TOPIC 2: MECHANICS
(17 HOURS)
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
Kinematics
Time Interval
6 hours
the time elapsed, t, between two defined events, t1 and t2
Time interval is a scalar quantity and always positive.
SYMBOL: t
(∆t)
UNIT: second, s
t = t2 – t 1
2.1.1
Define displacement, velocity, speed and acceleration.
Displacement
the change of position
SYMBOL: s, x, y, z
 the straight-line distance and direction that an object moves
between two points during time interval t
UNIT: metre, m
x = xfinal – xinitial
x = x2 – x1
As an object moves from point A to point B, the displacement is the vector difference
between the displacements from a reference point.
x = xB – xA
Velocity
the rate of change of position
 the displacement per unit of time
x x x
v  2 1
t
t 2  t1
SYMBOL: v
STANDARD UNIT: metre per second, m s-1 (m/s)
Velocity is the gradient of a position-time graph.

slope for y-x graph
Δy y2 -y1
m=
=
Δx x 2 -x1
Distance
length of path between two points – no direction
Speed
how fast the object is moving along the path
For position-time graph (x-t) graph v 
speed 
Acceleration
x x2  x1

t
t 2  t1

distance
t
the rate of change of velocity
STANDARD UNIT: metre per second per second, m s-2
 the velocity change per unit of time
(m/s/s = m/s2)
a
v v 2  v1

t t 2  t 1
a
vu
t
Acceleration is the gradient of a velocity-time graph.
slope for y-x graph
m=
 y2 -y1
Δy
=
Δx x 2 -x1

For velocity-time graph (d-t) graph a  v  v2  v1
t

t 2  t1
Constant, Average, and Instantaneous Velocity
2.1.2
Explain the difference between instantaneous and average values of speed, velocity and acceleration.
Average Velocity
the rate change in position between two points (definite time interval, t).
 the slope of the straight line between two points on a position-time graph
x x2  x1
v

t t 2  t1
Instantaneous Velocity
the velocity at one point – one instant in time (t →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
x
where x is the infinitesimal change in displacement
v
t
during the infinitesimal change in time, t
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
2.1.3
Non-Uniform Motion = Changing Velocity = Acceleration
 changing speed and/or direction
 curved line on position-time graph
Outline the conditions under which the equations for uniformly accelerated motion may be applied.
Uniform Motion
Non-Uniform Motion

moving with a constant
speed in a straight line

moving with a changing speed
and/or a changing direction
Velocity

constant velocity (∆v = 0)

changing velocity
Acceleration

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
Definition
Equations
x = vt
(x2 – x1) = v (t2 – t1)
v is constant, so v = vave = v1 = v2
a=0


x  12 (v  u)t
x  ut  12 at 2
v  u  at
v 2  u 2  2ax
acceleration
 must be constant

Acceleration of Free Fall
2.1.4
Identify the acceleration of a body falling in a vacuum near the Earth’s surface with the acceleration g of free fall.
g
Gravitational Field Strength
depends on : (1) the mass of the central body;
and (2) the distance from a selected point to the centre of mass of the central body.
g
Acceleration due to Gravity at Sea Level
g = 9.81 m s– 2 [toward Earth]
 for all objects at sea level
The magnitude decreases as the object moves upward, away from the centre of the Earth.
Free Fall and Air Resistance
Free fall is a state in which the only force acting on an object is gravity (its own weight).
This can only truly happen in a vacuum where there is no air and no air resistance.
It is assumed for simplicity that most objects launched or dropped near the surface of the Earth
will experience free fall and have uniform acceleration.
For multiple choice (paper 1)  use g = 10 m s–2
For calculations and word problems (paper 2)  use g = 9.81 m s–2
2.1.6 Describe the effects of air resistance on falling objects.
When air resistance is significant,
2.1.5

the negative acceleration of an object moving upward will be more than g (stopping sooner)

the negative acceleration of an object moving downward will be less than g (falling slower)
Solve problems involving the equations of uniformly accelerated motion.
Clue Words
“dropped” or “released”
“thrown” or “launched”
highest point
Meaning
initial velocity is zero; free fall
initial velocity is NOT zero; free fall
greatest vertical displacement;
instantaneous velocity is zero
Variables
u = 0 a = g = – 9.81 m s–2
a = g = – 9.81 m s–2
ymax v = 0
Each kinematics formula for uniform acceleration contains four of the five variables (1 is not in equation)
In each situation/stage of motion, there must be 3 known variables and 1 unknown to find.
Variable not in Equation
s
u
v
a
t




Equation
v  u  at
not in Data Booklet ( a  v  u  definition of acceleration)
s  vt  12 at 2
s  ut  12 at 2
s  12 (v  u)t
v 2  u 2  2as
not in Data Booklet
in Data Booklet

in Data Booklet
in Data Booklet
t
2.1.7 Draw and analyse distance-time graphs, displacement-time graphs, velocity-time graphs and
acceleration–time graphs.
2.1.8 Calculate and interpret the gradients of displacement–time graphs and velocity–time graphs, and
the areas under velocity–time graphs and acceleration–time graphs.
2.1.9 Determine relative velocity in one and in two dimensions.