Science Voyages, Level Blue,
Glencoe/McGraw-Hill, 2000
Science Interactions, Course 1,
Glencoe, 1993
Motion, Forces, and Energy,
Prentice Hall, 1993
If instructed to do so, please complete the worksheet for this presentation.
Click to move to the next page or to go to the previous page. Click underlined words for links to explanations.

• position
• reference point
• motion
• distance
• SI
• meter
• unit
• time
• speed
• speedometer
• odometer
• displacement
• velocity
• relative
• frame of reference
• average
• constant
• instantaneous

Speed =
Distance
Time
Velocity =
Displacement
Time

You and your friend agree to meet for a movie tonight. You decide to get together at the corner of Lee and
3 rd Streets in 30 minutes .
Where you will meet is the position as described by a reference point . A reference point is an object that does not move.
Click the object for a history of clocks
The time
you will meet is measured with a clock.
Time is defined as the interval between two events
(when you make the decision to meet and when you meet). Over the years, time has been measured with sundials, hour glasses, watches, and the most accurate clock, the atomic clock.

As you walk to meet your friend, you change your position with every step you take. Distance is how far you travel along a path while you change your position.
Distance is measured in units. This is a system of measurement that scientists all over the world use so they can communicate information clearly. SI stands for the International
System of Units called the metric system, a system based on the number ten. The basic unit of length is the meter (m).
Do you want to know more about units of length and the metric system?
Visit these links
Metric System SI Metric Numbers How Many?

While you were walking to meet your friend, you were in motion . Changing position creates motion.
You have gone a distance, and time has passed while you were changing your position.

To be able to meet your friend on schedule, you had to go a distance in a certain time.
Speed
is the distance traveled by an object during a given time interval (distance divided by time) .
You stop for a few minutes to watch a ball game at a local park, hurry to cross the street, and rush to get there on time. You do not maintain a constant speed. Even if your speed isn’t constant, you can find your average speed by using this equation---
Speed =
Distance
Time
=

A cheetah can maintain a constant speed for short periods of time. Its speed is the same at all times during its motion. In 4 minutes, the cheetah traveled 8 km.
speed =
8 km
4 min
= 2 km/min
0 1 2 3 4
Time
0 1 2 3 4
Time
Most objects do not move at constant speeds for very long. Dividing the total distance by the total time does not tell you the speed for every point of the journey. Instead it gives you the average speed.
The rabbit had a burst of speed during the second minute. What is going on between the 3 rd and 4 th minutes?
The rabbit is not moving.
Answer
Time passes but no distance is covered.
average speed = total distance time interval

v = 2 km/min t = 4 min d = 8 km
Suppose you were given the speed and the time .
Could you determine the distance ?
Yes. d = v t
This time you know the distance and the speed . Can you find out how long it took for the cheetah to go the distance?
Yes. t = d / v

Instantaneous speed is the rate of motion at any given instant. What does that mean?
The picture at the right gives you the speed of the runner at the instant the picture was taken.
The speedometer in your car gives you a reading of instantaneous speed.
8.5 meters/second
While the speedometer is showing the speed at any given moment, the odometer shows you how far you have gone. An odometer measures distance.
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6
6 6
7

Let’s work a problem or three-
A car travels 240 km in 3 hours. What is the speed of the car during that time?
80 km/hr
Check for your answer 
Speed = Distance/Time
The speed of a ship is 50 km/hr. How far will it travel in 10 hours?
(Something different here!)
Check for your answer 
500 km
Distance = speed x time
A friend lives 4 km away. Walking at 2 km/hr, how long will it take you to go this distance?
(Be careful---it’s another version of the formula!)
2 hrs
Check for your answer
 time = distance/speed

Have you ever ridden a roller coaster? You twist and turn for three or four minutes, but you get on and off at the same place. Even with all the turns, you really haven’t gone anywhere at all.
Displacement is the change in position of an object.
If you end up where you started, your displacement is zero.
Displacement is described by both a length and a direction.
Distance is described only by length.
To say that you live 6 km from the amusement park isn’t very helpful unless you state in which direction (6 km south).

When you know the speed and direction of an object’s motion, you know the velocity of the object.
The wind is out of the southwest at 10 km/hr.
For example, the meteorologist
(one who studies the weather) gives the velocity of the wind.
Velocity is the displacement of an object divided by time.

Velocity =
Displacement
Time
In the formula, v stands for average velocity and d for displacement. Time is t .
v = t d

Does it sound like speed and velocity are the same thing?
How about distance and displacement?
You live 10 km away from school. There is a heavy snow. It took you 5 hours to walk to school. So, what is the distance you traveled?
Well, if you walked in a straight line , through houses, trees, and fences, it would be 10 km . But if one connected all your footprints and measured the distance, it was actually 15 km.
The distance is 15 km.
Your displacement is 10 km in the direction of the school.
Your speed was 3 km/hr but your velocity was 2 km/hr in the direction of the school.
15 km/ 5 hr = 3 km/hr (speed) 10 km/ 5 hr = 2 km/hr in the direction of the school (velocity)
If you walk back home, your displacement is zero, and your velocity is 0 km/hr. Well, just think of it, all the years you walked to school and back home your displacement and velocity were both zero!

An airplane is flying from Bridgeport, WV, to
Washington, DC., a distance of 330 km. The trip takes 1.5 hours. What is the airplane’s velocity?
The total displacement divided by total time is a velocity of 220 km/hr east.
A girl walks 2 km west, she turns and walks 3 km east.
What distance does she walk? What is her displacement?
The distance she walked 5 km .
Her displacement is 1 km east .

Would the paper airplane’s distance traveled be the same as its velocity?

Suppose you are swimming downstream at 10 km/hr.
You are actually going faster. How? The water is moving at 10 km/hr. Since you are swimming downstream, both you and the river are going in the same direction. The velocities combine (add). You are moving at 20 km/hr. What would happen if you were swimming upstream against the flow of water?
Velocities must be in the same direction to add them. You must subtract when combining velocities in opposite directions. You would be swimming at 0 km/hr or simply going nowhere!
10 km/hr downstream – 10 km/hr upstream = 0 km/hr

When you describe something that is moving, you are comparing it with something that stationary or not moving. This stationary object is your frame of reference .
Think about an astronaut on a space walk. If you are the astronaut, the shuttle is your frame of reference (your relative velocity to the shuttle is zero). But, if you are on the ground, you would describe both the astronaut and the shuttle having a velocity of 40,000 km/hr. The
Earth is your frame of reference .

Speed =
Distance
Time
For example, if a car travels 50 kilometers in one hour, the car is traveling at a speed of 50 km/h.
An ant moving two centimeters in one second moves at a speed of two centimeters per second or 2 cm/s . The ant has a much slower speed.

Time is the interval (gap or break) between two events. It is a quantity that is measured with a clock. A clock is designed to mark off equal time intervals. The difference between two clock readings tells you how long something takes and is called a time interval.

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