Mechanical kinetic energy is also measured inft-lb. The amount of kinetic energy present at anyone time is directly related to the velocity of themoving object and to the weight of the movingobject.Mechanical potential energy can be changedinto mechanical kinetic energy. If you push that5-pound rock over the edge of the 100-foot cliff,it begins to fall, and as it falls, it loses potentialenergy and gains kinetic energy. At any givenmoment, the total mechanical energy (potentialplus kinetic) stored in the system is the same—500ft-lb. But the proportions of potential energy andkinetic energy are changing all the time as the rockis falling. Just before the rock hits the earth, allthe stored mechanical energy is kinetic energy. Asthe rock hits the earth, the kinetic energy ischanged into energy in transition—that is, workand heat.Mechanical kinetic energy can likewise bechanged into mechanical potential energy. Forexample, suppose you throw a baseball straightup in the air. The ball has kinetic energy whileit is in motion, but the kinetic energy decreasesand the potential energy increases as the balltravels upward. When the ball has reached itsuppermost position, just before it starts its fallback to earth, it has only potential energy. Then,as it falls back toward the earth, the potentialenergy is changed into kinetic energy again.Mechanical energy in transition is calledWORK. When an object is moved through adistance against a resisting force, we say that workhas been done. The formula for calculating workisW = F × D ,where:W = work,F = force, andD = distance.As you can see from this formula, you needto know how much force is exerted and thedistance through which the force acts before youcan find how much work is done. The unit offorce is the pound. When work is done againstgravity, the force required to move an object isequal to the weight of the object. Why? Becauseweight is a measure of the force of gravity or, inother words, a measure of the force of attractionbetween an object and the earth. How much workwill you do if you lift that 5-pound rock from thebottom of the 100-foot cliff to the top? You willdo 500 ft-lb of work—the weight of the object(5 pounds) times the distance ( 100 feet) that youmove it against gravity.We also do work against forces other than theforce of gravity. When you push an object acrossthe deck, you are doing work against friction. Inthis case, the force you work against is not onlythe weight of the object, but also the forcerequired to overcome friction and slide theobject over the surface of the deck.Notice that mechanical potential energy,mechanical kinetic energy, and work are allmeasured in the same unit, ft-lb. One ft-lb of workis done when a force of 1 pound acts through adistance of 1 foot. One ft-lb of mechanicalpotential energy or mechanical kinetic energy isthe amount of energy that is required toaccomplish 1 ft-lb of work.The amount of work done has nothing at allto do with how long it takes to do it. When youlift a weight of 1 pound through a distance of1 foot, you have done 1 ft-lb of work, regardlessof whether you do it in half a second or half anhour. The rate at which work is done is calledPOWER. The common unit of measurement forpower is the HORSEPOWER (hp). By definition,1 hp is equal to 33,000 ft-lb of work per minuteor 550 ft-lb of work per second. Thus a machinethat is capable of doing 550 ft-lb of work persecond is said to be a 1-hp machine. (As you cansee, your horsepower rating would not be veryimpressive if you did 1 ft-lb of work in half anhour. Figure it out. It works out to be just a littlemore than one-millionth of a horsepower. )THERMAL ENERGYEarlier in this chapter we discussed molecules.You should remember that all substances arecomposed of very small particles called molecules.The energy associated with molecules is calledthermal energy. Thermal energy, like mechanicalenergy, exists in two stored forms and in onetransitional form. The two stored forms ofthermal energy are (1) internal potential energyand (2) internal kinetic energy. Thermal energyin transition is called HEAT.Although molecules are too small to be seen,they behave in some ways pretty much like thelarger objects we considered in the discussion ofmechanical energy. Molecules have energy ofposition (internal potential energy) because of the2-8

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