Physical Science Coach Kelsoe Pages 410–440 C HAPTER 14: W ORK, P OWER, AND M ACHINES

Physical Science Coach Kelsoe Pages 412–416 S ECTION 14–1: W ORK AND P OWER

O BJECTIVES  Describe the conditions that must exist for a force to do work on an object.  Calculate the work done on an object.  Describe and calculate power.  Compare the units of watts and horsepower as they relate to power.

W HAT I S W ORK ?  An object begins moving only when an unbalanced force acts on it.  In science, work is the product of force and distance. Work is done when a force acts on an object in the direction the object moves.  For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is no movement, no work is done.

W ORK D EPENDS ON D IRECTION  The amount of work done on an object, if any, depends on the direction of the force and the direction of the movement.  A force does not have to act entirely in the direction of movement to do work. Only the part of the force that acts in the direction of motion does work.  Any part of a force that does not act in the direction of motion does no work on an object.

C ALCULATING W ORK  The work done on an object is calculated by multiplying the constant force acting in the direction of motion by the distance that the object moves. W = Fd Work = Force x distance  When using SI units in the work formula, the force is in newtons, and distance is in meters. One newton-meter is called a joule.  The joule (J) is the SI unit of work. When a force of 1 newton moves an object 1 meter in the direction of the force, 1 joule of work is done.

W HAT I S P OWER  Power is the rate of doing work.  During work at a faster rate requires more power. To increase power, you can increase the amount of work done in a given time, or you can do a given amount of work in less time.  When comparing someone shoveling snow and a snow blower, they both can do the same amount of work. But typically the snow blower can do it faster, therefore it is more powerful.

C ALCULATING P OWER  You can calculate power by dividing the amount of work done by the time needed to do the work. P = W/t Power = Work/Time  When using SI units in the power formula, work is done in joules and time is in seconds.  The SI unit of power is the watt (W), which is equal to one joule per second. Thus, a 40-watt light bulb requires 40 joules each second that it is lit.

C ALCULATING P OWER  You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?  Force (F) = 72 N  Distance (d) = 1.0 m  Time (t) = 2.0 s  Power = Work/Time = Force x Distance/Time Power = (72 N)(1.0 m)/(2.0 s) = 36 W

J AMES W ATT AND H ORSEPOWER  Besides the watt, another common unit of power is the horsepower.  One horsepower (hp) is equal to about 746 watts.  The horsepower was first defined by Scottish scientist James Watt. He was looking for a way to compare the power outputs of steam engines he had designed, and the most logical comparison was the work a horse could do.

V OCABULARY  Work  Joule  Power  Watt  Horsepower