MOTION. Do you have to see something move to know that motion has taken place?? All you need to know...
Transcript of MOTION. Do you have to see something move to know that motion has taken place?? All you need to know...
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MOTION
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Do you have to see something move to know that motion has
taken place??All you need to know is that its
position has changed!
Motion occurs when an object changes its
position RELATIVE TO A REFERENCE POINT.
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The dog’s position has changed
relative to the tree (reference point) so you
know motion must have occurred.
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After you choose your reference point, you can create what is called
a FRAME OF REFERENCE
This is basically just a GRAPH!!
X axis
Y axis
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1. DISTANCE• Describes how far something
has moved.
QUICK REVIEW: Distance is measured in what unit
when you use SI??
DESCRIBING MOTION
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DISTANCE V/S DISPLACEMENT
Distance is HOW FAR SOMETHING HAS TRAVELED from the reference
point (starting position)Whereas
2. Displacement is DISTANCE and the DIRECTION from the
reference point (starting position) something is.
DESCRIBING MOTION
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EXAMPLE
Total Distance Traveled =80 m
Displacement = 20 m North
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DESCRIBING MOTION
3. Speed = How far something travels (distance) in a given
amount of timeSpeed (in meters/second) = distance (in meters)
time (in seconds)
(You could also state speed in km/h or km/s or cm/yr)
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EX. OF A SPEED PROBLEM
A car traveling at a constant speed covers a distance of 750 m in 25 s.
What is the car’s speed?
Important info: Distance (d) = 750 m
Time (t) = 25 sSpeed = d/t
Speed = 750 m =
25 s
30 m/s
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Different Types of Speed
1.Constant Speed = when the speed hardly changes. Like when you use
cruise control on your car.
2.Changing Speed = when the speed changes over a given distance. Like
when you ride a bike or drive through downtown.
3.Average Speed = this describes speed of motion when you have changing speed. It is the total
distance traveled divided by the total time traveled.
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Different Types of Speed
4. Instantaneous Speed = The speed at a given point in time. Like going from 0 km/h to 60 km/h. Your speedometer shows you your instantaneous speed.
-Instantaneous Speed changes when you increase or decrease
your speed
-Instantaneous Speed stays the same when your speed is
constant
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How would these different types of speed look if your graphed them?
time
Distanc
e
time
Distanc
eCONSTANT SPEED
CHANGING SPEED
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GRAPHING MOTIONUse a DISTANCE/TIME GRAPH
-Time = X axis
-Distance = Y axis
A graph of constant speed will represent the motion with a straight
lineTo show two constant speeds which are
different, you use SLOPE = steepness of the line =
speed
Steeper the slope = faster the speed
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GRAPHING MOTION
A graph of changing speed will have a zig-zag or curved line
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Putting it all together
TIME
DIS
TA
NC
E
FASTEST CONSTANT SPEED
SLOWER CONSTANT SPEED
CHANGING SPEED
What is the slope of this part of the line?
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Putting it all together
10 20 300
200
600
1000
What is the average speed (slope) during the 10 min to 20 min period?
Change in distance = 1200-600 = 600
Amount of time = 20-10 = 10 min
Avg. Speed = change in distance (600 m)
Amount of time (10 min)= 60 m/min
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Putting it all together
10 20 300
200
600
1000
What is the instantaneous speed at t= 10 min? (it is the slope of the line at that point)
600m – 0m = 600 m
10min – 0min = 10min
600m/10min = 60m/min
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VELOCITY
Includes the SPEED of an object and the DIRECTION it is moving
-Velocity of an object can change even though the object’s speed doesn’t –
how??
-The velocity changes if the direction changes even though the
speed is constant
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How are Velocity and Speed different??
Velocity includes direction/speed doesn’t!
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Example Problem with Velocity
Describe the velocity of an object that travels north 6.9 m in 3 s, then turns and
travels south 2.8 m in 4 s.
Work this just like you did for finding average speed, but
now you include direction!
6.9 m / 3 s = 2.3m/s NORTH
Then
2.8 m / 4 s = 0.7m/s SOUTH
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ACCELERATION= Rate of change of velocity
-When velocity changes = object is accelerating
-So: Acceleration occurs when an object changes speed, changes direction, or changes both speed
and direction
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Acceleration due to a Change in Speed
-Increasing Speed = Positive Acceleration
-Decreasing Speed = Negative Acceleration (Deceleration)
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Graphing Positive and Negative Acceleration
Time
Speed
Positive Slope = Positive
Acceleration = like going downhill (speeding up)
Time
Speed
Negative Slope = Negative
Acceleration = like climbing a hill
(slowing down)
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Graphing Zero Acceleration
Time
Speed
Zero Slope = Zero Acceleration
This means No change in
Velocity = No change in Speed or direction
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Putting them all together
Time
Speed
+ A
0 A
- A
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Calculating Acceleration
Acceleration (A) m/s2 = Change in Velocity (m/s)
Time (s)Final Velocity = Vf
Initial Velocity = Vi
A (m/s2) = Vf-Vi (m/s)
Time (s)
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Calculating Acceleration
If there is no direction change and only speed changes, then a change in velocity = change in speed so you use:
A (m/s2) =Final speed – Initial Speed
Time
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Example Problems
1. A plane on a runway reaches 80 m/s in 20 s. What is the planes
acceleration?Important Info: Final Speed = 80m/s
Time = 20 s
A = Vf-Vi = 80m/s – 0m/s = 80 = 4m/s2
t 20s 20
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Example Problem
A skateboarder is going 3m/s and stops in 2 seconds.
Important Info: Final Speed = 0m/s
Initial Speed = 3m/s
Time = 2 s
A (m/s2) = Vf-Vi = 0m/s – 3m/s = -1.5 m/s2
t 2s
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Working Backwards
The acceleration of a car that comes to a stop in 5 seconds is -4m/s2. What was the car’s speed when it started accelerating
(decelerating)?
A = -4m/s2
t = 5sA = Vf-Vi
t
-4m/s2 = 0 – Vi
5s
X 5s 5s X
-20 m/s = -Vi
20 m/s = Vi
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Get your book: Work these problems!
pg. 51 # 6-7
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Force
= A push or pull
-Can cause the motion of an object to change (so it can change velocity)
EX: kicking a soccer ball, playing pool, hitting a tennis ball
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Does the Atmosphere have force? Do you feel it?
Does gravity have force? Do you feel it?
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Net Force
-When two or more forces act on an object at the same time, the forces combine and we call this
NET FORCE
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Balanced Forces
-Forces on an object that are equal in size and opposite in direction
-The Forces cancel each other out
-Net Force = 0
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Unbalanced Forces-When two or more unequal forces act on
an object
Situation #1
-Forces are unequal and in opposite directions
-Object is moved in the direction of the larger force
-Net force = difference between the two forces
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Unbalanced ForcesSituation #2
-Forces are applied in the same direction
=Forces are combined (added together)
-Net force = add two forces together
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Inertia= The tendency of an object to resist a
change in its motion
-The velocity (speed and direction) of an object remains constant unless a force
changes it
This said, if an object is at rest, it should
stay at rest unless a force causes it to
move. Time to test this out.
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Inertia and Mass
-The greater an object’s mass = The greater its inertia
(the more it resists moving)
DUH! It takes more force to stop a bowling ball by swatting it with a
paddle than it does to stop a tennis ball!
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Newton’s First Law of Motion
-An object moving at a constant velocity(constant speed and direction)
keeps moving at that velocity unless an unbalanced net force acts on it.
-AKA: The law of Inertia
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Two students push on a box in the same direction, and one pushed in the opposite direction. What is the net force on the box
if each pushes with a force of 50 N.
50N
50N
50N
Add your combined forces (50 + 50 = 100)
Subtract the opposite force from your combined forces
Net Force = 100 N – 50 N = 50 N in the direction the two students are pushing
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The downward force of gravity and the upward force of air resistance on a ball
are both 5N. What is the ball’s acceleration? (Remember acceleration = change in velocity divided by time)
5 N
5 NNet force = Difference between the two = 5 N – 5 N = 0
So is there any change in velocity (speed or direction of the object)?
NOPE. Acceleration = 0 m/s2