1. Sec. 11.2- Speed and Velocity

2. Looking out window- you will eventually observe things that are moving
Leaf in wind
Bird flying
Car driving
Grass/trees growing

3. How can we describe the differences between the types of motion we just discussed?

4. Speed
Speed: the ratio of the distance an object moves to the amount of time the object moves
When describing the speed of your car, you might say it is traveling at 45 kilometers per hour- what is the ratio?

5. SI unit for speed: Meters per second: m/s
Pick proper units for the motion you are describing:
Ex: For a moving worm: meters per second or kilometers per hour?
For a car: meters per second of kilometer per hour?

6. Two ways to express the speed of an object:
Average speed
Instantaneous speed
Average speed is calculated differently than instantaneous speed

7. Average speed- calculated for entire duration of a trip
Instantaneous speed- measured/calculated at a particular instant/ point in time

8. Average Speed
Describing speed of hiker isn’t as easy as describing speed along a straight line
Hikers change speed depending on terrain
Really rocky vs. really flat, downhill- how does the hiker’s speed differ?
For objects that change speed a lot, it is sometimes easier to express speed over the entire trip- so the average speed

9. Average speed is calculated by taking the total distance traveled divided by the time it takes to travel that distance
During the time an object is moving, it may change speed, but this equation tells you the average speed over the entire trip
Average speed equation:

10. Average Speed Calculation
While traveling on vaca, you measure and record the times and distances traveled. You travel 35 kilometers in 0.4 hours, and then you travel 53 kilometers in 0.6 hours. What was your average speed?

11. Instantaneous Speed
Sometimes it is more useful to know your instantaneous speed rather than the average speed
When driving a car, knowing your instantaneous speed is important- you need to make sure that you are going the speed limit!
Instantaneous speed: the rate at which an object is moving at a given moment in time
What does a car’s speedometer measure?

12. According to the speedometer pictured below, what is the vehicle’s instantaneous speed?
Average Speed?

13. Graphing Motion
Motion can be described and graphed using a distance vs. time graph
Horizontal x-axis: Time
Vertical y-axis: Distance
Slope=

14. The slope of a distance vs
The slope of a distance vs. time graph is the change in distance divided by the change in time
The slope of a distance vs. time graph is SPEED
If the slope is a straight line, what do you think is true about the motion of the particular object?

15. If the slope changes, what do you think is true about the motion of the object?
If the slope is zero, what is true about the motion of the object?
If the slope is steeper, what is true about the speed of the object?

16. Distance vs. Time Graph- Constant High Speed

17. Distance vs. Time Graph- Constant Low Speed

18. Distance vs. Time Graph- Varying Speed

19. Velocity
Knowing the speed of an object is not always enough information to answer a question
Ex: A cheetah runs at a speed of 90 km/hr, and there is an antelope standing still 30 m away from the cheetah
Can you tell how long it will take the cheetah to get to the antelope?

20. The answer is no- you don’t know which DIRECTION the cheetah is running in if you only know it’s speed
It is often important to know the speed at which an object moves, as well as the direction it moves (with that particular speed)

21. Velocity: the speed and direction in which an object is moving
Velocity describes both speed and direction of an object- what kind of quantity is velocity?

22. Vectors- represented by arrows
For velocity vectors: Length of arrow would be the relative speed, and arrow head would point in direction of movement
Longer arrow= higher speed
Shorter arrow= lower speed

23. Velocity is affected/changed by:
Change in direction
Change in speed
Both change in direction and speed

24. Sailboat: when moving in a straight line with a constant speed, how would you describe the velocity of the sailboat?
How could the velocity of the sail boat change?

25. Sometimes, the motion of an object involves more than one velocity
Combining Velocities
Sometimes, the motion of an object involves more than one velocity
Ex: Boat traveling with the river current, or against the river current
In order to figure out the velocity of the boat relative to the riverbank, what would you have to do?

26. Ex: Boat traveling with river current
A boat is traveling East (downriver) at 12 km/hr
The river flows at a velocity of 5 km/hr East
What is the velocity of the boat relative to the riverbank?

27. Ex: Boat traveling against river current
A boat is traveling West (upriver) at 12 km/hr
The river flows at a velocity of 5 km/hr East
What is the velocity of the boat relative to the riverbank?