The Lever

The lever may have been the first machine man learned to use. Cavemen almost certainly knew how to move a heavy rock with the help of a strong pole. All they had to do was to force one end of the pole under the rock and put a smaller rock on the ground under the pole near the same end. By pushing down on the other end of the pole, they could raise the rock. 

About 4000 years ago, the Great Pyramid was built in Egypt. It took about 30 years to build. There is no record of how it was built, but the lever may have been the principal machine used. Great teams of workmen may have levered the massive blocks of limestone out of quarries (workweek), dragged them to the site of construction, and then used inclined planes built of sand to raise them up onto the pyramid. Each block has a mass of about. Today, levers are more popular than ever. They are used in many household tools, such as scissors, pliers, the crowbar, and the claw hammer. 

Simple Machines

There are just six kinds of simple machines. Each kind has many uses. Big machines are merely many simple machines put together. The six kinds of simple machines are the lever, the pulley, the wheel and axle, the inclined plane, the wedge, and the screw.

The Meaning of Work

In everyday conversation, we use the word work in various ways. "Going to work", "doing homework", "a writer's work" these are some common expressions using the word. The meaning of "work" varies. But, to the scientist, "work" has the same meaning every time it is used. In science, we say that "work" has been done only if a force of some kind has moved an object from one place to another place. "Work is done when a force moves an object through a distance" is how scientists describe it. 

You are doing work, in the scientific sense, if you lift a bag of potatoes onto your shoulders. You use a force to raise the bag through the distance from the floor to your shoulders. Once the bag is on your shoulders, no more work is being done because there is no motion. The amount of work you did depends on the amount of force you had to use and the distance through which you moved the potatoes. For example, you do work whenever you stretch a spring or an elastic band. The more you stretch it, the more work you do.

Machines 

The modern world is filled with machines. Some of them are very simple, such as the can opener, the shovel, and the saw Others are more complicated, such as the bicycle, the lawn-mower, and the crane. All machines are designed to make work easier. There are three ways in which they can do this by increasing the force, by increasing the speed, and by changing the direction of the force. Here is an example of each of the three ways.

Musular Force

Muscles make it possible for animals to move. They are attached to the bones. Most muscles are controlled by the brain. Messages from the brain travel through nerves and tell the muscles when to act. Unused muscles go soft. Exercise and a proper diet that includes lean meat, milk, and cheese help to build strong muscles. 

The force Of Friction 

When two surfaces are rubbed against one another, one tends to stick to the other. Friction is the force that causes this. No surface is perfectly smooth. Even the smoothest looking surface, when examined with a microscope, will be seen to have bumps and hollows. These bumps and hollows resist one another when two surfaces are rubbed together. The force of resistance is called friction. Friction can be useful. Rub the palms of your hands together, pressing hard. Do this quickly several times. Your hands begin to feel warm. This shows that friction produces heat.

What Can Forces Do?

Forces can make things move. They can also act on stationary objects without causing motion. Sometimes we use a force on an object that is already moving, to stop it. A train uses a braking force as it approaches a station. Similarly, when astronauts were about to land on the moon, rockets were used to slow down their craft, so it could make a soft landing. Forces can also change the direction of motion. This happens in baseball when a batter hits the ball. The ball is first slowed down, then stopped for an instant, then shot off in a different direction. 

 The Force Of Gravity

This is the force that pulls objects down to the ground. when we say a rock is heavy, we mean that the force of gravity pulling down on it is a strong force. To lift the rock off the ground, you must use a force that is slightly greater than the pull of gravity. Once the rock is moving upwards, the lifting force becomes equal to the gravity force. The force of the Earth's gravity is what holds the moon in its nearly circular path around the Earth and prevents it from flying off into outer space. Similarly, it is the force of the sun's gravity that keeps the Earth and the other planets in their orbits.

Culture Media

A culture medium must contain a balanced mixture of the required nutrients, at concentrations sufficient to allow a good growth rate. If a nutrient is in excess, it can inhibit growth or even be toxic. Often one nutrient is present in limiting quantities, to allow adequate growth while preventing the microorganisms from multiplying at their maximum rate. By moderately slowing down the growth rate, the microorganisms are kept healthy. Usually a mineral base is made, containing all the possible nutrients in inorganic form. To this can be added a carbon source, energy source, nitrogen source and growth factors, to suit the organism being cultured. A medium composed entirely of chemically defined nutrients is called a synthetic medium. A medium which contains some ingredients of unknown chemical composition, such as malt extract, is called a complex medium.