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SPEEDBack seat of an F-18
Question #8
• Is motion relative? • Yes or No?
Are we ever really standing still?
Yes, All things are in motion.
• You may be perfectly still yet the earth is spinning on its axis and orbiting the sun.
• The sun orbits the center of the galaxy, our galaxy and Andromeda are moving toward one another, most of the other galaxies are moving away.
So to measure motion in a meaningful way, we measure it relative to the observer or another object we consider not to be moving.
When you pass a car on the highway moving in the same direction, you may be going 70mph, and the other car 65mph. But your speed relative to the other car is 5mph.
When the shuttle docks with the space station the two orbiting objects approach one another at a very slow speed relative to one another, but both are orbiting at 17,000mph relative to the surface of the earth.
•Motion is a change in position relative to your surroundings.
•An object moves when it speeds up, slows down or changes direction.
Main IDEAS
What’s a Frame of Reference?
A
B
C
BACKGROUND OR OBJECT USED FOR COMPARISON
SPEED rate at w/c an object moves
Distance (D)
Speed =
Time (T)
d
ts
Another way to express this is in a “formula triangle” like this:
SPEED IS….Inversely Proportional to TIMEAn ant crawled 12 inches in 10 seconds. A second ant crawled 24 inches in 10 seconds.
Compare the speed of the ants.
DIRECTLY PROPORTIONAL TO DISTANCE
A plane flew 220 meters in 1 second.
A second plane flew 220 meters in 2 seconds.
Compare their speeds.
DISTANCELength between two places
Distance = speed x time
D= ST
Expressed in km (1000m)
TIME
=
d s
T
STEPS IN PROBLEM SOLVING
STEP 1. Write the formula STEP 2. Substitute given numbers and units STEP 3. Solve for the unknown.
CONSTANT SPEED
SPEED THAT DOES NOT CHANGE
CONSTANT SPEED
SAMPLE PROBLEM:
At what speed did a plane fly if it traveled 1760 meters in 8 seconds?
SOLUTIONStep 1. Speed = Distance/Time
Step 2. Speed = 1760 meters/ 8sec
Step 3. Speed = 220 m/sec
SAMPLE PROBLEMS:1. A car travels 240 kilometers in 3 hours. What is the speed of the car during that time?
ANSWER: S= d/t S= 240 km/3 hr
80 Km/Hr
The speed of a cruise ship is 50 km/hr. How far will the ship travel in 14 hours?
Answer: D = ST 700 km
AVERAGE SPEED
TOTAL DISTANCE / TOTAL TIME
AVERAGE total distance traveled
SPEED time=
AVERAGE SPEED
A car travels 200 m in 10 seconds. What is the average speed of the car?
ANSWER: Ave speed= total d/total time
= 200m/10s
20 m/s
Definitions:
A change in position in a certain amount of time is __________.
__________________ is the rate at which an object moves and is determined by dividing ____________ by _____________.
_________________________ is speed that does not change.
_________________________ is calculated by dividing the total distance by the total time.
http://www.youtube.com/watch?v=crjU5hu2fag&feature=player_embedded
http://www.youtube.com/watch?v=arD374MFk4w&feature=player_embedded
SPEED WORKSHEET
Complete for homework
Ken goes for a walk
0123456789
10
20 40 60 80 100 120
Time (minutes)
Dis
tan
ce (
km)
1.Calculate the speed at each point on the y-axis
Time (min)
20 40 60 80 100 120
Distance (km)
Speed (km/min
)
2. Describe the motion shown in the graph:
3.Calculate the average speed for the graph – how does this value compare to your calculations above?
4. How far would Ken travel if he walked for another hour (60 minutes)?
Ken goes for a walk
0123456789
10
20 40 60 80 100 120
Time (minutes)
Dis
tan
ce (
km)
1.Calculate the speed at each point on the y-axis
Time (min)
20 40 60 80 100 120
Distance (km)
1.5 3 4.5 6 7.5 9
Speed (km/min)
0.075 0.075 0.075 0.075 0.075 0.075
2. Describe the motion shown in the graph:
3.Calculate the average speed for the graph – how does this value compare to your calculations above? 0.075 km/min
3. How far would Ken travel if he walked for another hour (60 minutes)? 13.5 km
Michelle Goes on a Bike Ride
010
2030
4050
6070
80
1 2 3 4 5 6
Time (hours)
Dis
tan
ce (
km)
5.Describe the motion shown in the graph
6.How many different rates are shown? Label them using A, B, . . . . .
Time (hours)
A B C D
Distance (km)
Speed (min/km)8. What is the average speed for the entire trip? How does the value compare to your calculations above?
9. Can you predict how far Michelle will travel if she travels for another 2 hours? Why or why not? What additional information do you need?
7. Calculate the speeds for the different segments of the journey (Hint - you will have to use change in distance!)
Michelle Goes on a Bike Ride
010
2030
4050
6070
80
1 2 3 4 5 6
Time (hours)
Dis
tan
ce (
km)
5.Describe the motion shown in the graph (moving at varying speed)
6.How many different rates are shown? Label them using A, B, . . . . .
Time (hours) A (1 h) B (1h) C (2 h) D (1h)
Distance (km) 10 0 40 10
Speed (km/h) 10 0 20 10
8. What is the average speed for the entire trip? How does the value compare to your calculations above? 11.7 or 12 km/h
9. Can you predict how far Michelle will travel if she travels for another 2 hours? Why or why not? What additional information do you need?
7. Calculate the speeds for the different segments of the journey (Hint - you will have to use change in distance!)
