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Motion and Speed
Chapter 2
Section 1
Describing Motion
A. Motion
1. Motion – change in position
2. Distance - how far something has moved
Motion
3. Displacement - distance and direction from
the starting point
If a football player runs 100yds in one direction
and turns around and runs 70 yds back, the
displacement is equal to the difference.
100yds – 70 yds = ?
B. Speed
1.
2.
3.
4.
Speed – Distance / time
Rate - any change over time
Calculation for speed:
speed = distance / time
Constant speed - speed that doesn’t change
5.
Instantaneous speed- speed at a specific point
S (meters/second)= d (meters)/ t (seconds)
1. A car traveling at a constant speed covers a
distance of 750m in 25s. What is the car’s speed?
D=750m
T=25s
S=“what we need to find”
What do we need to do to find the speed of the car?
C. Graphing Motion
1. The motion of an object over a period of time can
be shown on a distance-time graph
2. Distance is plotted on the vertical axis (y)
3. Time is plotted on the horizontal axis (x)
4. The slope of a line gives the
speed of an object in motion
D. Velocity
1. Velocity – speed and direction 5 m/s N
a. How are speed and velocity different ?
b. These 2 escalators have the same speed but why do
they have different velocities?
Section 2
Acceleration
A. Acceleration
1.
2.
3.
4.
Acceleration - change in velocity’s rate
Positive acceleration - speed is increasing
Negative acceleration - speed is decreasing
When an object changes speed or direction,
it is accelerating
Visualizing Acceleration
A
C
E
B
D
A to B=speeds up as gravity pulls it down
B to C=slows down as gravity is pulling down
and preventing it to climb back it
B. Calculating Acceleration
1.
Acceleration= final velocity - initial velocity over time
a = vf – vi / t
2.
Units of acceleration – m/s2
3.
A speed/time graph tells you if the acceleration is positive or
negative
•
•
Positive acceleration – “+” numbers with “+” slope on graph
Negative acceleration –”-” numbers with “-” slope on a graph
Review Problems:
Acceleration
1. A ball is dropped from a cliff and has an
acceleration of 9.8m/s2. How long will it
take the ball to reach a speed of 24.5m/s2?
2. A sprinter leaves the starting blocks with an
acceleration of 4.5m/s2. What is the
sprinter’s speed 2s later?
Discussion Question
Most sports rely on the ability of people to
make quick changes in acceleration.
What is some of the equipment used in
different sports to make acceleration easier?
Answer
Cleats - soccer, baseball, football
Rubber-soled shoes – basketball
Starting Blocks – runners, swimmers
Wind-resistant clothing – racers
Section 3
Motion and Forces
A. What is force?
1. Force – a push or a pull
2. Net force – when 2 or more forces combined
act on an object at the same time
3. If the 2 forces cancel each other out, what
do you think the net force will be?
B. Balanced vs. Unbalanced Forces
Fpus
h
W
N
Ffriction
1. Balanced Force – forces on an object that
are equal in size and opposite in direction
The net force = 0
2. Unbalanced Force – forces on an object that
are unequal in size and opposite in direction
The net force = the larger force
C. Inertia and Mass
1. Inertia – the tendency of an object to resist any
change in its motion
2. The larger the mass of an object, the greater its
inertia
3. Newton’s Laws of Motion – rules that describe the
effects of forces on the motion of objects
4. Newton’s 1st Law of Motion – an object moving at a
constant velocity keeps moving at that velocity
unless an unbalanced net force acts on it
D. What Happens in a Crash
The law of inertia (a.k.a. Newton’s 1st Law) can
explain what happens in a car crash
Inertia causes this unrestrained crash test
dummy to continue moving at the speed of
the car before the crash
Balanced or Unbalanced…
Two students push on a box in the same
direction, and another student pushes on the
box in the opposite direction.
1. Is this force that is being exerted balanced
or unbalanced?
2. What does the net force equal?