Work and force

Terms like work, force, and power are well-known among engineers, so it is necessary to define them precisely. Force is an ef­fort that results in motion or physical change. If you use your muscles to lift a box you are exerting force on that box. The water which strikes the blades of a turbine is exerting force on those blades, thereby setting them into motion.

In a technical sense, work is the combination of the force and the distance through which it was exerted. In the case of the box that you were lifting, work would be the force times the distance you raised the box. Work can be expressed mathematically in the formula: Work = Force x Distance. If you lift a ten-pound box to a table three feet high, you perform thirty foot-pounds of work. Note that the force is measured in terms of the resistance that must be overcome, in this case the weight of the box.

To produce work, a force must act through a distance. If you stand and hold a twenty-pound weight for any length of time, you may get very tired, but you are not doing work in the engineering sense because the force you exerted to hold up the weight was not acting through a distance. However, if you raised the weight, you would be doing work.

Note these two kinds of motion: linear and rotary. Linear mo­tion is movement in a straight line; the technical term for this kind of motion is translation. Reciprocating motion is a linear motion that goes back and forth or up and down in the same path, like the movement of the pistons in a car. Rotary motion is movement in a circular path. To produce rotary motion it is necessary to have torque, a force that can cause a twisting motion called torsion.Torque is the kind of effort that you exert to open a twist-off lid on a jar. In many machines there is a problem to change one kind of mo­tionto another.

Precisely	точно
Effort	зусилля
To exert	застосовувати
Force	сила
Strike	бити
Blade	лопать
Exerting force	діюча сила
Resistance	опір
Pound	фунт, одиниця ваги (0, 454 кг)
Motion	рух
Linear motion	лінійний рух
Reciprocating motion	зворотно-поступальний рух
Rotary motion	обертальний рух
Torque	обертальний момент
Torsion	кручення

Comprehension check.

1. What is force in a technical sense? Give some examples.

2. Give the definition of work.

3. In what way must a force be acting to produce work? What kind of effort would not produce work?

4. What are the two principal kinds of motion?

5. Which kind of motion is reciprocating motion? What is its principal characteristic?

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