1. Introduction to Motion
Distance vs. Displacement,
Speed vs. Velocity,
Acceleration,
Free-fall,
Average vs. Instantaneous quantities,
Motion diagrams,
Motion graphs
Introduction to Motion

2. Distance vs. Displacement
Tells “how far” an object is from a given reference point or “how far” it has traveled in a given time.
Is a scalar quantity which means it has magnitude only
Is measured in length units such as meters (m), centimeters (cm), feet (ft), inches (in), miles, etc.
Displacement
Tells “how far” and “which way” an object moves.
Is distance in a given direction
Is a vector quantity because it includes magnitude and direction
The magnitude is measured in the same units as distance, direction is usually measured as an angle.
“d” is the distance from A to B and the arrow indicates the direction of the displacement (from A to B).
“d” is the distance from point A to point B or from B to A A B A B d d

3. Distance vs. Displacement
Distance : =16 m 5m 2m
Displacement: 3m 5m
start
45°
@ 45° south of east 3m 1m N
Displacement 1m W E 2m
end S

4. Speed vs. Velocity Ex: speed = 60 mph Ex: velocity =60 mph, N Speed
Is the rate of change of position of an object
Tells “how fast” an object is moving
Is a scalar quantity because it only includes magnitude (size).
Measured in units of distance over time such as mi/hr, km/hr, or m/s
Velocity
Is speed in a given direction
Tells “how fast” and “which way”
Is a vector quantity because it includes magnitude (speed) and direction.
Magnitude is measured in same units as speed.
Ex: speed = 60 mph
Ex: velocity =60 mph, N

5. Acceleration The rate of change of velocity How fast the speed changes
Measured in meters per second per second (m/s/s) or meters per second squared ( )
A vector quantity (requires both magnitude and direction)
An object may accelerate (or decelerate) in three ways:
Speed up
Slow down
Change direction
If acceleration is…
…positive then it is in
the same direction as
the velocity causing velocity
to increase.
…negative then it is in the opposite
direction of the velocity causing velocity
to decrease (deceleration)

6. Free-fall
An object under the influence of only gravity is said to be in “free-fall”
We assume no air resistance for any free-fall problems…thus, the “only gravity” stipulation.
All objects fall at a constant acceleration… (9.8 m/s2 downward near earth’s surface)…regardless of mass. (For quick calculations and estimates we can round to 10 m/s/s)
This means that a falling object will gain 9.8 m/s of speed every second that it falls.
Also…an object that is thrown up will slow down or lose 9.8 m/s every second of its upward motion.
The velocity of the object at the tip-top of the path is zero, the acceleration is “g” the acceleration due to gravity.

7. Average vs. Instantaneous velocity
Average velocity is the calculated by dividing total distance traveled by total time
Constant velocity has the same value as average velocity
Instantaneous
Instantaneous velocity is the velocity that an object has at a specific instant in time.
It is NOT the velocity over a time interval.
Initial velocity and final velocity are examples of instantaneous velocities.

8. Motion Diagrams
A series of pictures illustrating the motion of the object including displacement, velocity, and acceleration.
The rules:
4 images at equal time intervals…pay attention to spacing
Include and label velocity vectors with relative lengths to represent relative speed on each image
Include and label a single acceleration vector
Example: Car moving to the right at constant velocity v v v v
a=0
The term “constant velocity” means that the car will cover equal distances in equal time intervals…thus equal spacing. Also all velocity vectors are the same length indicating the same speed. If there is no change in speed, there is no (zero) acceleration.

9. Position-Time (d-t) Graphs
To determine how fast an object is moving, look at the steepness of the line on the d-t
graph. (steeper is faster)
To determine which way an object is moving, look at whether the slope is positive (/) or negative (\).
A positive slope indicates that it is moving forward or away from the zero position (or detector).
A negative slope indicates that it is moving backwards or toward the zero position (or detector).

10. Motion Graphs: Finding velocity on a position-time graph
faster
Distance
Time
Acceleration (increasing slope)
slower
Time
The slope of the line on a distance-
time graph tells us about the speed.
Steeper slope means faster speed
Straight line means constant speed
For non-constant motion, the instantaneous speed can be calculated by finding the slope of the tangent line at that point.
Deceleration (decreasing slope)
Distance
Time

11. Velocity-Time (v-t) Graphs
To determine how fast an object is moving, look at how far the graph is from the x axis (farther means faster).
To determine which way an object is moving, look at whether the graph is above or below the x axis.
If above the x-axis, it is moving away from the zero position or detector.
If below the x-axis, it is moving toward the zero position or detector.

12. Velocity – time graphs (constant velocity)
Moving away at a constant medium speed.
Moving away at a constant slow speed.
Moving toward at constant fast speed.

13. Motion Graphs: Finding acceleration on a velocity-time graph
The slope of the line on a velocity-
time graph tells us about the acceleration.
Steeper slope  higher acceleration
A slanted straight line means constant acceleration
A flat or horizontal line would indicate constant velocity or zero acceleration.

14. Motion Graphs: Finding displacement on a velocity-time graph.
8 m/s
Velocity
Area
Time
10 sec
The area under the curve of a velocity-time graph for a particular time interval gives the displacement of the object during that time interval.
The seconds cancel out and the units of the area are meters, therefore area is a displacement.

15. Motion Graphs: acceleration-time graphs
Positive constant acceleration
Time
Zero acceleration (constant velocity)
acceleration
Time
Negative constant acceleration
Time
Horizontal line means constant acceleration
We will not do any calculations with changing accelerations, we assume all accelerations to be constant.

16. Interpreting graphs (standing still or not moving)
acceleration
distance
velocity
time
time
time
*On a d-t graph, a horizontal or flat line indicates standing still or not moving.
*Zero velocity since position is not changing.
*Zero acceleration since velocity is not changing.

17. Interpreting graphs (constant motion moving away from zero)
time
acceleration
time
distance
time
velocity
*On a d-t graph, a slanted straight line indicates constant speed.
*A positive slope (/) indicates moving away from the zero position (or detector)
*A horizontal line indicates constant speed.
*A positive velocity indicates moving away from the zero position (or detector)
*Zero acceleration since velocity is not changing (constant velocity).

18. Interpreting graphs (constant motion moving toward zero)
acceleration
distance
velocity
time
time
time
*On a d-t graph, a slanted straight line indicates constant speed.
*A negative slope ( \ ) indicates moving toward the zero position (or detector)
*A horizontal line indicates constant speed.
*A negative velocity indicates moving toward the zero position (or detector)
*Zero acceleration since velocity is not changing (constant velocity).

19. Interpreting graphs (accelerated motion – speeding up)
acceleration
velocity
distance
time
time
time
Slope from this gives you this
Slope from this gives you this
The slope of the d-t graph starts almost flat and steadily gets steeper over time. This indicates that it is speeding up.
An increasing slope on the d-t graph indicates an increasing velocity over time.
Since the slope of the v-t graph is a positive constant value, the acceleration is positive and constant. Therefore it is a horizontal line above the axis on the acceleration graph.

20. Interpreting graphs (accelerated motion – slowing down)
acceleration
velocity
distance
time
time
time
Slope from this gives you this
Slope from this gives you this
The slope of the green line starts out steep and gets less steep (flatter) over time. This indicates that it is slowing down.
A decreasing slope on the d-t graph indicates decreasing velocity over time.
Since the slope on the v-t graph is negative and constant, the acceleration is a constant negative value. Therefore it is a horizontal line below the axis.

21. Kinematics (motion) Formulas
Average or Constant Motion
Accelerated Motion

22. Word clues to numbers for problem solving
“free-fall”  acceleration due to gravity
a=9.8m/s2, down
“at rest”  not moving
v=0
“dropped”  starts at rest and free-fall
vi=0 and a=9.8m/s2, down
“constant velocity”  no acceleration
a=0
“stops”  final velocity is zero
vf=0
These are the most common but be on the lookout for more.