Your Power
Based on a lab from Physics: Principles and Problems, Glencoe, Zitzewitz
What is the relationship between effort and energy? Between energy and power? Does the amount of force you use to do something affect how much energy you use? Does the amount of time that you apply that force change the energy consumption?
We have many preconceived notions when we hear the words energy and power. More often then not these terms are used incorrectly. We are about to examine the relationship between these terms apply to what we call physical effort.
Preliminary Questions
1. Assume that the displacement and force vectors are acting in the same direction. Write the expression for work.
2. Now examine the equation you wrote. If you were doing work against gravity how would this change the equation? Write a new expression for work the work done by lifting an object against the gravitational force. Use only the variables W, x, m, & g.
3. If you are walking horizontally do you do work against gravity? You do have to lift your legs, but does the height of the center of mass for your body change? Explain.
4. In this lab you will be walking and running up a set of stairs. Would it matter if you use stairs or if you climb up a ladder? Carefully consider the definition of displacement AND the direction of gravitational force acting against you before answering.
5. Write the equation/definition for average power in two ways.
Materials
Tape Measure
Stopwatch
Calculator
Procedure WATCH YOUR SIG-FIGS!!!
1. Measure the height of the staircase from floor to floor in meters. Record your value in the data table. Use the conversion 1 in = 0.0254 m
2. Select a person to operate the stopwatch. Select a group member to be your designated “walker.”
3. Find the mass of the walker in kilograms and record it in your data table. You may simply convert from the person’s weight in pounds if it is known. Use the conversion 2.2 lbs = 1 kg.
4. Calculate the force of gravity being overcome by the walker.
5. Have the walker walk up the stairs at their normal walking pace (everybody’s will be a little bit different). Carefully time the walker as they do this.
6. Record the time in the data table.
7. Calculate the work done/energy used by the walker. Record.
8. Calculate the power of the walker. Record.
9. Repeat steps 5 – 8 for the same person walking at approximately the same speed two additional times.
10. Repeat the experiment 3 times except now having the “walker” run up the stairs. BE CAREFUL TO PLAN AHEAD FOR SAFETY AND MAKE SURE THE PATH IS CLEAR. WATCH YOUR FOOTING!!! Record your data and perform the calculations as you go.
Data & Analysis WATCH YOUR SIG-FIGS!!!
Height of StairsMass of Walker / Lbs / kg
Walking
Mass(kg) / Displacement
(m) / Force
(N) / Work
(J) / Time
(s) / Power
(Watts)
Running
Mass(kg) / Displacement
(m) / Force
(N) / Work
(J) / Time
(s) / Power
(Watts)
Drawing Conclusions
1. Why does the force variable remain constant in all trials? Don’t you use more force if you run rather than if you walk? Explain why this value is the same for all trials.
2. Does the energy expended depend on time? Does it’s value in this experiment change between the walking and running trials? Why? Hint: It is related to the answer to number 1.
3. Power does depend on time. Is more power utilized when walking or when running? What evidence from your data reveals this conclusion?
4. Finish the sentence: Power is the ______of energy consumption.
5. The effort your put forth is most closely related on what variable in the experiment?
6. Sometime the average power is written as P = W/t. It can also be written as P = Fv. Derive the second equation from the first.
7. Power depends inversely on time. This means as the time required for an action goes up, the power, or rate of energy consumption, goes ______.
Power depends directly on velocity. This means that the faster you run, the faster you produce power or consume ______.
8. True or False: Walking up the stairs “burns” as many calories (energy) as running up the stairs.
Explain with data from your experiment.
9. True or False: Running up the stairs “burns” calories (energy) faster than walking up the stairs.
Explain with data from your experiment.