Chapter 4 - Motion
Frames of Reference
• Is this train in motion?
• How do you know?
Frames of Reference
• Frames of reference help us to describe the
motion of something
Frames of Reference
• RELATIVE MOTION is movement in relation to
a frame of reference.
Frames of Reference
Frame of
Reference
You – on a
moving bus
You – on a
moving bus
Object
Your friend – in a
nearby seat
Your friend –
standing at the
bus stop
You – standing at Your friend – on
the bus stop
the moving bus
Relative Motion
Measuring Distance
• Distance is the length between 2 points
• When measuring the length, be sure to
include units (cm, km, m)
Measuring Displacement
• Displacement is direction + length between
starting and ending points
Displacement Along a Straight Line
• A VECTOR is a quantity the describes size,
length or amount, and direction
– Same direction = add them
– Opposite directions = subtract the difference
– Example: Figure 4.2 on page 78
Displacement That Isn’t Along a Straight
Path
• Use graphing to help determine displacement
• Displacement is always shorter than or equal
to the distance
QUICK LAB!!!
• Using graph paper and rulers, draw a path on
your paper with at least 3 turns. Measure the
total distance of the path and displacement.
Review!
• What’s a frame of reference?
• What’s is distance?
• How is displacement different than distance?
4.1 Vocabulary
•
•
•
•
Speed
Average speed
Instantaneous speed
velocity
Speed
• SPEED is the ratio between the distance an
object travels to the amount of time is takes
• Example: 50 miles/hour
Average Speed vs. Instantaneous
Speed
• Average speed = total distance/total time
– Formula: s = d/t
• Instantaneous Speed = how fast something is
moving at any given movement
– Example: a speedometer in a car measures
instantaneous speed
Graphing Motion
• The slope of a line on a distance-time graph is
speed
• The steeper the slope, the faster the speed!
• Check out page 83-84 for comparing graphs!
Velocity
• Speed in a given direction!
• Velocities in the same direction are added
• Velocities in opposite directions are
subtracted
4.3 Vocabulary
•
•
•
•
•
Acceleration
Free fall
Constant acceleration
Linear graph
Nonlinear graph
What is acceleration??
• Acceleration = rate at which velocity changes
• Can be described as a change in speed,
change in direction, or change in both
• The rate of acceleration, due to gravity is
9.8m/s/s
Changes in Speed
• FREE FALL – movement of an object toward
Earth solely because of gravity
Changes In Direction
• You can accelerate even if your speed is
constant!
• Example: the speed may be constant on a
carousel, but the direction is always changing
Changes in Speed and Direction
• Sometimes speed and direction can change at
the same time
• Example: riding a roller coaster!
Constant Acceleration
• A steady change in velocity
• The velocity of the object changes by the
same amount each second
Calculating Acceleration
• Acceleration = change in velocity/total time
– Formula = (vf – vi)
t
• Complete the Math Practice problems on page
346
Graphs of Accelerated Motion
• The slope of a speed-time graph is
acceleration
• Check out pages 86 & 89 for graphing
examples!