Fundamentals of physics

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Fundamentals of physics
Mechanics
Unit 5 Motion and Forces 5.6a Linear Motion ...
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Fundamentals of physics - Mechanics

• Motion Is Relative
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• Everything in the world moves even apparantly
  stationary objects.
• Everything moves relative to the Sun and stars.
• You're moving at about 1.1 X 105 kmh-1 relative
  to the Sun, and even faster relative to the
  center of our galaxy.
• When can only describe the motion of an object
  relative to something else.
• When you walk down the aisle of a moving bus,
  your speed relative to the floor of the bus is
  probably different from your speed relative to
  the road.
• When we say a high speed train reaches a speed
  of 300 kmh-1, we mean relative to the track.
• The speeds of things in our environment is
  measured relative to the surface of Earth.

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Fundamentals of physics - Mechanics

• Speed
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• Galileo defined speed as the distance covered
  per unit of time.
• Speed is measured in kilometers per hour (kmh-1)
  For shorter distances, eters per second (ms-1) is
  more appropriate.
• Driving 100m in 10s is a speed of 100m/10s
  10ms-1

When sitting, your speed is zero relative to
Earth but 30 kms-1 relative to the Sun.
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Fundamentals of physics - Mechanics

• Instantaneous Speed
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• When things move their speed may change.
• A car, for example, may travel along a street at
  25 kmh-1, slow to 0 kmh-1 at a red light, and
  speed up to 120 kmh-1 on a highway.
• When you look at the speedometer it gives you an
  instantaneous picture of the speed as it may move
  up and down.
• A speed of 120 kmh-1 means if the car travelled
  for one hour it would have covered 120km...

A speed of 100 kmh-1 means if the car travelled
for one hour it would have covered 100km... In ½
hr it would only have travelled 50km
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Fundamentals of physics - Mechanics

• Average Speed
• --------------------------------------------------
  --------------------------------------

• We are often more concerned with the average
  speed for the trip.
• For example in getting to work it is the time it
  takes for the entire trip and the entire distance
  that matters.

I drive 110km from Sangre Grande to Corinth
Campus in 2 hours. My average speed is
110km/2hr 55kmh-1. Along the way my
instantaneous sped may vary from 0 (at traffic
lights) to 160kmh-1 (on the Highway)
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Fundamentals of physics - Mechanics

• Average Speed
• --------------------------------------------------
  --------------------------------------

• We can rearrange this to get
• Total distance covered average speed X time.
• What is the average speed of a runner who covers
  100m in 10s?
• Average Speed 100m/10s
• 10ms-1

If you get a ticket is it for the average
speed or instantaneous speed?
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Fundamentals of physics - Mechanics

• Velocity
• --------------------------------------------------
  --------------------------------------

• Velocity is both the speed and the direction of
  an object.
• For example, if a car travels at 10ms-1, we know
  its speed.
• But if we know it moves at 10ms-1 to the east,
  we now know its velocity.
• Speed is simply how fast an object moves but
  velocity is how fast and also in which direction.
  

Recall Speed is a scalar... Velocity is a
vector...
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Fundamentals of physics - Mechanics

• Constant and Changing Velocity
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  --------------------------------------

• Constant speed means unchanging speed or no
  speeding up or slowing down.
• Constant velocity, however, means both constant
  speed and constant direction.
• So how can we change Velocity?
• 1. Change speed.
• 2. Change direction.
• When velocity changes we call that acceleration...

The velocity of these cars are constantly
changing because their direction changes
constantly... Even if the speed is constant...
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Fundamentals of physics - Mechanics

• Constant and Changing Velocity
• --------------------------------------------------
  --------------------------------------

• On the highway one car moves to the east at 80
  kmhr-1. Another car that moves to the east at 80
  kmhr-1.
• ?Same speed
• ? Same velocity
• 3. Around a bend, the speedometer of a car reads
  a constant 60 kmhr-1.
• ? Constant speed
• ? constant velocity

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Fundamentals of physics - Mechanics

• Acceleration
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  --------------------------------------

• We can change the velocity of something by
  changing its speed, by changing its direction, or
  by changing both its speed and its direction.
• How quickly (rate) the velocity changes is called
  acceleration

If a car on a level straight road changes its
velocity from 10ms-1 to 30ms-1 in 10s. What is
the acceleration? Acceleration (30ms-1 - 10ms-1
) / 10s 2ms-2
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Fundamentals of physics - Mechanics

• Acceleration
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  --------------------------------------

• Note when an object moving in a straight line
  slows down it also experiences an acceleration!
• Sometimes this is called a deceleration.
• However acceleration is a vector quantity and
  thus we can call a deceleration a negative
  acceleration.

If a motorbike on a level straight road changes
its velocity from 30ms-1 to 10ms-1 in 10s. What
is the acceleration? Acceleration (10ms-1 -
30ms-1 ) / 10s -2ms-2 The motorbike
experiences an acceleration of -2ms-2 or a
deceleration of 2ms-2 .
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Fundamentals of physics - Mechanics

• Free Fall
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  --------------------------------------

• Objects fall towards the centre of the earth
  because of the force of gravity.
• When a falling object falls under the influence
  of gravity alone, the object is in a state of
  free fall. At this stage we ignore air
  resistance.
• Free fall acceleration on earth is approximately
  10ms-2.
• We refer to this free fall acceleration as g.

Time /s Velocity/ms-1 0 0 1 10 2 20 3 30 4
40 5 50 t 10t
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End of Unit 5 Motion and Forces5.6a Linear
Motion...