EXPERIMENT 8.2

HOW FAST CAN YOU DO WORK?

Student Information:

PURPOSE:

To explore the relationship between work, energy, and power using your own body.

MATERIALS:

BACKGROUND:

Any time you use a force in the direction of motion to move an object a distance, you are doing work. In this investigation, you will use your arms to supply the force to lift a 1 lb object. You will also use your legs to lift yourself (you are the object) up a flight of stairs.

Since you will be doing your work against gravity, lifting your body from the first floor to the second floor, we will only need the vertical height of the stairs, not the horizontal distance. We'll compare the power of your arms and legs.

QUESTIONS:

What is the relationship between work, energy, and power?

HYPOTHESIS:

Write what you predict will have more power, your arms or your legs:

PROCEDURE—PART A, ARMS:

  1. Attach one end of the string to the 1 lb weight or box of spaghetti by tying the string around the center.
  2. Tie the other end of the string around the pencil and secure with tape.
  3. Measure the distance by measuring the string from the hanging weight to the pencil. You should have a distance of at least 50 cm. Record this measurement in your data table.
  4. Record the mass in kilograms of your weight or box of spaghetti (1 lb = 16 oz = 0.45 kg).
  5. With the stopwatch, have your helper time how quickly you can lift the 1 lb mass by rolling the string onto the pencil as you turn the pencil in your hands. Record the time in seconds.
  6. Calculate the force (in this case it is the weight, use W = mg, where g = 9.8 m/s²), work, and power (in watts and horsepower) that you produced as you lifted the 1 lb mass and record in the data table.

PROCEDURE—PART B, LEGS:

  1. Using a bathroom scale, measure your weight. You will need to convert your weight in pounds to kilograms (1 pound = 0.45 kg). Record your weight in kilograms.
  2. Since we are going to determine the work and power required to lift your weight from the first floor to the second floor, we are only concerned with the vertical height of the stairs, not the horizontal distance of the stairs. Remember that, although the force you will be applying is in the direction along the stairs, the only part of the force that is used to lift you is the vertical component. The horizontal component does not help to lift you up the stairs.
    Applied Force Vertical Component Horizontal Component

    FIGURE 8.25 - Force Components on Stairs

    ● Orange: Applied force along stairs

    ● Red: Vertical component (does work against gravity)

    ● Yellow: Horizontal component (no work against gravity)

    ● Blue: Individual step heights

    So to determine the vertical height of the stairs, measure the height of each step (blue arrows) and add them together. (You should find that each step is the same height, so once you measure one or two—to be certain they are the same height—you can simply count the number of steps and multiply the number of steps by the height of one step to find the total vertical height.) Record the distance (total vertical height of the stairs) in your data table.
  3. With the stopwatch, have your helper time how quickly you can get from the first floor to the second floor. Use common sense when running up the steps. Hold onto the railing and go up the steps quickly and safely. Record your time in the data table.
  4. Calculate the force (your weight, use W = mg, where g = 9.8 m/s²), work, and power (in watts and horsepower) that you produced as you lifted your body up the stairs and record in the data table.
  5. Clean up and put everything away.

DATA COLLECTION:

Part A: Arms

Measurement Value Units
Distance (string length) cm
Distance (converted) m
Mass of weight kg
Time to lift seconds
Force (W = mg) N
Work (Force × Distance) J
Power (Work ÷ Time) W
Power (in horsepower) hp

Part B: Legs

Measurement Value Units
Your weight (in pounds) lb
Your mass (converted) kg
Height of one step cm
Number of steps steps
Total vertical height cm
Total vertical height (converted) m
Time to climb stairs seconds
Force (W = mg) N
Work (Force × Distance) J
Power (Work ÷ Time) W
Power (in horsepower) hp

CONCLUSIONS:

In a paragraph or two, answer the following questions and make connections to the text:

  1. Which activity required the most work? Explain why using the terms force and distance.

  2. Which activity produced the most power? Explain why.

  3. If you wanted to produce more power in either part A or B, what could you do to maximize power?

  4. If you went up the stairs twice as fast as you did, would you be doing more work? Why or why not?