Throwing things
Have you used a bow and arrow? If you have, you will remember that you put the bowstring into the slot at the back of the arrow then pull the bowstring back. As you pull the bow bends but also pulls in the opposite direction. You may have felt this pull make the bowstring dig into your fingers. When you had pulled the arrow back, you looked along the shaft and took aim at the target and let the bowstring go. The pulling force of the bow was no longer balanced by the pull of your arm so the bow straightened. As it did so, the string pushed the arrow into the air. With luck it hit the bullseye.

The push given to the arrow by the bow is easy to understand but why does the arrow keep moving when the bowstring is no longer pushing it? If you had lived in Ancient Greece and asked your teacher you would have had a reply like this: "When the arrow leaves the bow it is no longer pushed by the bowstring but the air takes up the task. The air pushes the arrow along through the air." If you asked why the arrow eventually fell to the ground, your teacher would have replied, "Because the air got tired of pushing." For many hundreds of years people believed that these answers provided the correct explanation for how things move through the air.

The explanation, as in all scientific explanations, was based on observations and the ancient Greeks observed that the natural state of any object was being still or at rest. They came to this conclusion by observing that most things around them were not moving and that they had to be given a push if they were to move. It then seemed reasonable to think that once you stopped pushing something into the air - like an arrow or a thrown stone - the air would carry on the task of pushing. They also reasoned that when we got tired of pushing a heavy object it stopped moving so when the air got tired of pushing an object it would stop moving too.

However following the ancient Greek explanation it would seem that the moment the air grew tired and stopped pushing, the arrow would fall vertically to the ground. If you remember the time when you used a bow and arrow you may have seen the arrow gradually sink as it approached the target and fall to the ground in front of it. Using the Greek idea of 'tired air' you may explain your observation as the air getting gradually more tired so pushing with less and less force until the arrow hits the ground. Although you may be quite pleased with your explanation based on the ancient Greek idea your explanation would be completely wrong because the ancient Greek idea was completely wrong in the first place.

It was not until scientists began studying the way objects move through space that it was discovered how an arrow or any object moves through the air. When you look at objects in space you find that they are moving. It is as natural for them to keep moving as it is for most of the objects on the Earth to stay still. From these observations a scientist called Isaac Newton concluded that everything remains at rest or moves at a steady speed unless it is acted on by one or more forces. Using this information the movement of the arrow can be explained in the following way. When the arrow is being held in the bow it is at rest, which is a natural state. When the bow straightens it makes the string push on the arrow and sets it moving through the air. The arrow is now moving in a straight line at constant speed. This is another natural state, which was unknown to the ancient Greeks. However the arrow does not keep moving in a straight line so some other forces must be acting on it. These two forces are gravity and air resistance.

Gravity acts to pull the arrow down towards the centre of the Earth and air resistance pushes against the forward moving arrow to slow it down. The two forces work to bring the arrow to the ground. If the arrow comes down tip first it will stick in the ground and come to a stop but if it grazes the ground with its side it will keep going. When this happens, it experiences another force to stop its natural movement of travelling at constant speed in a straight line. This force is friction. It is caused by the surfaces of the arrow and the ground rubbing together. Eventually the force of friction brings the arrow to a stop. Next time you send something flying through the air think about the forces that act on it the moment it leaves your hand and see how they bring the object to the ground.

Were the ancient Greeks stupid?
No, they were not. They made observations and used reason to explain them. At the time this was the way they conducted all their affairs. They did not make experiments to test their explanations, as they believed that their power of reason was sufficient to provide an understanding of almost everything. In later times, scientists began to do experiments to check their observations carefully. It was then that many of the ancient Greek explanations were found to be incorrect.

Why didn't the ancient Greeks realise that objects in space moved naturally?
Ancient Greeks had their own explanation for how the stars and planets moved across the night sky. They thought that the stars and planets were set in crystal spheres. These could not be seen but enclosed the Earth. All the objects in the sky moved because there was something that pushed on the crystal spheres and made them move. As they were satisfied with this explanation they did not make any further observations, so they did not realise that there was another reason why the objects move as they do.

When scientists experiment does it mean that the results provide the correct explanation for something?
The results or data simply show what happened when the experiment was made. The scientists then describe what the results show and make suggestions to explain them. The suggestions may or may not give a correct explanation so all experiments are repeated by other scientists and they make their descriptions and suggestions. Scientists then meet to discuss the experiments and decide which suggestion is probably the best explanation for the results.

Does this mean that scientists sometimes are not sure how to explain something?
Yes, it does. When this happens they make new experiments to test their ideas to see if they can find a better explanation. A huge number of things have been explained by making scientific investigations but there are still a huge number of things to find out. This is what makes science a very attractive career for many people.

What happens in the body when you throw something?
The parts of the body which generate forces are the muscles. When you prepare to throw something, muscles in your arm raise your hand and muscles around the back of your shoulder pull your arm back. When you make your throw, muscles at the front of the shoulder pull the arm quickly forwards; muscles in your arm open your fingers and let the object go. Your hand pushes the object into the air and it would travel forwards in a straight line if the forces of gravity and air resistance did not slow it down and make it fall.

Is it best to throw something horizontally to make it travel the furthest?
No. The pull of gravity would soon pull it down after a short distance. An object should be thrown into the air at an angle of 45°. This provides the best path for the upward force to oppose the force of gravity. If the angle is less, gravity will pull down the object sooner, so the object will not travel as far. If the angle is higher, gravity will still pull down the object sooner so the object also will not travel as far.