TOPIC 2: PHYSICS – DESCRIBING MOTION
Introduction to Physics
Physics is the study of everything in the physical world.
“All science is either physics or stamp collecting.” – Ernest Rutherford
Physics is the study of matter and energy and their interactions
2.2.1 Define kinematics.
Areas of Physics
Classical Physics
Mechanics
(Newtonian Physics)
 for systems larger than atoms and much slower than the speed of light
the study of energy, forces, and their effects on bodies
Kinematics
– the study of how things move in terms of position, direction, velocity, and acceleration
Dynamics
– the study of why things move in terms of forces, momentum, and energy
Thermodynamics the study of heat transfer, temperature, and energy. Cool ideas: absolute zero, entropy.
Electromagnetism the study of electric and magnetic fields and their effects on charged bodies
Modern Physics
anything that cannot be explained by classical physics
 for subatomic systems and/or particles moving near the speed of light
Quantum Theory the study of fundamental particles that possess wave and particle properties
Atomic Physics the study of the parts of the atom (electrons, protons, neutrons) and their behaviours
Nuclear Physics the study of atomic nuclei and their changes (radioactivity, fission, fusion)
Relativity
the study of changes when relative speeds approach the speed of light.
2.1 Vectors and scalars
2.1.1 Distinguish between vector and scalar quantities, and give examples of each.
Every measurement in physics has a value (magnitude) and units.
Measurements without a direction of motion or force are scalars.
Vectors are measurements that have a direction of the motion or force indicated.
Scalars
quantities that are fully described by a magnitude (or numerical value) alone.
Vectors
Common Scalar Quantities
NO DIRECTION
time, t
temperature, T
mass, m
length, ℓ
t = 1.23 s
T = 293K = 25°C
m= 75.6 kg
ℓ = 1.14 m
quantities that are fully described by both a magnitude and a direction
 indicate direction in square brackets [ ] after the magnitude
Common Vector Quantities
DIRECTION in [ ]
position, d
velocity, v
displacement, Δd
acceleration, a
d = 12.5 m [65° clockwise]
v = 25 m s-1 [west]
Δd = 23 m [E23°N]
a = 9.81 m s-2 [toward ground]
In many IB documents, vector variables are indicated by symbols in bold.
When identifying variables in text or graph labels, name the variable, its symbol, and its accepted unit(s).
 Separate name and symbol using a comma.
 Separate symbol for units using a backslash (/) and use negative exponents when appropriate
ex.
velocity, v / m s-1
Frame of Reference and Directions
2.1.2 State the direction of a vector using angular conventions: RCS, compass directions
Frame of Reference
the fixed reference point and set of directional axis relative
All observations, descriptions, and measurements are relative to this frame of reference
Reference Point
the one fixed point from which all positions are measured
Coordinate System
the set of directions from which all directions are measured
Common Conventions for Directions
forward
up
right
East
North
counterclockwise (ccw) 
+
backward
down
left
West
South
clockwise (cw) 
–
For each situation, you can decide which direction you want to be positive.
Interpret the sign of the answer and state the direction according to your convention.
Angular Conventions
North = 90º RCS
West = 180º RCS
East = 0º RCS (360º RCS)
South = 270º RCS
or – 90º RCS
RCS Direction = Rectangular Coordinate System
All angles measured from positive x-axis (East).
155º
300º = – 60º
Compass Direction
Angles measured from one cardinal direction (NESW)
toward another cardinal direction
W 25º N = N 65º W
E 60º S = S 30º E
2.2 Kinematics
2.2.2 Define displacement, velocity, speed and acceleration.
Position
location relative to the origin (reference point)
 straight-line distance and direction from reference point
Use subscripts to label different variables. For three positions, use: d1 d2 d3 or dA dB dC
Displacement
the change of position
 the straight-line distance and direction between two positions
Δd = dfinal – dinitial
Δd = d2 – d1
Standard unit of displacement: metres, m
Velocity
the rate of change of position
 the displacement per unit of time
v=
Δd d 2 -d 1
=
Δt t 2 -t1
Standard unit of velocity: metres per second, m s-1 (m/s)
Velocity is the slope of a position-time graph.
Δy y2 -y1
=
Δx x2 -x1
slope for y-x graph
m=
For position-time graph (d-t) graph
Distance
length of path between two points – no direction
Speed
how fast the object is moving along the path
speed 
Acceleration
v=
Δd d 2 -d 1
=
Δt t 2 -t1
dis tan ce
t
the rate of change of velocity
 the velocity change per unit of time
a
v v2  v1

t t 2  t1
Standard unit of acceleration: metres per second per second, m s-2 (m/s/s = m/s2)
Acceleration is the slope of a velocity-time graph.

slope for y-x graph
m=
Δy y2 -y1
=
Δx x2 -x1
For velocity-time graph (d-t) graph a  v  v2  v1
t
t 2  t1
Constant, Average, and Instantaneous Velocity

2.2.3 Determine displacement, time interval, and average velocity.
2.2.4 Determine the average acceleration given two velocities and the time interval between readings.
2.2.5 Identify the conditions required for uniform motion.
2.2.6 Solve problems involving the equation of uniform motion: v = x / t.
2.2.7 Construct position-time graphs for sections of uniform motion
Average Velocity
the rate change in position between two points.
 the slope of the straight line between two points on a position-time graph
v=
Δd d 2 -d 1
=
Δt t 2 -t1
Instantaneous Velocity
the velocity at one point – one instant in time (Δt →0)
 the slope of the tangent line at that point
 approximated by the slope of a very short section of the line near the point
Extend the tangent line and select two distance points on it to find slope.
Uniform Motion = Constant Velocity
Constant Velocity
 same speed in a straight line (same direction)  no acceleration
 constant slope (straight line) on position-time graph
During periods of constant velocity,
 the instantaneous velocity at every point will remain the same
 the average velocity between any pair of points will remain the same
Constant Velocity Online Worksheet
Changing Velocity
Non-Uniform Motion = Changing Velocity = Acceleration
 changing speed and/or direction
 curved line on position-time graph
Uniform Motion
Non-Uniform Motion

moving with a changing
speed and/or a changing
direction
constant velocity (∆v = 0)

changing velocity

no acceleration (a = 0)

acceleration is not 0
Position-Time Graph

constant slope (straight line)

changing slope (curved line)
Velocity-Time Graph

no slope (∆v = 0)  horizontal line

slope ≠ 0  angled line

moving with a constant
speed in a straight line
Velocity

Acceleration
Definition
Equations
∆d = v ∆t
or x = vt
(d2 – d1) = v (t2 – t1)
v is constant, so v = vave = v1 = v2
d  12 (v2  v1 )t
d  v1t  12 at 2
v2  v1  at
v2 2  v12  2ad
a=0
acceleration must be constant