A lightning tour, qualitatively, of the physics of forces and motion. Intended as a backgrounder for middle years teachers.

2. MOTION
What does is mean for an object to be stationary?
Is there any such thing?
If an object is in motion, we can describe its
position, and how fast it is changing its position
We can also describe the direction it is moving
All motion is described in relative, not absolute
terms
Usually relative to the earth‟s surface in the local
area, but not for rockets, planets, stars

3. CONCEPTS - MOTION
Speed – rate of change of position (distance unit
per time unit, e.g. metre per second)
Velocity – same as speed but with a direction
specified (e.g. 30 m/s North)
Distance – total distance travelled relative to the
ground (distance unit, e.g. metre)
Displacement – current distance from point of
origin, including direction (e.g. 25 m West)

4. CONCEPTS - ACCELERATION
Acceleration – rate of change of velocity (not
speed)
Unit of distance per unit of time per unit of time
(e.g. metre per second per second, m/s/s, ms-2)
Can involve a change of speed or a change of
direction (or both)
Can be in the same direction or the opposite
direction compared to motion, or at an angle to
it

5. CONCEPT - FORCES
A push or pull that acts on an object
Measured in an SI unit called the newton (N) which
is a kilogram metre per second per second,
kgms-2)
How we arrive at this unit will become obvious
Can involve contact (e.g. a tug pulling a ship, a
bulldozer pushing dirt) or not (e.g. earth pulling
on the moon)

6. EXAMPLES OF FORCES
A golf club hitting a ball
People pushing a car
Magnets attracting or repelling
A book pushing down on a desk and the desk
pushing upward on the book
Gravity pulling downward on a flying seagull
Positively charged protons in the nucleus
attracting negatively charged electrons

7. GRAVITY
Gravity is a particular kind of force
It is a result of the nature of matter in this universe
(related to the Higgs field and the Higgs boson)
It causes all masses to be attracted to all other
masses
It is a very weak force (compared to electrostatic
force, for example), which means it becomes
important when masses are very large, but it
exists for all masses
It decreases with the square of separation
distance

8. CONCEPTS - MASS AND WEIGHT
Mass – measures the amount of matter in something
in mass units (e.g. kg)
Weight – the force that gravity, at a particular place,
exerts on a particular mass
Mass of an object is the same everywhere in the
universe
Weight of an object depends on the local
gravitational field
Objects have the same mass but about 1/6 the
weight on the moon

9. NEWTON’S FIRST LAW (OF MOTION)
First Law – “An object remains at rest, or in
motion in a straight line at constant speed,
unless acted on by an unbalanced external
force”
(since motion is relative, „rest‟ and „uniform
straight line motion‟ only depend on your point
of view!)
This is not intuitive to us, since things tend to
slow down – but this is because friction is
acting (see below)
Think about the situation in outer space…

10. NEWTON’S SECOND LAW
Second Law – “The acceleration of an object is
directly proportional to the (external,
unbalanced) force acting on it and inversely
proportional to its mass”
Usually written as an equation for calculations: a =
F/m
Alternatively, F = ma
It makes sense that the bigger the force the bigger
the acceleration
„Inertia‟ is the name we give to the idea that bigger
mass will lead to smaller acceleration with the
same force

11. NEWTON’S THIRD LAW
Third Law – “For every force there exists another
force of the same magnitude (size), that acts in
the opposite direction, on a different object”
People talk about „action‟ and „reaction‟ forces, but
that can be confusing – it‟s only a point of view
Earth pulls the moon, and the moon pulls back with
the same force: but because the earth has more
mass, the moon accelerates more (i.e. changes its
direction of motion, moving in (nearly) a circle)
There are no exceptions to the Third Law

12. FRICTION
Friction is a kind of force that occurs when
objects are in contact with each other, either
stationary or moving relative to each other
Frictional forces always oppose the motion that
causes them
Energy (which is a topic for another day) is lost to
frictional forces, causing heat
Different kinds of frictional: static (stationary
relative to each other), kinetic (sliding past each
other), rolling

13. AIR RESISTANCE
When an object is in „free fall‟ in an atmosphere,
as it gains speed the air will oppose the motion
– this is a form of friction
The gravitational force remains constant and the
air resistance increases with velocity – „terminal
velocity‟ is reached when the two forces
balance

14. BUOYANCY
When an object is placed in water it displaces
water
The weight force due to that displaced water acts
upward on the body – the force of buoyancy
If buoyancy balances the weight of the object, it
will float, if not, it will sink