Motion is change of position. A snail might change its position from one side of your garden to the other. It could take all day. You have to look closely to see its motion.
An airplane passes overhead. Its motion is much faster than the snail’s. In a few hours, the plane changes its position from New York to Los Angeles. We can see that an important thing to know about motion is the speed of motion.
To find the speed of something that is moving we need to know how far it traveled and how long it took to get there. If a snail is in a hurry, it can travel “three feet per minute”. In other words, we divided 3 feet by 1 minute to find the speed.
That is how to find speed – divide the distance traveled by the time it took to do it. We can use whatever is easiest to measure distance and time. Miles per hour works well for planes and cars. Feet per minute is better for snails. Inches, feet, miles, centimeters, meters, and kilometers are all ways of measuring distance. Seconds, minutes, and hours are used to measure time.
Airplanes usually travel 3,000 miles from New York to Los Angeles in about 6 hours. So the speed of a plane is 3,000 miles divided by 6 hours or 500 miles per hour. That is about 10,000 times faster than a snail.
So far, we have been talking about steady motion in a straight line. There are other kinds of motion. A moving thing can be slowing down or it can be speeding up. A car slows down as it comes to a stop sign and then speeds up as it leaves the stop sign. Motion can change direction too, as when a car goes around a corner.
Speeding up is called acceleration. Slowing down is called deceleration. Changing direction is also a kind of acceleration. Spinning is one way of changing direction. Spinning is called rotation. Things that move back and forth are said to be vibrating. Guitar strings make sound by vibrating very fast.
We can also use numbers to tell how much something accelerates. It is easiest if something has a regular, steady acceleration. By steady, we mean that every second, the speed of the accelerating object increases by the same amount. When things fall they accelerate in a regular, even way. Every second a falling rock increases its speed by about 22 miles per hour. A speed of 22 miles per hour is the same as 32 feet per second. So we say that things accelerate at 32 feet per second per second.
It took Jesse 15 minutes to ride her bike to school. What else would you need to know to find Jesse’s speed?