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Motion
PH 338
Time 1 second. Stop & Reset
Time 10 seconds. Stop & Reset
What is the shortest amount of time you can measure?
Start & StopReset
Practice using a stopwatch
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
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
Road Trip!
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
• Example: 32 km for 2 hours, then 13 km for 1 hourTotal distance = 32 km + 13 km = 45 kmTotal time = 2 h + 1 hr = 3 hrAverage speed = 45 km ÷ 3 hr
= 15 km/hr
Traffic!
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 WestA 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?
• By a distance versus time graph!
• X-axis = time
• Y-axis = distance
Dis
tanc
e (m
)
Time (min)5
50
Slope = slant of line
X
y
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.
speed
Time
A negative slope represents a decreasing speed.The object is decelerating.
Speed
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)
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