Now we want to discuss the properties of motion from a fundamental point of view:

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Now we want to discuss the properties of motion
from a fundamental point of view
What properties of the motion of an object do we
need to know in order to know its speed?
t2 5 s X2 20 m
t1 0 s X1 0 m
2
t2 5 s X2 20 m
t1 0 s X1 0 m
Distance rate x time
d r x t
In our case r d/t (20 m)/(5 s) 4 m/s
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The SPEED we just defined was the AVERAGE SPEED
over the interval. We have to be careful with
this
Example Suppose a person drives 60 miles in two
hours, but we check their times at various
checkpoints (1) calculate the average speed over
each interval (2) calculate the average speed
between the first and fifth intervals
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Note even though this persons AVERAGE SPEED was
only 60 mph, they were speeding during one
interval!
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We can measure the speed of an object over very
small time intervals. If the time interval is
VERY small, we call the speed we measure the
INSTANTANEOUS SPEED
INSTANTANEOUS SPEED The speed of an object at a
specific INSTANT in time. (As apposed to the
speed measured over a relatively large time
interval (average speed).
Example The speed you measure on the speedometer
in your car approximately gives you the
instantaneous speed.
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The Difference between speed and velocity
scalars vs. vectors
SCALAR a quantity that is specified by only a
NUMBER Example 12 oranges, 5 meters, 10 m/s
VECTOR a quantity that must be specified both a
NUMBER and a DIRECTION Example velocity (30
miles/hr SOUTH)
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ACCELERATION
ACCELERATION CHANGE OF VELOCITY WITH TIME
How does the law of inertia apply to objects that
are accelerating? A BODY THAT IS SUBJECT TO NO
EXTERNAL FORCES MUST MAINTAIN AN UNCHANGING
VELOCITY
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Example Suppose that a car is moving at a speed
of 5 m/s. 10 seconds later it is moving at a
speed of 20 m/s. What is its acceleration?
Its velocity changed by (20 m/s - 5 m/s) 15
m/s in 10 seconds. The acceleration must be
a (15 m/s) / (10 s) 1.5 m/s2
Notice the units m/s2 (read meters per
second, per second)
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Time
0s 1s 2s 3s
4s
Position 0m 5m 20m 45m
80m
Speed 0m/s 10m/s 20m/s 30m/s
40 m/s
Calculate the acceleration in each time interval
Answer (10 m/s)/(1 s) 10 m/s2
10
0s
1s
It is exactly like a falling object!!
2s
Galileos Law of Falling Neglecting Air
resistance, all objects fall with the same
acceleration.
3s
Acceleration due to gravity The acceleration of
any freely falling object. On earth this is
about 10 m/s2 (actual value 9.8 m/s2.)
4s
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V 0
What can we say about the SPEED of the ball at
the top and at the bottom?
V (up)
V (down)
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THE LAW OF INERTIA IF NO EXTERNAL INFLUENCES ACT
ON A MOVING OBJECT, THEN IT WILL MOVE IN A
STRAIGHT LINE (AT A CONSTANT VELOCITY) FOREVER.
Does an accelerating object move with a constant
velocity??
NO!!!
Therefore, if the velocity is changing, there
must be an external influence.
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What is the force due to the mass on the table?
It is the weight!
THIS IS VERY VERY VERY IMPORTANT! Most people
confuse mass and weight. They are RELATED by the
above relationship.
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Newtons Third Law (the law of force pairs)
FOR EVERY ACTION THERE IS AN EQUAL AND OPPOSITE
REACTION
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The Free-Body Diagram
Normal Force
W mg
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When an object is not accelerating (i.e.,
whenever the velocity is constant), the sum of
the forces must be ZERO.
In this case Sum of forces (normal force)
(weight) 0
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A ball is thrown straight upward. While it is
moving upward, the ball's acceleration is (a)
zero, i.e. the ball is not accelerating. (b) in
the downward direction. (c) in the upward
direction. (d) in the horizontal direction.
(e) none of the above.
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A one kilogram grapefruit weighs ten newtons.
Suppose we take this grapefruit to the moon. The
force of gravity on the grapefruit will then be
(a) ten newtons. (b) less than ten newtons.
(c) one kilogram. (d) less than one kilogram.
(e) none of the above.
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An auto weighing 3000 newtons has a drive force
of 1000 newtons. Air resistance on the car is 100
newtons, and road resistance Is 100 newtons.The
auto moves along a straight level road. The net
force on this auto is (a) 200 newtons backward.
(b) 800 newtons backward. (c) 800 newtons
forward. (d) 1000 newtons forward. (e) 1200
newtons forward.
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Continuing the preceding question, the force due
to gravity acting on the auto is (a) 3000
newtons upward. (b) 3000 newtons downward.
(c) zero. (d) 4200 newtons downward. (e) 800
newtons upward.