Relating Work to Power


How work, energy, and power are interelated.

How to use formulas to solve problems related to work, energy, and power.

1.       State the law of conservation of energy.   Why does a swinging pendulum eventually come to rest if this law is true? (ie, where does the energy go?)

The law of conservation of energy states that energy can be neither created nor destroyed.   A pendulum will eventually slow and stop because of the downward pull of gravity and air resistance.   The energy lost by the pendulum, however, is gained by the air molecules that are displaced by the pendulum.

2.      Describe the two conditions necessary for work to be done on an object.

Work is done when a force is applied to an object and that object moves in the same direction as the applied force

3.      What is the relationship between work and energy?

Work and energy are related in that there is a transfer of energy from the object applying a force to the object that moves as a result of that force.

4.     A 1000-newton force is applied to a stalled car, causing it to move 8 meters.   How much work is done on the car?

Work   =   force   x   distance


W   =   1000 N   x   8 m


            W   = 8000 Joules

5.      Two students, each weighing 500 newtons, must travel   700 meters to get from the parking lot to their first period class.   Student A walks and gets to his destination in 200 seconds.   Student B runs and gets there in 100 seconds.

a.      Which student does more work?   Support your answer with calculations

Both students do the same amount of work because their mass is the same and their distance to class is the same…

Work   =   force   x   distance

W   =   500 N   x   700 meters

W   =   350,000 Joules

b.      Which student has more power?   Support your answer with calculations.

Student B has more power because they did the same amount of work in half the time as student A.

Student A                                                                         Student B

            Power   =   work/time                                                             Power   =   work/time

            P   =   350,000 J/100 seconds                                                 P   =   350,000 J/200 seconds

            P   =   35,000 watts                                                    P   =   17,000 watts
Homeworkp 835 #40-55