1) Average speed is the total distance traveled divided by

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Transcript 1) Average speed is the total distance traveled divided by

1) Average speed is the total distance traveled divided by the
time taken to cover the distance.
• We use the letters s for
speed, d for distance, and t
for time whence:
• sav = d/t.
• The SI units of speed are
m/s.
• Usain Bolt traveled 100 m in
9.63 s. What was his
average speed?
2) A multi-exposure photograph is made in a totally dark room
with a strobe and a camera with an open shutter.
• If you travel 40 miles in 2
hours your average speed is
sav = 40 mi/2 h = 20 mi/h =
20 mph.
• Average speed tells you
nothing about the details of
the trip.
• What is happening to the
speed of the blue ball?
3) Instantaneous speed is equal to the average speed over a time
interval that is very, very small.
• For practical purposes we
need a time interval that is
small enough that the
average speed does not
change very much if we use
an even smaller time
interval.
• Think of instantaneous
speed as what a
speedometer says. How
does it work?
4) Velocity is speed and direction. The magnitude of velocity is
speed. Quantities that have both magnitude and direction are
called vectors.
• Average velocity is
displacement divided by
time. vav = Dx/t.
• Displacement is a vector
quantity whose magnitude
is the straight-line distance
from A to B and whose
direction is the direction
from A to B.
• Is the displacement vector
ever longer than the
distance traveled?
5) Average acceleration is the change in velocity divided by the
time it takes to make that change.
• We use the letters a for
acceleration, v for velocity,
and t for time whence:
• aav = Dv/Dt.
• The SI units of acceleration
are m/s2.
• When you travel in a
straight line at a constant
speed your acceleration is
zero.
• The falling ball accelerates!
6) According to Aristotle, a heavy physics text should fall
significantly faster than a crumpled piece of paper.
• According to Galileo, in a
vacuum, the falling speeds
of a cannonball and a
feather should be equal.
• Astronauts conducted an
ultramodern demonstration
of this on the Moon where
a hammer and a feather fell
at the same rate.
• Why does the hammer fall
faster on Earth?
7) As Galileo let balls roll down steeper ramps he discovered the
following rule: the acceleration in each case is constant.
• Constant acceleration means the speed changes by the
same amount each second.
• In the case of free fall the acceleration due to gravity is
about 9.8 m/s2.
• We use the letter g for this acceleration and usually round it
off whence:
• g = 10 m/s2.
8) If you throw a ball vertically upward with an initial speed of
20 m/s, one second later its instantaneous speed will be 10 m/s.
• As the ball continues to rise
against gravity it continues
to lose speed. It will reach
its maximum height in 2
seconds.
• At that moment the
instantaneous speed of the
ball will be zero.
• Is the ball rising or falling?
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Exercise 80