Physics third assignment Energy Name

Physics third assignment Energy Name ____________________________

Discussion Topic

A system is a group of parts that can be considered one unit. Storm systems, ecosystems, and our bodies

are natural systems. Cars and houses are human-made systems.

Choose an example of a system, and analyze the energy flow through the system. Describe in detail how

energy enters the system, how energy changes form while in the system, and how energy leaves the

system.

Imagine a girl climbing a rope ladder to ride a zip line. It takes effort for her to get to the top of the

ladder, but then she zips to the ground with ease. She travels down easily because she has stored

energy. What’s your guess about where this energy came from?

Gaining Potential Energy

Energy is the ability to do work. Anything that moves has motion energy, also called kinetic energy.

Objects can also have stored energy in them. This kind of energy is called potential energy. Any object

can store energy, depending on its position. As an object is raised higher above Earth’s surface, it stores

more potential energy. For example, the image shows two hikers. The hiker at the top of the cliff has

gained more potential energy than the hiker who’s still climbing.

We know that energy can’t be created or destroyed. However, energy can change its form. So when the

hikers climb, their motion energy is stored in them as potential energy. This energy transforms back to

motion energy as they climb down the rock. This energy transformation is why it’s easier to climb down

than up.

In some situations, the kinetic energy used to move an object can be stored by that object as potential

energy. In this activity, you’ll need a notebook or other small object. You’ll investigate two diff erent

situations. You’ll lift the notebook against the force of gravity and push the notebook against the force

of friction.

Lift a notebook a few inches off a table or other surface. As it moves up, it has kinetic energy. Which

force opposed the motion of the notebook when you lifted it?

Did the energy you used to lift the notebook get stored in the book as potential energy? Explain your

answer with reasons.

Place the notebook on a table, and push it a few inches with your finger. When it moves, it has k inetic

energy. Which kind of force opposed the motion of the notebook?

Did the energy you used to push the notebook get stored in the book as potential energy? Explain your

answer.

Based on this activity, is kinetic energy always transformed into potential energy?

Kinetic Energy to Potential Energy

In the activity, we saw that the kinetic energy of an object transforms to potential energy in some cases.

One way to know if an object has gained potential energy is to see if there’s a subsequent energy

transformation back into kinetic energy.

For example, in the image, the diver standing on the diving board used kinetic energy to climb a ladder.

This energy is now stored in him as potential energy. When he dives, he will move through the air and

into the water. During the dive, his potential energy will transform back to kinetic energy.

But sometimes, kinetic energy isn’t transformed into potential energy. For example, when the ignition of

a moving car is switched off, the car stops. The car won’t move again unless there’s an input of energy.

Gravitational Potential Energy

The potential energy of an object is based on its position. Objects gain potential energy when they’re

moved against a force field. The form of potential energy gained depends on the force. Gravitational

potential energy is one form. When an object is lifted against gravity, it gains gravitational potential

energy.

Think of a skateboarder. When he rolls toward the top of the ramp, his motion energy transforms into

gravitational potential energy. The higher the skateboarder goes, the more gravitational potential

energy he gains.

Electric and Magnetic Potential Energy

We’ve learned that moving an object against gravity causes it to gain gravitational potential energy.

Similarly, if we move a charged object against an electrical force, it gains electric potential energy. The

farther the object moves, the more electric potential energy it gains.

If we move a magnet against a magnetic force, it gains magnetic potential energy. The farther the

magnet moves, the more magnetic potential energy it gains.

Elastic Potential Energy

Elastic objects and materials include rubber bands, springs, bungee cords, air, and liquids. The molecules

in these materials exert electric forces on each other. When an elastic material is stretched or

compressed, the positions of its molecules change. The energy used to move the material is stored in

the form of elastic potential energy. For example, when a rubber band is stretched, it gains elastic

potential energy. When the rubber band is released, it moves and returns to its original shape. The

elastic potential energy transforms back into kinetic energy.

When a ball is bounced, it flattens as shown in the image. This flattening occurs because the air inside

the ball compresses. When the ball flattens, kinetic energy transforms into elastic potential energy. The

more the ball is flattened, the more elastic potential energy it has. When the ball returns to its original

shape, it bounces up. Its elastic potential energy transforms back into kinetic energy.

Chemical Potential Energy

Atoms exert electric forces on each other. When atoms rearrange in chemical reactions, energy is

needed to bring atoms close together to make molecules. The energy is stored in the molecules in the

form of chemical potential energy. The chemical potential energy then transforms into other forms of

energy in another chemical reaction.

For example, burning a fuel such as gasoline in a car transforms the fuel’s chemical potential energy into

thermal energy and light energy. Some of the transformed energy moves the car. When our bodies

digest food and break it down in chemical reactions, the food’s chemical potential energy transforms

into other forms of energy that our bodies need.

Fuel and food molecules come from plants. Plants transform radiant energy from the Sun into chemical

potential energy. This process is called photosynthesis. Almost all living organisms depend on the

chemical potential energy captured by plants during photosynthesis.

Each kind of molecule gains a specific amount of chemical potential energy when it’s made. If the same

mass of two substances are compared, the substance with larger molecules usually has more chemical

potential energy. Fuels and food both have large molecules.

Applications of Gravitational Potential Energy

Gravitational potential energy has many applications. One application is the use of water tanks to supply

water to communities. Water is stored in water tanks that are located high off the ground. The

increased height gives the water gravitational potential energy. This potential energy transforms to

kinetic energy when water flows down through pipes to reach homes.

Another important application is the generation of electricity in hydropower plants. Here, the

gravitational potential energy of water in a high place transforms to the motion of falling water. This

motion energy is then transferred to the generator’s fan blades.

Measuring Potential Energy

Although we can't see most forms of energy, we can measure it, either directly or indirectly.

To understand how we measure gravitational potential energy, we need to know what factors affect it.

Gravitational potential energy (PE) is affected by height (h) and mass (m). Another factor that affects

gravitational potential energy is the strength of the gravitational field, also referred to as gravitational

acceleration (g). These factors are combined in the gravitational potential energy equation. The

equation is PE = m × g × h.

When mass is expressed in kilograms (kg), height is expressed in meters (m), and gravitational force is in

newtons/kilogram (N/kg), the SI unit for potential energy is joules (J). Near the surface of Earth, the

gravitational field is about the same. So, we use a constant value for g, 9.8 newtons/kilogram.

We can find the gravitational potential energy of something if we know its mass and the height to which

it’s raised. For example, in the image, the paint can is sitting on the top step of a ladder. The mass of the

paint can is 5 kilograms, and its height from the ground is 7 meters.

The formula to find potential energy is PE = m × g × h. Plug the values of m and h into the formula. Use

9.8 for the value of gravity, g:

PE = 5 × 9.8 × 7

= 343 joules.

The paint can has 343 joules of potential energy at the top of the ladder.

Careers in Science:

Fluid Power Engineering

When we pump air into a tire, we raise the air pressure inside the tire. The air is compressed. An

important application of elastic potential energy is compressed ai r and liquids.

The use of compressed air is called pneumatics, and the use of compressed liquids is called hydraulics.

The scientific name for gases and liquids is fluids. The ability of compressed fluids to do work is called

fluid power. The fluid power industry is a fast-growing field. Compressed fluids are used in automobiles

parts, construction equipment, airplanes, agricultural equipment, and medical devices. In the image, a

hydraulic system filled with compressed oil controls the movement of the bucke t on a backhoe.

A career in fluid power appeals to people who enjoy working with equipment and know how things

move. Engineers research, design, and build controls for hand tools and heavy equipment. They solve

mechanical problems and are physically active. They also work with different customers in many types

of industries. Four years of college are needed to become an engineer. Engineers who gain a lot

experience become specialists or consultants who give advice on designing new equipment.

Technicians are also needed in this industry. They assemble, install, and maintain equipment that uses

fluid power. Technicians can be trained on the job and then can become certified.

luid power is based on compressed fluids. Special machines called compressors raise the pressure of

fluids. This process changes the volume and shape of the fluids. When the fluid is allowed to expand

again, its stored energy becomes kinetic energy. The expanding fluid applies a force where needed.

Applications of pneumatics and hydraulics include:

power washers and jackhammers

car lifts at auto repair shops

papermaking presses

vehicle brakes

automatic doors in buses and trains

glass-making equipment

Warm-Up

Every day, we see objects moving around us. We see people walking, children playing, and vehicles

traveling down the street. For every activity we do, some amount of energy is needed. This energy of

motion is called kinetic energy. One type of motion is moving from one position to another. For

example, we move positions when we walk around inside a house or when we go from one destination

to the next.

What Is Kinetic Energy?

Energy is the ability to move or change matter. When we look around, we might see people working or

exercising. We might even see objects flying through the air. Each of these situations is made possible

with kinetic energy.

There are different types and forms of energy. Kinetic energy is found in a body or an object in motion.

Kinetic energy makes objects vibrate, rotate, and move from place to place. It’s found everywhere from

tiny atoms to huge stars, and in everything in between. If kinetic energy didn’t exist, everything around

us would be motionless. The universe wouldn’t have heat because heat is also related to kinetic energy.

The name “kinetic energy” comes from the Greek words kinesis, which means “motion,” and energeia,

which means “activity.”

What factors affect the amount of kinetic energy an object has? In the image, notice that the man has

thrown a stone into the water. The water splashed to a certain height based on his throw. The water

splashed because the kinetic energy of the stone was transferred to the water.

What would happen to the water if he threw a heavier stone? How would the water react if he threw

the same stone, but with greater speed? Complete the activity on the next screen to find answers to

questions like these.

Kinetic energy of a moving object depends on two factors, mass and speed. Whenever mass and speed

vary, kinetic energy also varies.

When an object moves from one position to another, it’s called translational motion. A skateboarder

coasting down a ramp is an example of translational motion. This kind of motion is different from

vibrational and rotational motion. When objects vibrate or rotate, they return to the position they

started at. The relationships between kinetic energy, mass, and speed that we’ll study in this lesson

apply to translational motion.

Calculating Kinetic Energy

How can we determine exactly how much kinetic energy an object has? We know that speed and mass

affect the amount of kinetic energy.

In the following activity, you’ll look more closely at these relationships. You’ll use graphs and data to find

more information and draw conclusions about the mathematical relationships between these three

quantities.

Part B

Think about multiplying the mass of each student by a factor to calculate each student’s kinetic energy.

Is there a common factor that works for every student? If so, what’s this factor?

Part C

Predict what the shape of the line will be when you graph kinetic energy against mass.

Part A

Based on the data, what overall trend or relationship between speed and kinetic energy do you see?

Part B

Think about multiplying each speed by a factor to calculate kinetic energy at that speed. Is there a

common factor that works for every speed? If so, what’s this factor?

Part C

Predict what the shape of the line will be when you graph kinetic energy against speed.

Part D

Draw a graph of kinetic energy against speed

So far, we’ve seen that the kinetic energy of an object depends on two things: the mass of the object

and its speed. We’ve also learned that a change in the object’s speed has a greater effect on kinetic

energy than a change in its mass.

Points of Reference

Motion and kinetic energy are measured relative to a reference point. These points help us decide

whether an object is moving. If an object isn’t moving relative to the reference point, it doesn’t have

kinetic energy. If it is moving relative to the reference point, the object does have kinetic energy.

Suppose a boy throws a dart at a dartboard. Compared to the boy, the dart is moving and has kinetic

energy. But compared to an organism sitting on the dart, the dart isn’t moving and doesn’t have kinetic

energy. The reference point, whether it’s an organism, a boy, or something else, helps us determine

whether an object has kinetic energy.

Braking and Kinetic Energy

Drivers have to be alert. Sometimes drivers have to suddenly apply their car brakes. There might be an

obstacle in the road or the car ahead of them might have stopped unexpectedly. Braking is never

instantaneous. It takes time for the car to come to a stop. The car continues to move while the driver is

braking.

When a car moves, the chemical energy from the fuel is transformed into kinetic energy. When a car

brakes, it loses its kinetic energy. Brakes use friction to stop the car. During braking, the car’s kinetic

energy is transformed into thermal energy at the brakes, the tires, and the road. But these things can

absorb only so much thermal energy at any moment in time. This limitation is why there’s a relationship

between speed, kinetic energy, and braking distance, as the video showed.

Think about two cars, each with a different amount of kinetic energy, 500 joules and 700 joules. Each car

comes to an abrupt stop in the same way. Which car will require more time to convert its kinetic energy

to heat energy? The answer is the car with the greater kinetic energy, 700 joules. The higher the kinetic

energy, the longer it takes to bring the car to a stop. In fact, if the speed doubles, kinetic energy and

braking distance quadruple, which means they’re four times greater.