Transcript Motion 10sci

Physics
:motion and speed
Understanding
motion
Look at the pictures of a soccer ball
photographed as it moved over a distance.
Write an explanation of what is in the picture.
What did you assume while writing your
description?
Numbers
In your own words, describe a
procedure you could use to calculate
how fast an object moves.
Discuss your interpretations with the
person next to you
Reach a consensus about the
interpretation.
Speed
The “speed” of an object tells us
something about how far the object
will move in a particular amount of
time.
– The faster an object is moving, the
more distance it will travel in a given
amount of time.
5 cm/s
How fast is the
hare moving
relative to the
tortoise?
Speed is a Rate
In this case, rate implies a time
dependence.
Examples:
– The number of laps you swim per hour
– The number of times you sneeze per day
– The number of vacations taken per year
So speed is defined as the rate of
motion
With our definition of motion we have:
Speed = rate of change in position
Explanation
Speed Equation
The relationship between speed,
distance and time is given by:
Unit
Symbol
Speed
v
Measured Symbol
in
Metres per
second
Or km per
hour
ms-1
Kmh-1
Distance
d
metres
Or
kilometres
m
km
time
t
seconds or
hours
S
or
hr
Speed = Distance
Time
How fast did you
go?
The distance from Wellington to
Palmerston North is 145 km.
How fast should you go if you want
to make the trip in exactly 2 hours?
v = d/t
v = 145km/2hr
v = 90km/h
If you ran a speed of 20ms-1 over 100m what
time did it take?
Average speed can be calculated by total distance divided by
total time but journeys over distance are not travelled at a
constant speed but change over time.
They can be calculated in segments
Distance time graphs can be drawn from the data gathered to
show speed at different points in the journey
distance
Work out what is
happening from
1. 0 – 4 secs
distance
2. 4 – 6 secs
3. 6 – 10 secs
4. 10 – 12 secs
What would the line
look like it acceleration
(getting faster over
time)was occurring?
Decelerating?
ACCELERATION = CHANGE IN SPEED (ms-1)
(ms-2 )
--------------------v
TIME (s)
a
t
SPEED TIME GRAPHS
Draw and state what these graphs show
v
v
v
t
t
t
Why do objects not continue to accelerate indefinitely ?
They reach terminal velocity. This is is the term for the state an
object reaches when the force of drag acting on it is equal to the
force of gravity acting on it. When an object reaches its terminal
velocity, it no longer accelerates, remaining at whatever velocity it
was already traveling or else slowing down.
Different objects will have drastically different terminal velocities.
A person in freefall, for example, has a terminal velocity of
approximately 56.08 m/s or roughly 200 kph. A raindrop, in
contrast, although very aerodynamic, is also not very dense, and so
has a terminal velocity of around 7.62 m/s or roughly 27 kph. A
lead bullet shot up straight in the air, on the other hand, has a
terminal velocity of around 67.97 m/s or 245 kph.
Calculating the terminal velocity of an object means calculating
the drag exerted on the object, and then comparing that to the
weight of the object to determine the net force. To do this,
you need to know the frontal area of the object, the gas
density it is falling through, and a drag coefficient. It’s not a
particularly easy calculation to make.
where
Vt = terminal velocity,
m = mass of the falling object,
g = acceleration due to gravity,
Cd = drag coefficient,
ρ = density of the fluid through which the object is falling, and
A = projected area of the object.