Transcript 1.5 Frames of reference

1.5 Frames of reference
Imagine…
• Your friend and yourself have set up a
small experiment on your spare time,
because you have nothing better to do
• Your friend stands in front of a forest –
and in front of the forest is a railway track
upon which is a special railcar
• This railcar contains a large window and
through it you can see your friend
Imagine…
Movement
• Now, imagine that the rail car is moving at
10 km/h
• How will the scene look from your
perspective (on the ground) vs. your
friend’s perspective?
From the ground…
From the car…
Various views
• If you notice – depending on the
viewpoint of the person involved, the
velocity of the various objects in the scene
will differ
• For example – take two objects in the
scene – the trees, and the car itself
Are they the same?
• How would you describe the speed of the
trees and the car if you were:
• Standing on the ground?
• Standing in the car?
• Take the right as positive and the left as
negative
What are the values?
• If you were standing on the ground:
• tVg = 0 km/h
• cVg = 10 km/h
• If you were standing on the railway car:
• tVg = -10 km/h
• cVg = 0 km/h
Frame of reference(FOR)
• Notice that the velocity of the objects
were different depending on where you
are observing the event
• A frame of reference refers to the point
of view that you are choosing to analyze
an event
Important things to consider
regarding a frame of reference
• In a given FOR, the type that we will be
focusing on are ones in which no
acceleration takes place
• This is known as an INERTIAL FOR
• In an inertial FOR, all the laws of physics
will apply
For example…
• Imagine you find yourself in a room – you
are sitting in apparatus that cushions you
from all movement
• There is a ball sitting still on a table
• The ball doesn’t move
• And you notice that when you drop a ball
from where you sit, it falls to the ground
But what if…
• In the same room, the ball on the table
suddenly flies forwards?
• Or if when you try to drop a ball on the
ground, it doesn’t take a straight path to
the floor?
• HOW WOULD YOU EXPLAIN THIS?
What if the car was moving?
• In the first situation, the room that you
were sitting in could be either completely
still or moving at a constant speed – both
would produce those results
• Think about sitting in a car that is moving
smoothly at a constant speed – and
throwing a ball up and down or placing it
on the seat
What if the car were to stop?
• Now imagine sitting in that same car, and
suddenly, the driver slams on the brakes
• What would happen if you were trying to
catch a ball that you just threw upwards,
or a ball that was placed on the edge of
the seat?
Non-inertial FOR
• In non-inertial FOR, the FOR experiences a
change in motion
• Since the laws of physics don’t apply in noninertial FOR (think – objects don’t suddenly
slide off tables without a visible force applied) it
becomes hard to compare it with an inertial
FOR
• For many of the relative velocity questions we
deal with, we assume that all FOR’s are inertial
Relative velocity
• Therefore, velocity of an object can
change depending on the FOR that you
choose to analyze it from
Moving within a FOR
• There are 2 basic types of relative velocity
that we are going to look at – and
therefore, 3 basic types of relative velocity
questions that you are going to come
across
• One type is relative velocity in ONE
DIMENSION – that can be thought about
by looking at the railway track example
again
Imagine…
• What would happen if your friend began
walking towards the back of the railway
car at 6 km/h as it passed you at 10
km/h?
• Would your friend’s speed look the same
as from your point of view as it did
before?
One dimensional FOR
Notice…
• That your friend’s velocity relative to the
ground DECREASED
• Because they were moving towards the
back of the car, their backwards velocity is
subtracted from the forward velocity of the
car
• How would your friend’s velocity appear to
you if your friend was moving towards the
front of the car?
One dimensional FOR
Two-dimensional FOR
• In a 2D FOR, motion can occur on a plane
• An outside “force” pushes the object, and
contributes a velocity vector that provides
a perpindicular direction to the object’s
motion
If
swimmer
– what
isand
your
If you
you are
are the
standing
on the
shore
direction
to happen
the water?
watching,relative
what will
to a swimmer
What
is the
direction
that tries
to water’s
travel across
therelative
water? to the
ground?
wVg
sVw
sVg
Notice…
• In 2D vector questions, there are 3
vectors that are described
• fVg: The force’s (water, wind, another
moving FOR, etc.) velocity relative to the
ground
• oVf: The object’s (ship, boat, swimmer, etc.)
velocity relative to the force
• oVg: The object’s velocity relative to the
ground
Limited situations…
• In general, most of the questions you will
encounter are limited in how these
vectors can be oriented relative to each
other
• Ofcourse, like all basic vector questions, 3
vectors means that you can create either
a right angled or non-right angled triangle
And in real life…
• There is only a few ways that a force and
an object can interact to create the
various types of vector questions that
you’re going to see
• Therefore, there are 2 basic situations
that you will come across when solving
relative velocity questions
“Push”
• In these types of questions, the force
pushes the object away from the original
path that the object wanted to take
• This is the example that we have seen
earlier
Notice how the swimmer’s body is oriented –
they face the shore and swim – so although
the body faces towards the shore, the body
takes a diagonal path across the water
fVg
oVf
oVg
“Fight”
• In these types of questions, the object is
trying to oppose the force to follow a
particular path
Notice how the swimmer’s body is oriented –
they turn their body so that they swim
diagonally relative to the water – but the
water pushes the swimmer back so that their
velocity relative to the ground is directly
across the river
fVg
oVf
oVg