Mechanical advantage and friction

People use simple machines, such as levers and pulleys, to make manual chores easier. The mechanical energy in a person's muscles makes the machine do work. Not all machines use muscle power, however, to do work. A complex machine, such as an airplane engine or an elevator, is made up of many simple machines. Airplane engines and elevators are not powered by hand. Complex machines often use the energy stored in chemical substances, such as airplane fuel or the energy stored in electricity, to provide the necessary force to do work. An airplane engine uses the combustion, or rapid burning, of airplane fuel to power the engine that turns the propeller. An elevator uses large engines, usually powered by electricity, to pull cables that raise and lower the elevator car. Electricity also powers the levers that help open and shut the elevator doors.

Measuring the mechanical advantage (MA) is a mathematical way to determine how much a machine affects the amount of force needed to do work. Scientists find the mechanical advantage of a machine by dividing the force the machine delivers by the effort put into the machine. The theoretical, or ideal, mechanical advantage of a machine is the advantage it would produce if the machine were perfect. In simple machines, the main source of imperfection is friction. Friction results from two bodies moving against each other in different directions. Friction always opposes motion and makes doing work harder. Since friction is present in almost every machine, the actual mechanical advantage is always less than the theoretical mechanical advantage.

Because simple machines increase mechanical advantage by increasing the distance over which the effort is applied, one way to compute theoretical mechanical advantage is to divide the distance the effort is applied by the distance the load actually travels.

This means that the work was twice as easy, or that only half as much effort was needed to raise the load. Because of the inclined plane, however, the load needed to be pushed twice as far to end up 5 meters above the ground.

Efficiency

Another factor that people sometimes compute for machines is their efficiency, or the ratio of the work that results to the amount of work put into the machine. The efficiency of a machine is usually expressed as a percentage and can vary from 5 percent to 95 percent. A perfect machine would be 100 percent efficient. Most simple machines are very efficient, but they always lose some efficiency due to friction. An automobile engine is much less efficient because much of the energy used to move. the crankshaft is lost to friction in the form of heat dissipating from the engine.

2. Make a plan in the form of questions.

3. Give the summary of the text according to your plan in a written form.

IV Comprehensive skills.

1. Read and remember

1. emphasis – наголос, виразність

2. diminish – зменшуватись

3. persist – наполегливо продовжувати

4. traction – тяга

5. dynamos – динамо-машина