela cd (luminous intensity is not tested at our level) Prefixes for SI units -Using decimal notation, 1 km can be expressed as 1000 m -This is not efficient, so prefixes are more commonly used Order of magnitude Prefix Symbol Amount 10^12 tera- T 1000000000000 (trillion) 10^9 giga- G 1000000000 (billion) 10^6 mega- M 1000000 (million) 10^3 kilo- k 1000 (thousand) 10^-1 deci- d 0.1 10^-2 centi- c 0.01 10^-3 milli- m 0.001 10^-6 micro- µ 0.000001 10^-9 nano- n 0.000000001

Standard form -Other than using prefixes, standard form is also a method of writing physical quantities in which 1 or 10 is multiplied by an appropriate power of 10 -e.g. 1 km = 1 x 10^3 m -The order of magnitude of some measure is the 10^x part -e.g. Order of magnitude of radius of earth 10^6 m Order of magnitude of diameter of earth 10^7 m Order of magnitude of size of atom 10^-10 m Measurement of length Metre ruler and measuring tape -The metre ruler and measuring tape are commonly used to measure length -A metre ruler can measure distances of up to 1 metre -The measuring tape can measure straight distances longer than 1 metre -A cloth measuring tape can measure the length along a curved surface Digital calipers -Digital calipers can measure the internal and external diameter of an object and the depth of an object accurately Digital micrometer screw gauge -The digital micrometer screw gauge is used to measure objects which are too small to be measured using the digtial calipers

Measurement of time Pendulum -A pendulum consists of a heavy object called a bob tied to a string -When a pendulum is swung, it will move back and forth at regular intervals -Each complete back and forth motion of the pendulum is called an oscillation Pendulum clock -A pendulum clock is clock which keeps time using a pendulum Clocks and stopwatches -The oscillations of springs and the natural vibrations of crystals are other motions used to keep track of time -Most clocks and stopwatches today use quartz crystals as they are small, accurate and require little electric power Precision of an instrument -The smallest unit an instrument can measure is known as its precision Instrument Measuring range Precision measuring tape 0 to several metres 0.1 cm or 1 mm metre ruler 0 to 1 metres 0.1 cm or 1 mm digital calipers 0 to 15 centimetres 0.001 cm or 0.01 mm digital micrometer screw gauge 0 to 2.5 centimetres 0.0001 cm or 0.001 mm

Avoiding errors of measurment Parallax error -Parallax error occurs when our eyes are not positioned perpendicularly above the rule of a ruler Zero error -Zero error occurs when a digital instrument is not set at 0 when it is fully closed Human reaction time -Most stopwatches can measure time to a precision of 0.01 seconds, and digital stopwatches show time up to 2 decimal places -However we usually take readings from a stopwatch up to 1 decimal place, as stopwatches need to be started and stopped by hand -This introduces a random error called human reaction time -Human reaction time is around 0.3 to 0.5 seconds for most people What are scalars and vectors? -Scalar quantities are quantities that have only magnitude -Vector quantities have both magnitude and direction Scalar Vector distance displacement speed velocity time weight mass acceleration energy force Distance and displacement Distance -Distance is the total length covered by a moving object regardless of the direction of motion -It is a scalar quantity, as it only has a magnitude -SI unit: m (metre) Displacement -Displacement is the distance measured in a straight line in a specified direction -It is a vector quantity, as it has both a magnitude and a direction -SI unit: m (metre)

Speed and velocity Speed -Speed is the distance moved per unit time -It is a scalar quantity, as it only has a magnitude -SI unit: m/s (metre per second) Velocity -Velocity is the rate of change of displacement -It is a vector quantity, as it has both a magnitude and a direction -SI unit: m/s (metre per second) Adding two vectors by the graphical method -We can use a vector diagram to add up two vectors -In a vector diagram, a vector quantity is represented by an arrow -The length of the arrow is proportional to the magnitude of the vector -The direction of the arrow indicates the direction of the vector -Unlike scalars, vectors have magnitude and direction, and thus when we add two vectors, we cannot add only their magntiudes -We need to find a single vector that produces the same effect as the vectors combined -This single vector is known as the resultant vector -It must be equivalent to the individual vectors combined in terms of magnitude and direction Adding two parallel vectors -If a girl walks 100 m due east and then 40 m due west, what is her displacement? -We can find her displacement by adding her vectors graphically Adding two non-parallel vectors -A 6 N force is exerted on a block, pushing it due east -A 4 N force is the exerted on the same block, pushing it due north -We can find the resultant force using the head and tail method

-The resultant force should originate from the tail and end at the tip of the final vector

Chapter 2: Kinematics

Speed -Speed is the distance travelled per unit time -Formula for speed: distance / time -Formula for average speed: total distance / total time -Speed is a scalar quantity as it does not have a direction -SI unit: m/s (metre per second) Uniform speed -When the change of distance for every unit of time is the same, the object undergoes uniform speed Velocity -Velocity is the rate of change of displacement -Formula for velocity: displacement / time -Formula for average velocity: total displacement / total time -Velocity is a vector quantity -SI unit: m/s (metre per second) Acceleration -Acceleration is the rate of change of velocity -Formula for acceleration: change of velocity / time -Acceleration is a vector quantity -SI unit: m/s^2 (metre per second squared) Uniform acceleration -When the change of velocity of an object for every unit of time is the same, the object undergoes uniform acceleration -Formula for uniform acceleration: (final velocity - initial velocity) / (time at which an object is at final velocity - time at which object is at initial velocity) -Non uniform acceleration occurs if the change of velocity of an object for every unit of time is not the same Deceleration and decrease in acceleration -Deceleration is when the acceleration of an object decreases -Decrease in acceleration is when the rate of acceleration decreases -e.g. If an object is slowing down, it is decelerating, however if that object is still moving faster but at a slower rate, it is a decrease in acceleration

How do we analyse motion graphically? Displacement time graphs -A displacement time graph can give us some info about the motion of an object -The displacement of an object is recorded every second and a graph is then plotted, with the displacement (m) as the y axis and the time (s) as the x axis -The gradient of the graph is the velocity of the object Velocity time graphs -A velocity time graph can be used to show uniform and non uniform of an object travelling in a straight line in one direction -Non uniform velocity occurs if the change of displacement of an object for every unit of time is not the same -The velocity of the object (m/s) is the y axis and the