Work, Energy and Power

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Roderick Wilkins
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1 Name: KEY Work, Energy and Power Objectives: 1. To understand work and its relation to energy. 2. To understand how energy can be transformed from one form into another. 3. To compute the power from the rate at which work is done. Suggested Reading: Hewitt, Paul, John Suchocki and Leslie Hewitt. Conceptual Physical Science, 3 rd ed. San Francisco, CA: Pearson Education, Inc., (Ch. 3, pages 75-84) Equipment and Supplies: Balloons, golf/tennis/super balls, meter stick, and a timer. Introduction: In the physical world, the possession of energy by an object means that it has an ability to do work. Work done is a measure of the effect the application of a force produces. If the applied force and the displacement of the object are in the same direction, then the work done is given by, Work Done = Force x Distance. Mechanical energy has several different forms. Elastic Potential Energy is the stored energy by virtue of an object s configuration. When you stretch a spring, you are doing work on the spring and in turn the spring stores that work in the form of elastic potential energy. Gravitational Potential Energy, on the other hand, is the stored energy by virtue of an object s height (position). When the gravitational force is the only force on an object, the gravitational potential energy is calculated from, Gravitational Potential Energy = Weight x Height. Energy that exists by virtue of an object s motion is called the Kinetic Energy. The law of conservation of energy is a universal principle that says that the total energy of a system always remains constant. In other words, energy cannot be created or destroyed but it can be converted from one form into another. When work is done on an object, any of the following things can happen: The object may, in turn, do work on another object, The object s speed may increase (gain in kinetic energy), The object s temperature may rise (gain in thermal energy), The object may store the energy for later use (gain in potential energy), The object may rise in the earth s gravitational field (gain in gravitational potential energy). In many situations not only is the amount of work done important, but also how slowly or how quickly it is done is also important. The rate at which work is done or energy is transformed is called Power. Therefore, the power is calculated as follows.

San Francisco, CA: Pearson Education, Inc., 2004. (Ch. 3, pages 75-84) Equipment and Supplies: Balloons, golf/tennis/super balls, meter stick, and a timer.

2 Power = Work Done = Energy used time time In the first part of this lab you will learn how the work done on an object is stored as potential energy of that object. In the second part you will figure out the work done during various activities and compute the power expended. Procedure: Part I Answer the questions given in your data sheet after doing the following: Blow up a balloon and then release it. From the ground, raise the ball to a height of about 1-meter and then release it. Important: Be very careful when handling stored energy!! Part II 1. Go to the nearest stairway. Measure the height of one step and the number of steps to the 2 nd floor. Record your measurements. 2. Have one of your partners climb the stairs slowly. Record the time taken. 3. Next, climb at a faster speed and record the time. 4. Complete the calculations. Finally, answer the summative questions at the end.

Procedure: Part I Answer the questions given in your data sheet after doing the following: Blow up a balloon and then release it.

3 PH-218 Laboratory Data Sheet 1 Energy Experiment Name: Partners: Part I Answer the following questions. Energy storage in a balloon: 1. Do you do work when you blow up a balloon? How do you know? Yes, you are applying a force through a distance. Work = Force x Distance. Your diaphragm muscles push the air out of your lungs and into the balloon this is the force, and the balloon expands outward this is the distance. The force and distance are both in the same direction. 2. In what form is energy stored in a blown up balloon? Elastic Potential Energy the balloon has changed shape. It has stretched and would like to return to its original size and shape. 3. How can you get the stored energy out of the balloon? Where does it go? Energy may be released by letting go of the balloon, or by popping it. The elastic skin of the balloon contracts back to its relaxed state and the energy is transformed from one form to another and transferred to other objects. For example, the elastic potential energy can become KE of the balloon as it flies around the room, sound energy, or thermal energy of the balloon skin and the air. Energy storage in a ball: 1. Do you do work when you raise the ball to 1 meter height? How do you know? Yes. Work is force times distance, and in this case the force is exerted on the ball by your hand to raise the ball a distance of 1 meter. 2. In what form is the energy stored at 1 m height? Gravitational Potential Energy If the ball is released it will begin to fall to the ground.

Your diaphragm muscles push the air out of your lungs and into the balloon this is the force, and the balloon expands outward this is the distance.

4 PH-218 Laboratory Data Sheet 2 Energy Experiment 3. What happens to this stored energy as the ball falls to the ground? Explain. As the ball falls the stored energy decreases and the kinetic energy increases. Energy is transformed from gravitational potential energy to kinetic energy. Part III (Answers will vary.) Your weight: 556 N (Newtons) Use this table to estimate your weight in Newtons: under 100 lb use 90 lb = 400 N lb use 125 lb = 556 N lb use 175 lb = 778 N over 200 lb use 225 lb = 1001 N Height of one step: 0.18 m (meters) Number of steps to 2 nd floor: 18 Total distance to 2 nd floor: 3.24 m (meters) (Show calculations, write equation first.) Total distance = (height of 1 step) x (# of steps) = (0.18 m) x (18) = 3.24 m Work done in climbing to 2 nd floor: 1,801 J (Joules) (Show calculations, write equations first.) Work = Force x distance = (556 N) x (3.24 m) = 1,801 J Time taken while climbing slowly: 9.28 s (seconds) Time taken while climbing faster: 5.51 s (seconds) Power output (slow): Power output (fast): (Show power calculations, write equations first) Power (fast) = Work done = 1,801 J = 327 W time 5.51 s (Watts) (Watts) Power (slow) = Work done = 1,801 J = 194 W Time 9.28 s

) Your weight: 556 N (Newtons) Use this table to estimate your weight in Newtons: under 100 lb use 90 lb = 400 N 100-150 lb use 125 lb = 556 N 150-200 lb use 175 lb = 778 N over 200 lb use 225 lb =

5 Questions: 1. When you drop the golf ball from a certain height, does it return to the same height after bouncing from the floor? Does that violate the conservation of energy principle? Explain. No, the ball does not rise to the same height after bouncing from the floor, but this does not violate the conservation of energy principle. The conservation of energy principle says that Energy is neither created nor destroyed, but may be transformed from one form to another, or transferred from one object to another. In this case, some of the KE of the ball right before it hits is turned into thermal energy of the ball, some is turned into thermal energy of the floor, and some is transferred to the surrounding air as sound energy (KE of the air molecules). 2. In Part II, is the work done greater, smaller or the same when you climb fast or slow? What about the power output? Explain. The work done is the same whether you climb fast or slow. Work is Force times distance, and you do not change either the Force or the distance, therefore the work done remains the same. When you climb fast you do work (or use energy) more quickly. Power is the work done (which is the same each time) divided by the time it takes to do the work. The faster you do work, the more power it takes. Surprisingly, the energy converted to gravitational PE is the same each time also. You are still raising the same mass to the same height. The energy you expend may be greater when you climb fast because there may be greater motion of the muscles when you climb faster and you may convert more to KE.

The conservation of energy principle says that Energy is neither created nor destroyed, but may be transformed from one form to another, or transferred from one object to another.

6 3. Given that 4200 Joules = 1 Food Calorie, calculate the number of Calories used by you in climbing to the 2 nd floor. How many stories (approximately) would you have to climb to turn 100 Calories (energy equivalent of 1 cookie!) of energy into gravitational potential energy? Comment on these results. One solution is as follows: First, convert your energy in Joules to Calories: 1 Calorie Energy to climb 1story = 1,801J = 4200 J 0.43 Calories Then, since the number of Calories used is proportional to the height you climb, make a ratio: Energy to climb 1 story = Energy to climb x stories 0.43 Calories 100 Calories 1 story = X 0.43 Cal 100 Cal Cross multiply to solve for X: (1 story)(100 Cal) = (0.43 Cal)(X) X = 100 story Cal = 233 stories! 0.43 Cal This seems discouraging, but you must remember that this is only the energy that is converted into gravitational potential energy and does not take into account the energy necessary to power your muscles, nourish and repair cells, and to keep you warm. These other processes consume the majority of your energy, so if you do climb that high you will actually burn many more than 100 Calories. Go for it!

One solution is as follows: First, convert your energy in Joules to Calories: 1 Calorie Energy to climb 1story = 1,801J = 4200 J 0.

Bounce! Name. Be very careful with the balls. Do not throw them DROP the balls as instructed in the procedure.

Bounce! Name. Be very careful with the balls. Do not throw them DROP the balls as instructed in the procedure. Bounce 1 Name Bounce! Be very careful with the balls. Do not throw them DROP the balls as instructed in the procedure. Background information: Energy causes things to happen. During the day, the sun gives