Work and Power
Introduction
The word work represents a concept that has a special meaning in Physics that is different from the everyday use of the word. In Physics, the concept of work is concerned with the application of a force to an object, and the distance the object moves while the force acts. We talk about the work done by the force. In essence, work is the transfer of energy from the thing making the force to the object that the force acts on.
If the force that acts on a moving object is in the same direction that the object moves we can find the work (W) done by the force by multiplying the magnitude of the force times the distance that the object moves: W = Fd. The unit of work is force times distance, which is Nm or joules (J), the unit of energy in the SI system of units.
You are doing work when you walk up a stairway since you are lifting yourself through a distance (the vertical height of the stairs). In this lab you find the work done to go up a stairway. A person going up stairs transfers chemical energy into the gravitational potential energy associated with the height attained.
Another related concept is power. Power is the rate of doing work, or how fast the energy is transferred by the force. We find power by dividing the work done by the time it takes to do the work:
The unit of power is J/s or watts (W) in the SI system.
Required Materials
Ruler
One flight of stairs (one floor to the next)
Bathroom Scale (anything that can measure the weight of a person)
Stop Watch (any phone should have a stop watch app)
Another person
Procedure
All measurements should be done very carefully. Be sure to include units with all measurements and calculations.
1. Find the weight of yourself and the other person. This needs to be in newtons. If your scale weighs in pounds, convert the weight in pounds to newtons. You can easily find the conversion factor by searching the internet (eg. how many newtons are in a pound?). Record the weights in the data table below. You are Walker 1 and Runner 1 – so same weight for both. The other person is Walker 2 and Runner 2.
2. Measure the height of the stairs by measuring the height of one step and counting the number of steps. You should measure the height of the step in centimeters. Then divide the measurement by 100 to get meters. Enter the height value and the number of stairs in the data table. Note that you put the same measurement into each of the four cells under “Height of Step”, and the same number of steps into each cell under “Number of Steps”.
3. Measure the time for each person to walk normally up the stairs. Record the times in seconds in the data table.
4. Measure the time for each person to run up the stairs (as fast as can be safely done). Record the times in seconds in the data table.
Weight Height of Step Number of Steps Time
Walker 1
Walker 2
Runner 1
Runner 2
Analysis
1. Calculate the total vertical height of the stairs in meters. Put your answer in the text box below:
2. Calculate the work done by each person in walking and running. In this case, the force would be the weight of the person (that is what has to be lifted) and the distance would be the height of the stairs. Put your answers in the table below.
3. Calculate the power for each person in walking and running. Put your answers in the table below.
Work Power
Walker 1
Walker 2
Runner 1
Runner 2
Conclusions
1. How does the work done in walking compare to the work done in running? Explain why you would expect it to be this way.
2. How does the power used in walking compare to the power used in running? Explain why you would expect it to be this way.
3. Could the power developed by a slower moving person ever be greater than the power developed by a faster moving person? Explain your answer.
