motion physical science chapter 11. 2 motion can be described as a change in position. physical...
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Motion
Physical ScienceChapter 11
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Physical Science chapter 11 2
Motion
Can be described as a change in position.
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Physical Science chapter 11 3
Distance vs. displacement
Distance is how far you traveled What your odometer reads
Displacement is the distance between your starting point and your ending point, regardless of your path As the crow flies
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Physical Science chapter 11 4
Speed
How fast something moves The rate of change in position The rate of motion
Rate means change over time
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Physical Science chapter 11 5
Velocity vs. speed
Velocity describes both speed and direction.
Both “how fast” and “which way” something is moving
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Physical Science chapter 11 6
Constant speed
Speed that does not vary Cruise control
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Physical Science chapter 11 7
Average speed
Total distance traveled divided by total time of travel
Used when the object in motions makes changes in speed, including starts and stops.
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Physical Science chapter 11 8
Calculating speed
If you know distance and time, you can calculate average speed.
distancespeed
time
t
dv
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Physical Science chapter 11 9
Example
You skate a distance of 100 m in 67 s. What is your average speed?
Known: d = 100 m t = 67 s
Unknown v = ?
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Physical Science chapter 11 10
Example continued
t
dv
s 67
m 100v
s
m 5.1v
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Physical Science chapter 11 11
You try
Find the speed in meters per second of a baseball thrown 38 m from third base to first base in 1.7 s.
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Physical Science chapter 11 12
Example
Find the velocity in meters per second of a swimmer who swims 110 m toward the shore in 72 s.
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Physical Science chapter 11 13
You try
Calculate the displacement in meters that a cyclist would travel in 5.00 h at an average velocity of 12.0 km/h to the southwest. Remember to include direction.
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Physical Science chapter 11 14
Instantaneous speed
The rate of motion at a given instant Speedometer
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Physical Science chapter 11 15
Graphing speed
When distance is graphed on the y-axis and time is graphed on the x-axis, we call this a distance-time graph.
The slope of a distance-time graph is the speed.
See pages 370–371 Discuss the practice problems
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Physical Science chapter 11 16
Acceleration The rate of change of velocity. Acceleration occurs if either the
speed or direction changes, or if both change.
“Slowing down” is a change in speed, so when something slows down, it is accelerating.
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Physical Science chapter 11 17
Acceleration
The size of acceleration depends on both the change in velocity and the time interval over which the change takes place
If the change in velocity is large, the acceleration will be large.
If the time interval is short, then acceleration will be large.
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Physical Science chapter 11 18
Calculating acceleration
The triangle is the Greek letter delta, and stands for “change in”
final initialv va
tv
at
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Physical Science chapter 11 19
Units on acceleration
Written as m/s2
second
ondmeters/sec
second
1
second
meters
2second
meters
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Physical Science chapter 11 20
Examples
1. Natalie accelerates her skateboard along a straight path from 0.0 m/s to 4.0 m/s in 2.5 s. Find her average acceleration.
2. Find the average acceleration of a northbound subway train that slows down from 12. m/s to 9.6 m/s in 0.8 s.
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Physical Science chapter 11 21
Acceleration on graphs
On a speed vs. time graph (speed on the y-axis and time on the x-axis) Acceleration is the slope
On a distance vs. time graph (distance on the y-axis and time on the x-axis) A curved line indicates acceleration A straight line indicates a constant
velocity (no acceleration)
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Physical Science chapter 11 22
Discuss
1. Identify the straight-line accelerations below as either speeding up or slowing downa. 5.7 m/s2b. -29.8 m/s2c. -2.43 m/s2d. 9.8 m/s2
2. A turtle swimming in a straight line twoard shore ahs a speed of 0.50 m/s. After 4.0 s, its speed is 0.80 m/s. What is the turtle’s average acceleration?
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Physical Science chapter 11 23
Force
A push or a pull one object exerts on another
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Physical Science chapter 11 24
Fundamental forces
Gravity Electromagnetic force
Like charges repel; opposite charges attract
Strong force Holds nucleus together
Weak force Involved with radioactivity
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Physical Science chapter 11 25
Contact forces vs. field forces
Contact forces Objects must be touching
Field forces Don’t need to be touching Gravity, magnets, electric charges
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Physical Science chapter 11 26
Balanced forces
Forces that are equal in size and opposite in direction are called balanced forces.
Tug-of-war tie
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Physical Science chapter 11 27
Net Force
When forces are unbalanced, there is a net force.
A net force always causes a change in velocity.
Could be a change is speed, a change in direction, or both.
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Physical Science chapter 11 28
Friction
The force that opposes motion between two surfaces that are touching each other.
Enables us to walk, hold things, etc.
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Physical Science chapter 11 29
Friction
The amount of friction depends on two things The kinds of surfaces The force pressing the surfaces together
More force more friction
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Physical Science chapter 11 30
Static Friction
Between surfaces that aren’t moving Largest frictional force between any
two objects
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Physical Science chapter 11 31
Kinetic Friction
Friction between moving surfaces Sliding friction Rolling friction
Usually less than sliding friction
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Physical Science chapter 11 32
Discuss
What is a force? What are balanced forces? What is friction? Does a net force of zero mean that
there is no force at all?