Water resistance
Imagine that you are on the shore of a lake. As you look out across the lake you see an island. It has trees and huge rocks on it and looks very interesting. Resting on the shore next to you is a canoe. You put on your life jacket and drag the canoe along the shore on the first part of your journey.

The underside of the canoe scrapes along the ground. It seems that the ground is pushing in the opposite way to you. What is causing this push? If you looked at the underside of the canoe and at the ground with a powerful microscope, you would see that their surfaces are made from grooves and ridges. When the canoe was resting on the shore the ridges in its surface rested in the grooves in the ground. At the same time, the ridges in the ground projected up into the grooves on the surface of the canoe. At first when you pushed the canoe it did not move. This was due to the way the grooves and ridges in the two surfaces locked together. The pushing force against you, which stopped the canoe moving, was caused by a force made by the interlocked surfaces. This force is called holding or static friction. As you increased your push on the canoe the ridges rose out of the grooves and bounced along the opposite surface. They made another force called sliding friction. This is the force that made the ground seem to push against you.

When you pushed the canoe into the water, something different happened. The underside of the canoe was no longer in touch with the surface of the ground. The water of the lake had come between them. Suddenly the canoe was much easier to push. Why did this happen?

The canoe and the ground are made from solids. All solids are made of tiny particles, which can only be seen by the most powerful microscopes. In a solid, these particles lock together like bricks in a wall. They are the reason a solid keeps its shape. Liquids, like water, are also made of very tiny particles. The particles in a liquid are not held as strongly together as the particles in a solid. This means that they can slide over each other. The way the particles slide allows the liquid to flow. When the surface of the canoe is pushed into the water the particles of the liquid flow over its surface. They push on the surface, but with much less force than the particles in the solid ground, so the canoe is easier to move.

Once the canoe is afloat, you climb in and find the paddle. You push the paddle into the shallow water and make contact with the shore under the water. As you push on the rocks and pebbles below you, the canoe moves easily away from the shore. The pointed front of the canoe cuts through the water and the water slides underneath the boat and along its sides. The push of the water, called water resistance, is so gentle that the canoe moves a long way off shore before it slows down to a stop. When this happens, you dip the paddle in the water again. This time you hold the paddle so that one of its broad surfaces faces backwards.

You grip the paddle handle firmly and push the paddle backwards in the water. The water cannot flow as easily round the broad flat surface as it can round the pointed front of the canoe. This means that the water resistance is much higher and it pushes back strongly on the paddle. The force of the water resistance pushing forwards on the paddle makes the canoe move forwards in the water and you sail closer to the island.

At first you let the canoe come to a stop after each paddle stroke but as you see the island coming closer you want to reach it more quickly. You begin paddling faster and not letting the canoe slow down. Soon you are at the island's shore. You drive the front of the canoe into the gravel. The solid surfaces of the canoe and shore grip each other and create a force of friction, which is strong enough to hold the canoe in place while you climb out. You push the canoe a little way up the shore to stop it floating away. Take off your life jacket and begin exploring.

Why do ships and boats have pointed fronts?
The pointed front acts like a knife and cuts its way through the water. It separates some of the particles in front of it and they then flow along each side of the hull.

Why does water push so strongly on a paddle?
The paddle has a broad, flat surface. It cannot cut through the water like the pointed front of a boat. When you push a paddle through the water the particles of water cannot separate and flow round it easily. They move more slowly and build up a strong force, which pushes back on the paddle.

How does the pushing force on a paddle make a canoe move forwards?
The force is transferred from the paddle to the canoeist's arm and body. As the canoeist is sitting in the canoe the forward pushing force is transferred to the canoe and moves it forward.

Why are the sides of a boat curved?
The curved surfaces let the water flow easily over them. If there were flat surfaces facing forwards, they would push on the water and the water would push back on them in the same way as it pushes on the surface of a paddle.

Why is the back of a canoe pointed?
The point at the back does not cut through the water. It allows the water flowing down each side of the boat to come together gently. If the back of the boat were flat, the water from each side would come together quickly as it flowed past the end. This would make the water form tiny whirlpools called eddies. The eddies would make the water pull on the back of the boat and slow it down.

Why does the canoe still move forwards when you aren't paddling it?
All things behave in two ways. If they are still they will remain still until they are pushed or pulled. If they are moving they will keep moving until they are pushed or pulled. When you make one stroke of the paddle you push the canoe forwards. The moment you take your paddle from the water there is no further force pushing the canoe forwards. As the canoe is moving it will just naturally keep on moving.

Why does a canoe eventually slow down and stop?
The water resistance on the boat makes a pushing force on the hull. This makes the boat slow down and stop. When you paddle again you make a force which is much stronger than the water resistance and the canoe moves forwards again. If you keep paddling, you keep giving the canoe a strong forward force which is much stronger than the water resistance. This allows you to build up speed across the lake.

Does a shape which moves easily through water have a special name?
Yes, it does. It is called a streamlined shape. An object which has a streamlined shape has a rounded or pointed front and curved sides, which taper to the back and form a point. A spindle or a teardrop shape are examples of streamlined shapes.

Does a duck have a streamlined shape for paddling on the surface of the water?
Yes, it does. The front of the body just below the neck forms a small area in contact with the water. When the water flows past this front area, it flows over a wider area of the lower body, then a narrower area of the underside of the body. The feathers are arranged so that they form a smooth, curved surface. This allows the water to flow easily over the duck's body.

Do fish have streamlined shapes?
Many of them do. They have a spindle shape. The scales are arranged to provide a smooth surface and the surface is covered in a slimy substance. This substance makes the water flow over the surface even faster.