16/09/2013
What is Physics?
Physics in Everyday Life
The Physics of Motion
Andrew Robinson
• The study of objects and their interactions
• Observational science – why do things work
they way they do
• Predictive science – mathematical models are
developed to predict behaviour
• Experimental science – mathematical models
are tested to see if they reflect reality
• If they do and are accepted as a generally
robust model, then they become a Theory
Physics is Everywhere
• By looking around our
surroundings, we can see lots
of physics happening
• There is also lots of physics
happening which our own
senses cannot detect
Course Objectives
• To show you physics at work around us
• To explain why some things we observe
happen the way they do
• To show you that physics does not need
complicated mathematics to be
understandable
• To make you think
Position
Position
Acceleration
Describing
Motion
Displacement
• Where are we?
• To describe position, we need a Reference
Point, sometimes called the Origin
Velocity
(and Speed)
Equator
Greenwich Meridian
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• René Descartes
• The Cartesian Coordinate System
Position
Acceleration
Describing
Motion
Displacement
Velocity
(and Speed)
Velocity and Speed
• These are similar concepts, the rate of change
of distance with respect to time
• I drove at 80 km/hour describes a speed
• I drove due North at 80 km/hour describes a
velocity
• In Physics there is a distinction between
velocity and speed
• Velocity is the speed, but also has a
designated direction
– This is an example of a vector quantity
Acceleration
• The rate of change of velocity
• A vehicle with a higher acceleration travels
further in any given time
• At the end of that time it has a higher speed
than the vehicle with the lower acceleration
• How much faster are we getting?
– Accelerating
• How much slower are we getting?
– Decelerating
The concept of
separate velocity and
acceleration is a
difficult one to
visualise
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The Aristotelians
• The Greek Scholar Aristotle considered what
made objects move
• He stated that an object required a Force to
keep it moving
• This definition of a “Force being an agent
causing motion” lasted until the Renaissance
period in western Europe.
• Aristotle did not test his theory of motion with
experiments.
• Nevertheless, his ideas on force and motion
dominated Western science for thousands of
years.
Galileo Galelei
• Vincenzo’s son Galileo applied the scientific
method to the study of motion
• His experiments, some rolling marbles down
slopes and others, clearly distinguished
between velocity (speed) and acceleration
• Some of his results contradicted Aristotle and
were very controversial
Observe
Reject or
Accept
Hypothesis
Form
The Great Greek
thinkers failed to
develop the
Scientific Method
Compare
Observation
with
Experiments
Hypothesis
Perform
Experiment
Vincenzo Galilei
and the Scientific Method
• Vincenzo Galilei (1520-1591)
was an accomplished Lutenist
and composer.
• He produced the first
description of how the tension
in a lute string changed the
pitch of the note.
• He did experiments, analysed
the results and produced a
mathematical formula to
describe the relationship
• Aristotle had hypothesized
that the heavier an object,
the faster it would fall
• Galileo dropped
cannonballs of different
weight from the top of the
leaning tower of Pisa.
• They hit the ground at the
same time!
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Isaac Newton and the Laws of Motion
• In 1686, the English scientist
Isaac Newton, took work on
motion by Galileo and
Descartes, added his own
observations and
formulated three laws of
motion and a law of gravity
which describe most
observable motion.
The Second Law of Motion
• The force required to change the velocity of
an object is proportional to the mass of the
body and the acceleration
The First Law of Motion
• An object continues at constant speed in a
straight line (i.e. at constant velocity) unless
acted on by an external force.
The natural state of
nature is either no
motion or motion in a
straight line with
constant speed
Aristotle was
wrong! Aristotle
assumed that
objects were
stationary when
no force was
exerted
𝐹𝑜𝑟𝑐𝑒 = 𝑚𝑎𝑠𝑠 × 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
• If you apply more force to an object, it
accelerates more
• 𝐹𝑜𝑟𝑐𝑒 = 𝑚𝑎𝑠𝑠 × 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
So What is Mass?
• Sometimes called Inertia or Inertial Mass
• The resistance of a body to a force – a large
mass will resist a force more than a small
mass, and produce a smaller change in motion
(the acceleration)
Mass and Weight
• Don’t confuse them!
• In physics, a weight is a force due to gravity
acting on a mass
• The mass is constant
• The weight depends where you are in the
universe
Dictionary Definition:
inertness, especially with regard to effort, motion, a
ction, and the like; inactivity; sluggishness.
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Walking On The Moon
The Third Law of Motion
• On the Moon, the gravitational force is a sixth
that on Earth
• The astronauts from Apollo 17 (Gene Cernan
and Harrison Schmitt) weigh less than they do
on Earth
• Their inertia (mass) is still the same
• When a force is exerted on an object, the
object exerts an equal and opposite force
back.
• Known as the Reaction Force
• This law is sometimes quoted as
• “Action and reaction are equal and opposite”
http://www.youtube.com/watch?v=wo3-fuYKWB4
Video: NASA
How Do We See It In Action?
• The Third Law was Newton’s major
contribution to development of the laws of
motion.
• If you push on an object, the object pushes
back on you with an equal force which is in
the opposite direction
Gravity pulls
down on
grandmother,
pushing her
towards the floor
“Child pushing Grandmother on tricycle”
Grandmother’s back
pushes on boy’s hands
Boy pushes
grandmother
Forces Can Cancel Out
• Force is a vector quantity (with magnitude and
direction).
• Vector quantities can be added together, so
that they cancel each other out, and the net
effect is zero
Floor pushes back
on grandmother,
stopping her from
sinking into the
ground
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Vase with Irises (Van Gogh, 1890)
We know gravity is
pulling the vase down:
things fall towards the
Earth
• In a tug of war contest, the teams are pulling
on the rope in opposite directions
Why doesn’t the vase
fall through the table?
The table pushes back on the vase, with a force equal to
the force of gravity. There is no force, no acceleration,
and a stationary object remains stationary
• If both teams are evenly matched, then the
two forces exerted on the rope are equal and
opposite. They cancel out.
• There is no net force on the rope, so the rope
does not change its motion
• If the rope was stationary, it remains
stationary
• Adding force vectors and cancelling them out
works with any number of vectors
If each of these
three force vectors
are equal, the net
force is zero
• If one team is slightly stronger than the other, the
forces do not cancel out, and there is a small
force acting on the rope in the direction that the
stronger team are pulling
• Newton’s Second Law says that if there is a force,
then there must be acceleration.
• The stationary rope must start to move in the
direction of the acceleration
Aircraft Climbing
Engine Thrust
Lift
Air
resistance
(drag)
Gravity
• This is a free body diagram, it shows the
main forces acting on the aircraft
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Apple tree at
Woolsthorpe Manor,
Grantham, where
Newton formulated the
Law of Gravitation
Gravity
• In the Principia Newton also
formulated a Law Of Gravity
to describe the easily
observable fact that things
fall down towards the
surface of the Earth
Newton’s Law of Gravity
Kepler’s Laws
• The gravitational force is exerted between any
two masses and depends on the value of each
of the masses.
• It also gets weaker with distance
• Newton’s Law of Gravitation was able to
explain the earlier observations of planetary
motion made by Johannes Kepler (1571-1630)
With distance
squared to be exact
Earth Blue
Mars Red
• Earth and Moon
seen from the
Galileo Probe
• Earth attracts the
Moon
• Moon attracts the
Earth
• The forces are
equal and opposite
Lunar Tides
• The gravitational pull of the moon is one of
the two contributing factors to explaining
tides in the seas and oceans
•
The gravitational
pull of the sun is
the other
influencing factor
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Circular Motion
Direction of Velocity for a Rotating
Object
• The velocity vector is always at a tangent to
the spinning object
• If the Earth and
Moon are pulling
on each other,
why don’t they
collide?
• The velocity vector is always at a tangent to
the spinning object
• So the vector points in a different direction
• This counts as a change in velocity over time
• A rotating object is always subjected to a
force, because its velocity is always changing
• The force always acts to push the object into a
circular path – it is always towards the centre
This is a
definition of
acceleration
Centripetal Force
• From Latin centrum
"centre" and petere
"to seek“
• The Centripetal force
is any force which
always acts to make
an object move in a
circular path
Where Does the Centripetal Force
Come From?
• It is not a new force of
nature, it has to come from
a force which is already
acting on the object
Bolas: South American Lasso
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• For a ball on a string, it’s the tension force in
the taut string that pulls the ball around in a
circle
• For the moon going
round the sun, it’s gravity
that provides the
centripetal force
Tension Force
Gravitational Force
Turning a Corner in Your Car
You turn in a
circular path, so
where does the
centripetal force
come from?
• Sideways friction forces from the tires (tyres)
provides the centripetal force
Canadian Grand
Prix 2006,
Montreal
• If the friction force is not sufficient to hold you
in the circle, then you slide off the road
Ice
Slide off, if
there is not
enough
centripetal
force
• Whether friction can generate sufficient force
to hold you on the road depends on
1. The speed you are going – there is a
maximum safe speed
2. The radius of the turn (tighter turns – must
go slower)
3. How good your tires are (tread and rubber
compound)
4. The nature of the road surface – slippery
surfaces generate much less friction force
(ice, puddles of water)
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• Why?
Banked Turns
– If the turn is banked, then part of another force
can provide an extra contribution to the
centripetal force
– More force: easier to turn more quickly and in a
tighter turn
• In many cases of
turning, the object
turns more easily if
it is banked
Force of road
pushing on car
Friction
Weight
• Centripetal force has contributions from
friction and normal force
• It is larger than it would be for a flat turn
• The car can take the banked turn faster than it
can take a flat turn
• There is a similar effect for aircraft, where
banking uses part of the lifting force
generated by the wings to provide centripetal
force
Centripetal
force
• The vertical part of the list force has to
counterbalance gravity, otherwise the plane
loses height!
gravity
• The part of the lift force which is parallel to
the ground provides the centripetal force.
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Modes of Transport
Sail Power
• Moving ourselves around is mostly about
providing a force to push against something in
order to change our state of motion
The exception to this statement is the
rocket motor, which depends upon a
different physical principle, the
Conservation of Momentum. We will
discuss this principle in another class.
• We use the wind to provide a force
• The sails change the direction of the wind
velocity (which means the wind is accelerating
around the sail)
• The wind exerts a force on the sail
• The Sail exerts a force on the air. This force is
the one which moves the boat
http://www.phys.unsw.edu.au/~jw/sailing.html
• Wind changes direction
Force on
air
making it
change
direction
Force of sail on
air. The sail
pushes on the
air and this is
the force which
can move the
boat
• The sideways force (which is not wanted) is
counteracted by a drag force from the keel
under the hull
Drag force from water
Sideways force
• The force of the sail on the wind is applied to
the hull of the boat through the mast.
• The hull has a force on it, which changes its
motion
Propulsive
force, moves
the boat
forward
through the
water
Unwanted sideways force
Flettner Rotor Ship
• In the 1920’s the German engineer Anton
Flettner, used the Magnus effect to power
ships, by using wind power coupled to rotating
cylinders to generate a propulsive force more
efficiently than sails
Flettner Rotors
Drag force
from keel
Propulsive force
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• The Flettner rotors were powered by low power
electric motors
Force generated on rotor
Wind velocity
Gondolas in Venice
• The oar pushes on the water
• The water pushes back on the oar
• This force is transmitted through the oar and
gondolier to the gondola
Oar Power
• Use the oar to push on the water
• Newton’s Third Law, means that the water
pushes back on the oar.
• The oar is held by the crew, who sit in the
boat. The push of the water on the oar
propels the boat
Propellers or Paddlewheels
• The paddlewheel or propeller in a ship turns
and exerts a force on the water
• The water pushes back on the propulsion
system, moving the boat
It actually depends on
the grip of the
gondolier’s shoes to the
deck!
Walking, Jogging or Running
• Our muscles move our skeletal
system so that we exert a force
on the road.
• The road pushes back on our
feet
• We move along
• Force is efficiently transferred to the ground if
there is plenty of grip
– A large friction force is applied between the
ground and the foot
– We wear shoes which are optimized to give good
grip in various conditions
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• Wheelchair basketball (Canada vs Australia,
Sept 2012, Paralympics)
• Athlete pushes on the wheel
Wheel pushes on the floor
• To propel a car forward is similar, except that
the force to turn the wheel comes from the
engine
• Forward progress depends on there being
efficient transmission of force from wheel to
ground.
• This requires a large friction force.
Floor pushes on the
wheel, wheelchair
moves forward
Conclusions
• By studying motion, we can deduce many
things about the physical universe.
• We can see Newton’s Laws of Motion
operating all around us, even on objects which
do not appear to be moving.
– In slippery conditions, the wheel turns, but slips
on the road, and so no forward progress is made
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