Motion
PH 338
Practice using a stopwatch
Reset
Start &
Stop
Time 1 second.
Stop & Reset
Time 10 seconds.
Stop & Reset
What is the shortest
amount of time you
can measure?
1. What is motion?
• A way of describing how fast something is
moving
• Our definition: Motion is a change in
position.
2. What is a reference point?
• A point motion is measured against
• Usually a stationary object
3. How do you know
if an object is in
motion?
• An object is in motion if it
changes position relative
to a reference point
4. What is the difference between
distance and displacement?
• Distance is the total
length of the actual path
between two points.
Displacement is the
length and direction of a
straight line between
starting and ending
points.
• Displacement and
distance can be the
same if the direction is in
one direction only.
What is my
displacement?
5. What is speed?
• The distance an object travels per unit of
time.
miles per hour
miles/hour
6. How do you calculate speed?
• Divide the distance the object traveled by the
amount of time it takes to travel that distance.
…some useful speeds
•
•
•
•
•
Growing fingernails = 4 cm/year
Walking speed = 3 miles/hr or 1.3 m/s
Driving speed = 60 miles/hr or 26 m/s
Earth’s revolution = 30 km/s
Speed of light = 300,000 km/s
Road Trip!
Example: The white car travels120 miles in 2
hours. What is the car’s speed?
S=D
S = 120 miles S = 60 miles/hour
T
2 hours
7. Why would you calculate
average speed?
• Because most moving objects do not
move at a constant speed.
• Think about biking uphill and downhill
…or think of traffic
8. How do you calculate average
speed?
• Average speed = Total Distance Traveled
Total Time of Travel
Traffic!
• Example: 32 km for 2 hours, then 13 km
for 1 hour
Total distance = 32 km + 13 km = 45 km
Total time = 2 h + 1 hr = 3 hr
Average speed = 45 km ÷ 3 hr
= 15 km/hr
9. What is instantaneous speed?
• The rate at which an object is moving at a
given instant in time.
• …or what your speedometer says!
10. What is velocity?
• Speed in a given direction
• Ex: 20 miles/hour West
A storm is
moving 25 km/h
north. Should
you prepare?
11. How can you describe
changes in velocity?
• Change in velocity may be due to change
in speed, change in direction, or both!
Acceleration
• Acceleration is when an object speeds up,
slows down or changes direction.
• Acceleration is a change in velocity.
12. How do you graph speed?
Distance (m)
• By a distance versus time graph!
• X-axis = time
Slope = slant of line
• Y-axis = distance
y
50
5
Time (min)
X
13. How can you interpret graphs
of distance versus time?
• The slope represents speed
• Slope = slant of line = speed!
• The steeper the slope the faster the speed!
Steep = Fast!
Shallow = Slow!
• What if it is flat?
Flat = Not moving!
Speed-Time Graphs
Speed-Time Graphs look much like
Distance-Time graphs.
Be sure to read the labels!
• Time is plotted on the X-axis.
• Speed or velocity is plotted on the Y-axis
A straight horizontal line on a speed-time graph
means the speed is constant. This means the
speed is not changing over time.
How is this different from a distance-time graph?
A positive slope represents an
increasing speed. The object is
accelerating.
s
p
e
e
d
Time
A negative slope represents a decreasing speed.
The object is decelerating.
S
p
e
e
d
Time
Match the part of the graph to
the description
Steady acceleration ________
Steady speed
_________
Gradual acceleration _________
Steady deceleration _________
Summary
• The steeper the line, the greater the
acceleration.
• A horizontal (sunrise) line means the
object is moving at a constant speed
• A downward (negative) sloping line means
the object is slowing down (decelerating)