Main->Readings->4th Grade Readings->Matter in Motion- >Part 1: Straight Line Motion
Vocabulary |
In the last unit, we talked about how matter acts when it stays in one place. Now we are going to talk about how matter acts when it moves.
Scientists who study the ways matter and energy act are called physicists (FIZZ-uh-sissts), One of the things physicists study is motion.
The first people to wonder about matter in motion were the Greeks. They thought matter naturally wanted to be stopped. They thought if you made an object move, it would not keep going. It would slow down and stop, because its nature was to be "at rest."
They were not looking close enough at what really happened. A scientist called Galileo was the first to realize their mistake.
Galileo noticed what happened when you rolled a ball down a track which went down and then up again. He realized that if the nature of matter was to be stopped, it would just stop when it got to the bottom part of the track. Instead, it kept going up again, even against the force of gravity. He realized that a rolling ball, once it was moving, tended to keep moving.
It is true that things do tend to slow down, though. Why? Galileo thought it was friction which kept a ball from rolling forever. He was right. When two pieces of matter are touching one another, and moving against each other, friction is a force that works against the movement. Friction does not have to be caused by solid matter. It can be caused by a gas, like air, or a liquid, like water.
If there were no friction, matter in motion would keep moving forever. Matter does what it is already doing. If it is stopped, it stays stopped. If it is moving, it keeps moving in the direction it is already going. This is called Newton's First Law, even though Galileo thought of it first, because Isaac Newton put it together with some other laws. Here is how Newton put it:
An object in motion tends to stay in motion in a straight line unless acted on by an unbalanced force. An object at rest tends to stay at rest.
Galileo's and Newton's ideas began the science of studying motion. With this science, people can do things like design cars, send a rocket to the Moon, or make a better skateboard. In the next sections, you will learn some words scientists use to talk about motion.
You have to use a force to make things start moving. In the news, when people talk about "using force," they mean violence. To a scientist, however, blowing on something is using a force, and so is patting somebody on the back, and so is friction. A force is any push or a pull on an object caused by another object.
A force can:
- make an object start moving
- make an object stop moving
- make an object go faster
- make an object go slower
- make an object change directions
Speed
Speed is the amount of length an object travels in a particular amount of time. We always give units with numbers when we talk about speed. If you said a lacrosse ball was going "10," that would not mean anything. It could be 10 meters per second (10 m/s) or 10 kilometers per hour (10 km/hr).
It could be 1 centimeter per hour for a tiny ant. It could even be 300,000 km/s, which is the speed of light. That is the fastest speed in the universe! As long as something is moving, it has speed. Speed does not have to be fast.
It is not enough to say an object is moving, though. That is not much information. An object moving in a straight line could be doing all kinds of things. It could be moving quickly. It could be moving slowly. It could be headed backwards.
Velocity
"Duck!" somebody yells. "That bowling ball is going 20 meters per second!" So you duck.
But you didn't need to. The bowling ball wasn't going in your direction. It was headed down the lane toward the pins at the bowling alley.
The direction of motion is important. Therefore, when physicists talk about motion, they tell its direction. They have a special name for speed-and-direction, which is velocity. They also have a way of showing velocity: vectors. A vector is an arrow which shows direction and speed at the same time. A vector which is twice as long as another one shows twice the speed.
In the pictures below, there are several vectors. Which one do you think shows the greatest speed? Which one shows the least speed? Which one shows the greatest speed in the leftwards direction?
Acceleration
There is a pedal in a car called an accelerator. When you push down on it with your foot, it feeds fuel to the engine. The engine applies force to the wheels, and keeps applying the force as long as you have your foot pushed down. What happens if you keep your foot pushed down? The car keeps getting faster, and faster, and faster. Its velocity changes. It accelerates. Acceleration happens when the velocity of an object changes.
If you push something and then let go, it will keep going at a steady velocity in a straight line. That is Newton's Law. If instead, you keep applying force, the object will accelerate. It will go faster and faster.
Wait a minute. If you are on a bicycle, don't you have to keep pedaling just to stay at the same speed?
Yes, but don't forget friction. Friction is working against the bicycle. If there were no friction, and you were on a flat surface, your bicycle would keep rolling forever with just one kick of the pedals. But there is friction, and it is working against you.
In this section, you have learned that:
In the next section, you will learn what happens when there are two objects moving.
Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:
Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.
Return to Table of Contents
Go on to Part 2: Collisions
This page last modified August 15, 2002
Copyright ©2000 Delia Marshall Turner. All rights reserved.
Questions? Send me a note at dturner@haverford.org
