[Figure 1]

The wings of this hummingbird are moving so fast that they're just a blur of motion. You can probably think of many other examples of things in motion. If you can't, just look around you. It's likely that you'll see something moving, and if nothing else, your eyes will be moving. So you know from experience what motion is. No doubt it seems like a fairly simple concept. However, when you read this article, you'll find out that it's not quite as simple as it seems.

Defining Motion

In science, motion is defined as a change in position. An object's position is its location. Besides the wings of the hummingbird in the opening image, you can see other examples of motion in the Figure below. In each case, the position of something is changing.

[Figure 2]

Q: In each picture in the Figure above, what is moving and how is its position changing?

A: The train and all its passengers are speeding straight down a track to the next station. The man and his bike are racing along a curving highway. The geese are flying over their wetland environment. The meteor is shooting through the atmosphere toward Earth, burning up as it goes.

Frame of Reference

There's more to motion than objects simply changing position. You'll see why when you consider the following example. Assume that the school bus pictured in the Figure below passes by you as you stand on the sidewalk. It's obvious to you that the bus is moving, but what about the children inside the bus? The bus isn't moving relative to them, and if they look at the other children sitting on the bus, they won't appear to be moving either. If the ride is really smooth, the children may only be able to tell that the bus is moving by looking out the window and seeing you and the trees whizzing by.

[Figure 3]

This example shows that how we perceive motion depends on our frame of reference. Frame of reference refers to something that is not moving with respect to an observer that can be used to detect motion. For the children on the bus, if they use other children riding the bus as their frame of reference, they do not appear to be moving. But if they use objects outside the bus as their frame of reference, they can tell they are moving. 

Q: What is your frame of reference if you are standing on the sidewalk and see the bus go by? How can you tell that the bus is moving?

A: Your frame of reference might be the trees and other stationary objects across the street. As the bus goes by, it momentarily blocks your view of these objects, and this helps you detect the bus's motion.

Direction

Direction is the location of something relative to something else. Direction can be described in relative terms that compare the position of something to another object, such as up, down, in, out, left, right, forward, backward, or sideways. Direction can also be described with the cardinal directions: north, south, east, or west. On maps, cardinal directions are indicated with a compass rose. You can see one in the bottom left corner of the map in the Figure below. You can use the compass rose to find directions on the map. For example, to go to the school from Jordan’s house, you would travel from east to west. If you wanted to go on to the post office, you would change direction at the school and then travel from south to north.

[Figure 2]

Why Direction Is Important

Look again at the map in the Figure above. The distance from Jordan’s house to the post office is 3 km. But if Jordan told a friend how to reach the post office from his house, he couldn’t just say “go 3 kilometers.” The friend might end up at the park instead of the post office. Jordan would have to include direction as well as distance. He could say, “go west for 2 kilometers and then go north for 1 kilometer."

Motion and Vectors

When both distance and direction are considered, motion can be represented by a vector. A vector is a measurement that has both size and direction. It may be represented by an arrow. If you are representing motion with an arrow, the length of the arrow represents distance, and the way the arrow points represents direction.

The red arrows on the map in the Figure above are vectors for Jordan’s route from his house to the school and from the school to the post office. 

Q: How would you draw arrows to represent the distances and directions from the post office to the park on the map in the Figure above?

A: The vectors would look like this:

[Figure 3]

Speed

How fast or slow something moves is its speed. Speed determines how far something travels in a time interval. SI units for speed are meters per second (m/s). Speed may be constant, but often it varies from moment to moment. 

Average Speed

Even if speed varies during the course of a trip, the average speed can be calculated using this formula:

For example, assume you go on a car trip with your family. The total distance you travel is 120 miles, and it takes 3 hours to travel that far. The average speed for the trip is:

Q: Terri rode her bike very slowly to the top of a big hill. Then she coasted back down the hill at a much faster speed. The distance from the bottom to the top of the hill is 3 kilometers. It took Terri ¼ hour to make the round trip. What was her average speed for the entire trip? (Hint: The round-trip distance is 6 km.)

A: Terri’s speed can be calculated as follows:

Instantaneous Speed

When you travel by car, you usually don’t move at a constant speed. Instead you go faster or slower depending on speed limits, traffic lights, the number of vehicles on the road, and other factors. For example, you might travel 65 miles per hour on a highway but only 20 miles per hour on a city street (see the pictures in the Figure below.) You might come to a complete stop at traffic lights, slow down as you turn corners, and speed up to pass other cars. Therefore, your speed at any given instant, or your instantaneous speed, may be very different than your speed at other times. Instantaneous speed is much more difficult to calculate than average speed. 

[Figure 2] Cars race by in a blur of motion on an open highway but crawl at a snail’s pace when they hit city traffic.

Calculating Distance or Time from Speed

If you know the average speed of a moving object, you can calculate the distance it will travel in a given period of time or the time it will take to travel a given distance. To calculate distance from speed and time, use this version of the average speed formula given above:

For example, if a car travels at an average speed of 60 km/h for 5 hours, then the distance it travels is:

To calculate time from speed and distance, use this version of the formula:

Q: If you walk 6 km at an average speed of 3 km/h, how much time does it take?

A: Use the formula for time as follows: