Although forces occur naturally, they can also be produced by people or machines. Often these forces are comprised of pushes, pulls or twists. You probably use forces like these every day — like when you push a door bell, open a drawer or twist a bottle top.
We also use machines to produce forces on our behalf. Machines can be made to exert a greater force than we can produce ourselves. For example, a tractor can pull a trailer full of hay, a bulldozer can push forward a mound of earth and a potter’s wheel turns and helps to shape a vase out of a lump of clay. Look around you and you will see many forces in action.
You can exert a force by pulling. Pulling a rope attached to a sledge will make the sledge move forwards. You use many pulling forces every day-like doing up a zip, pulling the curtains, putting socks on and pulling a door closed.
Sometimes you exert a force by pushing – you can push a table across a room, push a swing door to open it, push a trolley or push a parcel through letter box. When you push and pull you use your weight to exert a force onto an object.
You can also exert a number of forces by twisting – you might wring out wet clothes to dry them, turn a wheel, open a jam jar, and turn a screw – driver or wind up a clock. Twisting forces are usually conducted with your arms and your hands.
A watermill uses a large wooden or metal wheel to generate power. It consists of many small blades, over which the flowing water of a stream runs; this wheel is called the ‘water wheel’. The flowing water rotates the wheel. This rotational movement produces electricity when connected to a generator. Although mills were used to generate hydropower in earlier times in all countries, people in the underdeveloped countries use them even today. Electricity produced in such a manner is pollution free and does not harm our environment. 
Dam walls are mostly made of a material called concrete. These dams retain a large amount of water. The retained water puts very high pressure on the lowermost layers of water. If the water in these lowermost layers is diverted, it flows at a very high speed, and hence a high energy is produced with the help of a device called turbine. In the turbine, the energy is used to generate electricity. 
part of the coast, and in low tides it flows away again. This energy of the tides is used to run turbines. Special water turbines are used in tidal power plants. Water – rising in case of high tides and receding in case of low tides – can flow through these turbines in both the directions. Electricity is generated in tidal power plants due to the rotary movement of the turbines. These power plants need a tidal hub – the difference in the height of water between high and low tides – of more than 5 m to be able to work economically. Such a power plant is in the French Bretagne, in St. Malo. Here, the tidal hub is at 12-15 m. 
Ferries without engines are driven by the flow of water. They are attached to a wire cable, which is spanned across the river by means of two short ropes. When the ferryman shortens one of the two short ropes with the help of a device called ‘winch’, the ferry is tilted at an angle to the current, and the force of the current propels the ferry across the river.
At room temperature, there is always some air dissolved in water. The colder the water, the more air is dissolved. Air becomes less soluble in water as the temperature rises. When the water boils the air is released in the form of bubbles. Two thousand years ago, an attempt was made to use the power of steam with Heron’s ball, but the great breakthrough came only with the steam engines. The steam engine improved by James Watt is the most well-known. A little later the steam engines were used to drive locomotives and paddle steamers – the modern steam turbines followed later. 
James Watt is often called the inventor of the steam engine, but in reality he improved upon the working steam engine of Thomas Newcomen and got his design patented in 1769. In the steam engine of James Watt, hot steam is passed inside the cylinder not just from the side but alternatively from top and bottom. The expansion force of the steam causes a piston in the cylinder to rise and fall. Watt converted this up and down movement into the rotational movement of a flywheel. From there, the force generated by steam could be transferred to machines – to sewing machines or weaving looms.