what is speed? • how can you use a distance-time graph to ... · pdf file• what are...
TRANSCRIPT
• What is speed?
• How can you use a distance-time graph to calculate average speed?
• What are ways velocity can change?
Speed and Velocity
• speed
• constant speed
• instantaneous speed
• average speed
• velocity
Speed and Velocity
Speed is a measure of the distance an object travels in a unit of time.
What is speed?
What is speed?
• Calculate speed by dividing the distance traveled by the time it takes to go that distance.
• speed = distance/time (speed = d/t)
What is speed? (cont.)
•The SI unit for speed is meters/second; kilometers/hour; miles/hour
What is speed? (cont.)
Constant speed: the rate of change of position in which the same distance is traveled each second.
What is speed? (cont.)
Page 19
• What distance has the car traveled at 2 s?
• How long will it take the car to travel 132 m?
When the car’s speed changes, it moves a different distance each period of time.
What is speed? (cont.)
• Instantaneous speed: speed at a specific instant in time.
Questions
• What is instantaneous speed?
• How would the distance the car travels each second change if it were slowing down?
What is speed? (cont.)
• Average speed: the total distance traveled divided by the total time taken to travel that distance.
What is speed? (cont.)
Questions
• What is meant by total time in the definition of average speed?
• How is a car’s average speed different from its instantaneous speed?
• When might it be useful for a driver to know the average speed of his or her car?
• When might it be useful to know the instantaneous speed?
• distance-time graphs: graphs that compare distance and time
• Constant speed is shown as a straight line on a distance-time graph.
Distance-Time Graphs
• How would the d-t graph be different if the horse’s speed changed over time?
Questions
• What are distance-time graphs?
• How is constant speed shown on a d-t graph?
• Do you think distance & time could also be represented on a circle graph? Why or why not?
• You can use d-t graphs to compare the motion of two different objects.
• The steeper line indicates a faster speed.
Distance-Time Graphs (cont.)
• How far behind was horse B when horse A reached the finish line?
• If horse B’s speed remains constant, at what time will it cross the finish line?
Distance-Time Graphs (cont.)
• You can use distance-time graphs to calculate the average speed of an object.
• The difference in distance between two points is divided by the difference in timebetween the same points.
Distance-Time Graphs (cont.)
diff in distance/diff in time = avg speed
Distance-Time Graphs (cont.)
• What is the first step in calculating average speed using a d-t graph?
• How does avg speed of the horse from 60 s to 120s compare to its avg speed from 120 s to 180 s?
Distance-Time Graphs (cont.)
How can you use a distance-time graph to calculate average speed?
Distance-Time Graphs (cont.)
If the speed of an object changes instead of being constant, its motion on a distance-time graph is a curved line.
Changing Speed
• Slowing: the slope of a line on the d-t graph decreases; meaning the speed of the obj is decreasing
• Stopping: a horizontal line on the d-t graph indicates no motion
• Speeding up: upward curve of a line on the d-t graph
Distance-Time Graphs (cont.)
• Even when the speed of an object isn’t constant, its avg speed can be calculated from a d-t graph.
• How is this graph the same as the graph in Fig 9?
• How is this graph different from the graph in Fig 9?
• How far has the train traveled when it reaches the ending point? How long does it take the train to make this trip?
•What is the avg speed of the train for the trip show in Fig 10?•What might account for the lack of motion that occurs between 3 min and 5 min?
Velocity is the speed and the direction of a moving object.
• The velocity of an object can be represented by an arrow.
• A greater speed is shown by a longer arrow.
• The arrow points in the direction of the object’s movement
Velocity
Velocity (cont.)
Velocity changes when the speed of an object changes, when the direction that the object moves changes, or when both the speed and the direction change.
• Which symbol is used to represent velocity?
Figure 11 (page 23)
• Why are the two arrows show in Fig 11 the same length?
• If, instead, the girl walks in the same direction as the boy at a speed of s m/s, how would the arrows be drawn? (Fig 11)
Figure 12 (page 23)• Are there two positions of the bouncing ball in
which the velocity is the same? Explain.
• When does the ball have the greatest speed?
• Speed is a measure of the distance an object travels in a unit of time. You can describe an object’s constant speed, instantaneous speed, or average speed.
• A distance-time graph shows the speed of an object.
A. speedB. constant speedC. instantaneous speedD. average speed
The total distance traveled divided by the total time taken to travel that distance is known as which of these?
A. units of speedB. constant speedC. units of timeD. average speed
Units of speed are units of distance divided by which of these?
A. a wavy lineB. a crooked lineC. a straight line D. a curved line
How is constant speed represented on a distance-time graph?
3. Constant speed is the same thing as average speed.
4. Velocity is another name for speed.
Do you agree or disagree?