INVESTIGATION #1: GETTING TO WORK WITH ENERGY



Investigation #1: Getting To Work With Energy—

Investigating How Forces Transfer Energy

ENERGY – what do you think of when you hear the word energy? Do you

picture a speeding racecar or perhaps a spinning windmill? Maybe you imagine a street luge participant speeding down a twisting mountain road or you think of an explosion. We frequently read about energy in the newspaper. In the news, ENERGY usually refers to electrical energy or the energy stored in oil/gasoline. These are two good examples of energy that dramatically affect our daily lives. Are there other examples? How do we make use of this energy? Why are these two forms so important to us?

Energy is a topic that makes its way into all of the sciences. As you move through this year’s science class you will learn about energy in many different forms and in many different contexts. Energy is hard to define, so we generally define it by providing examples of energy. Energy can move from one place to another and it can also change forms. Keeping track of the energy in an example can help you understand and explain the science involved.

• Why is energy so important to us?

• What types of energy do you encounter most often?

• How does energy get from one place to another?

GOALS: In this lab investigation, you will …

• Review the concepts of energy transfer and energy transformation and the basic forms of energy (from the grade 6-8 science content standards)

• Discuss the significance of the Law of Conservation of Energy.

• Describe how force and distance combine mathematically to quantify the energy transfer in a process called work.

• Quantify the work done (energy transferred or transformed) in examples.

• Create and explain energy chains.

INVESTIGATION OVERVIEW: A synopsis of this lesson is as follows…

In this lesson you will begin by reviewing key energy concepts from earlier grades, primarily middle school. These ideas will be expanded upon, arriving at the concept of work and the Law of Conservation of Energy. Energy chains will be used as a tool to analyze energy systems. You will be asked to create crash barriers from dominoes to investigate how forces transfer energy from a moving car. The final task it to apply what you’ve learned to create a crash barrier from assorted materials to stop a fast moving car in a short distance.

wing Key Energy Concepts

Energy is the most central concept in all of science. Energy is the thread that ties the physical, life, and earth sciences together. Matter and energy make up the universe. While matter is tangible and something that we can hold in our hands, energy is far more abstract.

We commonly say that objects have energy, but we can’t really see this energy. We recognize energy mainly through the effects it has on objects. We see the changes that occur when an object or substance has energy and shares that energy with other objects.

Energy is not easily defined. So scientists study energy by looking at the effects energy has on other things. A definition that is often used for energy is “the ability to bring about some sort of change.” If something has energy, then that energy can cause a change in the object or in its surroundings. By designing experiments to study these changes, scientists learn more about energy.

Energy comes in many forms. You may remember some of these forms from previous studies of energy. It is important for us to be able to identify the forms of energy. It is also important to be able to describe how energy moves from one place to another and changes forms.

In some cases, the same type of energy is simply passed along from one object to another; this is called an energy transfer. In other cases, the energy changes forms (converted from one form of energy to another form of energy); this process is called energy transformation. It is common for many energy transfers and transformations to happen together. It is often helpful, in order to understand a sequence of events, to create a description of how the energy moved. This is generally done using words and pictures; the result is called an energy chain.

Part A: Investigating gravitational potential energy and kinetic energy

Question 1: Does the golf ball have energy while it is sitting on the top of the sand? (Assume that the sand represents the ground)

1. Pick up the golf ball and hold it about 25 cm above the pan.

Question 2: What type of energy does the golf ball have while the ball is being held at a height of 25 cm above the pan?

Question 3: How did the golf ball get its energy? Where did this energy come from?

2. Release the ball and discuss the crater produced by the golf ball.

3. Repeat the process from 50 cm, 75 cm, and 100 cm (1 m). Drop each trial’s ball in a different spot in the sand so that they can be compared in the end.

Question 4: Which trial created the most change in the sand (the largest crater)?

4.. Repeat the experiment with a hollow golf ball.

Question 5: What variable was changed in this part of the investigation? What effect did this change have on the crater in the sand?

Question #6: Did the hollow ball and the solid ball impact the sand with the same speed? In other words, did gravity speed them up both golf balls at the same rate?

Question #7: What can you conclude from this investigation?

Mechanical Energy (ME) – the combination of potential and kinetic energies.

Mechanical energy is a broad category of energy that includes the two main forms of energy, potential and kinetic. In many cases the combination of the two forms is an important quantity to keep track of, so the term mechanical energy refers to the sum of these two forms.

Kinetic Energy (KE) – the energy of motion.

The energy associated with moving objects is called kinetic energy, and is often referred to as the most fundamental form of energy. The size of the kinetic energy is determined by an object’s speed and its mass. A moving hockey puck has kinetic energy. If you have ever been hit by one, you are aware of the energy a moving object can have.

Gravitational Potential Energy (GPE) – the energy of position.

This is energy that an object possesses due to its

position. The size of the gravitational potential energy is

determined by the object’s mass and its height above the

ground. As a volleyball rises, increasing its height above the

ground, it increases its gravitational potential energy.

Heat Energy (HE) – the random kinetic energy of particles.

Heat energy is the random, and very disorganized, kinetic energy

of the particles in a substance. Thermal energy is another term often used as a synonym for heat energy. In most cases the distinction between the exact definitions of heat energy and thermal energy is not made. Due to the random nature of this form of energy, it is difficult to make heat energy a useful form of energy. For this reason it is usually the form of energy that appears at the end of energy chains. It happens so often that scientists refer to heat energy as the “graveyard of energy”. For example, if you pound a nail into a piece of wood, the nail gets hot due to the energy transferred to it by the hammer and the force of friction with the wood.

Chemical Potential Energy (CPE) – the energy of bonds.

Chemical potential energy, sometimes just called chemical

energy, is the energy stored in the bonds that hold the particles in a substance together. When these bonds are formed, or are broken, energy transfers and/or transformations take place. In many cases, the energy stored in the bonds of substances is transformed into other forms of energy. Food

is a source of chemical energy for our bodies, so we sometimes use

‘food energy’ in place of chemical energy in energy chains that involve

people. In most cases this chemical potential energy is later transformed into heat energy

Electromagnetic Energy – the energy of waves.

This form of energy is often referred to as solar energy or light energy. Electromagnetic energy is the energy that is carried by electromagnetic waves. The most common form of electromagnetic energy is “light”. Light energy is a term that

can be used to describe the energy ranges that our human eyes are sensitive to and it may include some forms of ‘light’ that we can not see with our eyes, such as infrared and ultraviolet. The sun is the most important source of electromagnetic energy for the Earth, supplying the vast majority of our planet’s energy. In some cases, chemical potential energy can be transformed into electromagnetic energy. This form of energy is very important in the scientific field of astronomy.

Electrical energy is a subset of electromagnetic energy, characterized by moving charges. It is used to run appliances and make artificial light. When the charged particles vibrate, they transfer energy by electromagnetic waves.

Sound Energy – the energy of vibrating particles.

This form of energy is transferred by mechanical waves. The particles that make up a

substance vibrate in a highly organized manner and transfer energy through the substance. The particles in the substance vibrate, but do not change their location.

In most cases, sound energy is classified into three categories;

Infrasonic is the sound that is below our human hearing level,

sonic is the sound that our human ears are sensitive to, and

ultrasonic is the sound that is above our human hearing level. Have you ever made a tin can telephone? If so, you have already experimented with sound waves and how vibrations are involved in the energytransfer process.

A good example of the use of sound waves is sonar. Humans have created devices that enable us to send out a sound wave and listen for the echo so that we can determine how far away something is to the source. Seismic waves or “earthquake” waves also fit into this category because they involve the transfer of energy through vibrating matter in the form of mechanical waves. Ultrasounds in the medical field are used for a variety of purposes. Perhaps you have seen an ultrasound image of a baby. In most energy chains, the sound energy is transformed into heat energy (the disorganized and random kinetic energy of particles).

Elastic Potential Energy (EPE) – energy of deformed materials.

This form of energy comes from the stretching or compressing of elastic materials. When an elastic material is deformed (by stretching or compressing), it exerts a force, called the elastic force, to return to its original shape. In many cases, the elastic material is held temporarily in this deformed position and the material has a stored amount of energy.

Bow hunters make use of elastic potential energy to shoot their arrows. The elastic potential energy of the bowstring is converted to the kinetic energy of the arrow. Catapults and slingshots also operate in this manner. Tennis players rely on the elastic properties of their tennis racquets and the tennis ball. Pole vaulters depend on the stored energy in the bent pole to help them get over the bar. The science behind the design of the pole relies on knowledge of how materials store elastic potential energy. Surprisingly, certain types of rock can have elastic properties. They can be stretched or compressed under huge forces.

1. Energy transfer is the passing of energy from one object to another.

2. Energy transformation is the changing of energy from one form of energy to another form of energy.

We have done the entire investigation based upon the assumption that the total energy in the example is constant (not changing). Is this true? Can we account for all of the energy? Scientific evidence leads us to believe that all of the energy in any example can be accounted for; it changes forms (it is transferred and transformed) but it does not go away. This idea is called the

Law of Conservation of Energy.

Most of the events in our daily lives though tend to tell us

that this law is not true. How many times have you heard

about energy being “lost” or that a machine wastes energy”?

In most cases, what we mean is that energy has been

transformed into a form that is not useful to us so we say that

some of the energy was “lost”. What we really mean is that

the energy was transformed into a non-useful form of energy. Heat energy is the ending form of all energy chains.

What is an energy chain?

Energy is transferred and transformed all the time. It is helpful to be able to track the ‘flow’ of energy in our everyday life. A map of what happens to the energy, where it goes, and how it changes, is called an energy chain. Energy chains can be created using words, pictures, arrows, or any combination of things that illustrate the movement of energy in an example. A good chain should include the forms of energy and any transfers or transformations that happen in the example. It will also be helpful to identify a starting point and an ending point.

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In automobile collisions, a great deal of energy must be transferred from the car (and its occupants) to other objects and transformed into other forms of energy. Major automobile manufacturers spend millions of dollars each year testing new safety features. Engineers will talk about “energy absorbing materials” and “crumple zones” in new car designs. They take

advantage of the fact that materials transfer energy in different ways and at different rates.

Your ultimate task in this activity is to engineer a crash barrier that will stop the car without causing the passenger (represented by a small wooden block or domino) to be ejected from the car. Prior to this task though, we will need to explore and discuss how the concepts of energy transfer and energy transformation can be used to prepare us for this activity.

In order for a moving car to stop, its kinetic energy must be transferred to another object and/or transformed into another form of energy. When the car stops, it has no kinetic energy. Where does the energy go? What can transfer energy to an object or away from an object? Forces are responsible for the transfer of energy. The forces are defined as pushes or pulls. In many cases the direction of the force is important in determining whether the force will transfer energy to an object or away from an object. You will be investigating this relationship between force and energy transfer in the following parts of the investigation.

FOCUS QUESTION: What barrier design will stop the car in the shortest distance?

You are expected to answer this question by conducting a scientific investigation. We know that when conducting an investigation it is important to clearly identify which variables will be changed and which variables will not be allowed to change.

Design of the Investigation

• You will be investigating how the barrier you build affects the stopping distance. You can change the number of blocks in the barrier and the way the blocks are arranged, but the barrier is the only thing that should be changed from trial to trial.

• If we want to make fair judgments about how effectively each barrier stops the car, than we need to ‘control’ all the other possible variables. Only the barrier should change. Everything else (the car, the ramp, the car’s kinetic energy, etc.) needs to remain the same.

Preparing the Ramp for Your Investigation

• Start this investigation by building a ramp. Place a block of wood under the track on one end so that end is elevated about two inches from the table surface (the 1-1/2 inch 2 x 4 width is sufficient if laid flat on table).

• Start the car from the top of the ramp each time. Allow the car to roll freely down the ramp until the 60 cm (1/2 way mark), which is where your barrier design will begin to stop the moving car.

• You will be given 15 dominoes to build your barrier. Design, build, and test a barrier that you think will stop the moving car in the shortest distance. You do not have to use all of the blocks, but as in any experiment, you will need to keep careful records of how you arranged the blocks for each trial. You will want to record the stopping distance and design.

Pre-Investigation Questions

Question 1: What form of energy is present when the car is sitting at the top of the ramp? How do you know this?

Question 2: What will happen to the energy of the car as it moves down the ramp? What evidence could you collect to justify your answer?

Question 3: When the car strikes the barrier what will happen to the energy of the car? How do you know this?

Question 4: Let’s assume we release the car from rest at the top of your ramp. What can you do to be sure that the car strikes your barrier with the same kinetic energy in each trial? Explain.

Conduct your Investigation

Record your results carefully including the designs of each barrier you tried and the stopping distance for each design.

Question 5: What forces are causing the car to stop?

Question 6: Why is the stopping distance shorter for some arrangements of blocks than for other arrangements?

FOCUS QUESTION: What is the shortest distance that your barrier needs to safely stop the moving car?

Background:

Using barriers to stop a real moving car can be a challenge. Using barriers to stop a real moving car safely is an even bigger challenge. Suppose the brakes of a car or a truck fail while the vehicle is traveling downhill on a long stretch of highway. We know that the gravitational potential energy of the vehicle will be transformed into kinetic energy as it moves down the inclined road. Without brakes, the vehicle will leave the road and crash, or worse yet, collide with other vehicles. The number one challenge in cases such as this is to stop the vehicle before it can cause any damage. But it is equally important to stop the vehicle safely. If a barrier is created to stop the vehicle quickly (in a short distance) the stopping forces acting on the vehicle will need to be very large. The driver and passengers of the vehicle will also be stopped in a short distance so they too must experience large stopping forces.

If the stopping force acting on a person is too large, the force will cause injury. So your new challenge is to stop the car in as short a distance as possible, but safely! We can simulate whether the stopping force is too small by standing a block up in the car. If the stopping force is too large, the impact will knock the block over. As long as the block remains upright, the stopping force will be considered small enough to be ‘safe’.

Conduct Your Investigation

Use the blocks provided to you to construct barriers to safely bring the car to rest. Be sure to control all the other variables in the experiment and carefully record all of your results. Start your investigation by determining whether or not your stopping barrier from Part A is a “safe” barrier.

Once again, record your results carefully and be prepared to report to the class, including the designs and distances. On poster paper draw and label your most successful design for both challenges (shortest stopping distance and safest stopping distance).

Question 7: How did the smallest “safe” stopping distance from Part B compare to the stopping distance in Part A?

Question 8: Can you think of other materials that would make safer barriers than the ones you made out of blocks? Explain why you think these other materials would make safer barriers?

To stop a moving car, all of its kinetic energy must be transferred away from the car. This usually involves transforming (changing) the car’s kinetic energy into another form of energy. You learned in middle school that forces transfer energy. A push or a pull can transfer energy to a car, increasing its kinetic energy. But a force can also transfer energy away from a moving car, slowing it down by reducing its kinetic energy. Scientists call this energy transfer process work. Whenever a force increases or decreases an object’s kinetic energy, the force does work. When investigating the properties of simple machines in middle school you learned that the work done by a force when it pushes or pulls on an object could be easily calculated.

In simpler terms: Work done = Force x distance

or W = F · d

The units used for these measurements and calculations are important. In the metric system, force is measured in Newtons (N), distance is measured in meters (m), and the calculated value for work is given in Joules (J).

How does this knowledge help us design a crash barrier that will safely stop a car? When a car that is traveling down a road is brought to a stop, we can say that a force did work to remove all of the car’s kinetic energy. This force transferred the car’s energy to some other object.

Work done by the force = KE of the car

(stopping force) x (stopping distance) = KE of the car

Even without knowing the details of the stopping force, this equation helps us understand why some ‘stops’ are more dangerous than others. Suppose there are two cars moving down a road with the same kinetic energy. The first car is forced to stop suddenly, in a very short distance. The second car stops slowly, taking a large distance to stop. Will the forces that stop the two cars be the same size? We can find the answer in the equation for ‘work done’ by a stopping force.

For the first car, the stopping distance was small (d) so

(stopping force) x (d ) = KE of car 2

For the second car, the stopping distance was large (d) so

(stopping force) X (d ) = KE of car 1

The initial kinetic energy of the two cars is the same size, but if the two stopping distances are different, then the stopping forces must be different in size too. But which stopping force (Fs ) is larger? Looking at the equation for work can help us.

(Fs) x (d ) = (Fs) x (d ) = KE

If one of the cars stopped in a very short distance by colliding with a large object, the stopping force would be dangerously large. The real problem is that the driver and the passengers of this car also must stop in a very short distance. That means that they too will experience very large stopping forces. These forces can be large enough to cause bodily injury, or worse.

Write a concise summary of this investigation. Be sure to address the following questions and use your data to support your responses.

❑ What is the difference between an energy transfer and an energy transformation?

❑ How can the same amount of energy be transferred (work done) if the forces acting on the object are different?

❑ Can energy ever be “lost”? What is meant by the Law of Conservation of Energy?

❑ How can an energy chain or an energy diagram be useful in our everyday life?

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Energy cannot be created nor destroyed. Energy can be transferred from one object to another and can be transformed from one form to another, but the total amount of energy never changes.

MAKING SENSE OF ENERGY … Work – The Transfer of Energy by Forces

CONNECTIONS

Scientific Content –

Energy can exist in different forms.

• Energy can be categorized into several forms (kinetic, potential, elastic, heat, electrical, etc.) Since energy cannot be seen, it is defined by the change that is produced when energy is transferred and/or transformed.

• Mechanical energy is the sum of both the kinetic energy and the gravitational potential energy of an object. Kinetic energy is dependent upon the mass and speed of the object; gravitational potential energy is dependent upon the mass and the height of the object.

• Chemical potential energy (or chemical energy) is the potential energy stored in the bonding of the particles that make up matter.

• Heat energy is the random kinetic energy of the particles that make up a substance.

• Electromagnetic energy is the energy transferred by electromagnetic waves

• Elastic potential energy is the energy stored in elastic materials by compressing or stretching them beyond their normal position.

• Sound energy is the energy carried by the organized vibrations of particles as a mechanical wave.

Energy can be transferred from one object to another.

• An energy transfer occurs when energy is passed from one object to another object but stays in the same form.

• When one object pushes or pulls on another object often (but not always) energy is transferred from one of the objects to the other. As a result of this transfer, the motion of one or both of the objects will change.

• When a force transfers energy, the process is called work. The work done by the force involved in the energy transfer can be quantified by multiplying the force by the distance over which that force acted during the energy transfer process.

• When kinetic energy is transferred to a large stationary solid this energy is transferred to the particles that make up the object. The kinetic energy may take the form of organized vibrations within the solid (a mechanical wave), but will ultimately become the random vibrational kinetic energy of the particles. This collective random kinetic energy of the particles is called heat energy.

TWO IMPORTANT PROPERTIES OF ENERGY ARE:

MAKING SENSE OF ENERGY …

Let’s Investigate … Energy as a “change”

Energy can be changed from one form to another. This process is called the transformation of energy.

• An energy transformation occurs when energy is passed from one object to another and it changes forms.

• In most cases, the energy of objects eventually becomes the random kinetic energy of the particles that make up the object. We call this form of energy heat energy.

By understanding energy transformation and energy transfer, we can begin to understand that the energy of an object can change forms and be passed to other objects but cannot be created nor destroyed.

• Energy cannot be created nor destroyed. Energy can be transferred from one object to another and can be transformed from one form to another, but the total amount of energy never changes. This concept is known as the Law of Conservation of Energy and is one of the most significant laws in science.

• Identifying the energy transformations and transfers that occur when an event takes place helps us to better understand what factors influence these changes. This understanding helps us to

appreciate how objects interact and enable us to make predictions about the outcomes of these changes.

• Energy chains help to understand the Law of Conservation of Energy. Identifying where energy comes from and where it goes builds an acceptance that energy can change form, or be transferred from object to object, but cannot disappear. When energy seems to go away, it really is just changing forms and spreading out. These concepts are the foundation for the Law of Conservation of Energy.

Scientific Process –

• Identifying the different forms of energy, and where it is transferred and/or transformed during an event are important skills that lead to a richer understanding of the flow of energy in everyday phenomena.

• Creating energy chains requires an understanding of the flow of energy that takes place during the changes in any physical system. Accurate completion of an energy chain and/or diagram reflects understanding of the flow of energy in systems.

Math/Graphing –

• Calculations of work will be completed. Graphical displays of energy flow will be created and analyzed.

A REVIEW OF SOME COMMON FORMS OF ENERGY …

MAKING SENSE OF ENERGY … the Law of Conservation of Energy

MAKING SENSE OF ENERGY … Creating Energy Chains

Section II: Investigating How Forces Transfer Energy

Let’s Investigate … Making a Crash Barrier

Section I: Reviewing Key Energy Concepts

Part A – Creating a Barrier to Stop the Car in the Shortest Distance

Part B – Creating a Barrier to Stop the Car Safely

Summary of Investigation 1…

Investigating Further …

SAFER CRASH BARRIERS

An excellent application of these concepts is the “soft walls” used by major racing facilities across the nation (Dover International Speedway being one of these). The new SAFER (Steel And Foam Energy Reduction) barriers have revolutionized the sport of automobile racing and made it much safer for both the drivers and the fans.

So how do SAFER barriers absorb energy? The barriers move upon impact so that the kinetic energy of the car is transferred to a very large area of the wall (a large portion of the wall flexes upon impact). The key idea is that no one portion of the wall receives a large amount of the car’s kinetic energy. The kinetic energy of the flexing soft wall is then transferred to the outer permanent wall and support structure. The materials that make up the wall are not elastic. Imagine what the collision would be like if the wall was elastic! Still other portions of the car’s initial kinetic energy are transformed into heat energy and sound energy.

[pic]

The illustration above is a cut-away drawing of a soft wall at a racetrack used by the major forms of racing. Access the site and view the link discussing how the Indianapolis Motor Speedway uses this technology.

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