Friction and Motion

In this unit, students expand their understanding of the need for energy among living things to include non living energy systems. They build sleds to figure out how the phenomena of friction transfers energy out of systems. This page highlights components of this lesson.

Science Background for Teachers:

Science background gives teachers more in-depth information about the phenomena students explore in this unit on energy and forces on Earth.

Martin Buser has such a passion for the Iditarod that he named his two children after checkpoints in the race—Nikolai and Rohn. He also runs a kennel, called Happy Trails Kennel, that raises and trains dogs to race.

Preparing for the Iditarod can take months, if not years. It is physically grueling for both the dogs and the musher, and those who make it to the end are usually exhausted and sleep- deprived. There are rules that have to be followed to make sure both the musher and the dogs are safe.

Once the race begins, the dogs need an almost-constant supply of energy. Energy is the ability to do work. Work is any change in position, speed, or state of matter due to force. Remember that consumers such as dogs get energy when they eat food. Racing dogs have a diet that often includes lamb, chicken, beef, moose, or salmon. They need between 10,000 and 12,000 calories a day to help pull the sled. Calories are the measure of energy that fuels your body.

Calories are a form of potential energy, which is energy that is stored. There are different forms of potential energy. For example, the energy in food is a form of potential energy called chemical energy, which is energy stored in the bonds of atoms and molecules. Because food is matter, it stores chemical energy.

Dogs can pull sleds because like all living things, their bodies turn the potential chemical energy stored in food into different forms of kinetic energy. Kinetic energy is the energy of motion.

We can understand how dogs use energy by thinking of them as energy systems. An energy system is a set of connected parts that change an input of energy to a different output of energy. When you eat, you are getting an input of energy in the form of calories (chemical energy). Your body stores some of that energy, and it turns the rest of it into different forms of kinetic energy. This is why it is important to eat enough calories. Without enough calories, you wouldn’t have enough energy for your heart to beat, your muscles to move, or your brain to regulate your body. These actions all require outputs of kinetic energy.

Not all energy systems are living things. For example, the dogs that pull the sled are part of another energy system that includes the dogs, the sled, and the musher.

Picture a dog sled. When the dogs are attached to the sled and they begin to run, they pull the sled. This input of energy is a pulling force that transfers the dogs’ kinetic energy to the sled. A force is a push or pull that acts on an object, changing its speed, direction, or shape. This transfer of energy causes the sled to move because it now also has kinetic energy. The movement of the sled is an output of energy.

This dog sled is an energy system, made up of the sled, the dogs, and the musher. Any time energy transforms (changes) from one form to another, it forms an energy system. For example, when you ride a bike, you and the bike form an energy system. You have potential energy in your body in the form of chemical energy stored in the bonds that hold food molecules together. When you move your legs, that chemical energy transforms into kinetic energy that powers your movement. The kinetic energy transfers to the bike, causing the bike’s wheels to turn.

Energy systems can also be human-made. Many of the technologies that power modern society are human-made energy systems that change one form of energy into different forms that do specific kinds of work. For example, an automobile is an energy system that takes chemical energy stored in a battery or fuel such as gasoline and changes it into kinetic energy that moves the car. As energy changes from one form to another, it is always conserved. This means that it is never created or destroyed. Because of this, sources of energy do not create energy. A source either stores energy or releases it in a way that can be changed to more useful forms. Fossil fuels store energy in their chemical bonds. A car engine burns gasoline, converting the fuel’s chemical energy into kinetic energy to make the car move. Wind turbines use the motion of the wind to turn turbines, which then produce electricity. Solar cells transfer light energy into electrical energy.

The conservation of energy means that in a perfect system, the total amount of energy will be conserved as it changes from one form to another. Let’s go back to the dog sled. The conservation of energy means that in a perfect system, however much kinetic energy the dogs transfer to the sled, the sled will have the same amount as it moves over the ground.

However, in the real world, some of that energy transfers out of the system. Friction is one way that energy is transferred out of a system. Friction is a force that slows motion whenever two objects rub against each other. Friction slows motion because it causes some of the energy of the moving object to change into heat. Friction is why your hands feel hot after you rub them together. It’s important to point out that energy isn’t disappearing as a result of friction. Instead, it’s transferring out of the system. Heat is evidence that energy has transferred out of the system and into the surrounding environment.

Science Lesson: Exploring Friction and Motion

In this lesson, students build on their knowledge of energy transfer by exploring how energy can change forms and be transferred into and out of energy systems. Students explore how sled dogs need energy to pull a sled over the snow, and how energy changes as it transfers from the dogs to the sled.

Science Big Ideas

• Energy is the ability to do work (any change in position, speed, or state of matter due to force). Energy can be stored or in motion.
• Potential energy is stored energy. It has the potential, or the ability, to do work, but it isn’t actively doing work. Kinetic energy is the energy of motion, which means it is energy that is actively doing work.
• All living things are energy systems because they are sets of connected parts that change an input of energy to a different output of energy.
• There are many examples of energy systems in the world.
• For an object to change its motion (such as beginning to move, stopping, or changing its direction or speed), a force has to transfer energy to or from the object.
• Friction happens whenever two objects rub against one another. As they move against one another, the force of their rubbing causes energy to transfer into the environment.

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Science Essential Questions

• How is kinetic energy different from potential energy?
• Why is chemical energy a form of potential, not kinetic energy?
• Why does a running dog have kinetic energy?
• Why do all living things need food?
• How is chemical energy changed within the body of a living thing, such as a dog or a person?
• Why is a dog sled an example of an energy system?
• Why is force necessary to change the motion of an object, such as a dog sled?
• What causes friction to occur? Why does friction affect the motion of an object?
• Why do dog sleds work well in the snow and ice but not on rougher surfaces like grass or gravel?
• What evidence do you have that ice causes less friction than grass or gravel?

Common Science Misconceptions

Misconception: Friction is always “bad” because it slows motion.
Fact: Friction isn’t good or bad. It does slow motion, but it also helps us move. Friction between our feet and the ground allows us to walk easily. This is why walking on ice is so hard—there is very little friction between our feet and the ground.

Science Vocabulary

Chemical Energy : energy stored in the bonds of atoms and molecules

Energy : the ability to do work

Energy System :  a set of connected parts that change an input of energy to a different output of energy

Force :  a push or pull that acts on an object, changing its speed, direction, or shape

Friction :  a force that slows motion whenever two objects rub against each other

Gravitational Energy :  the energy stored in an object as a result of its vertical position or height above the ground

Kinetic Energy :  the energy of motion

Potential Energy :  energy that is stored

Work :  any change in position, speed, or state of matter due to force

Lexile(R) Certified Non-Fiction Science Reading (Excerpt)

Making the Sled Move

The dogs that pull the sled form part of an energy system that includes the dogs, the sled, and the driver of the sled. This person is called a musher.

The number of dogs on a team is important. Every sled is pulled by a team of 12 to 16 dogs. It takes a large amount of force to pull a grown adult, as well as their supplies, across the ground. A force is a push or pull that acts on an object, changing its speed, direction, or shape. Force is always necessary to transfer energy into or out of a system.

Picture a dog sled. If nothing pushes or pulls the sled, it cannot move. When the dogs are attached to the sled and they begin to run, they pull the sled. This input of energy is a pulling force that transfers the dogs’ kinetic energy to the sled. This transfer of energy causes the sled to move because it now also has kinetic energy. The movement of the sled is an output of energy.

Transferring Energy

Energy is never created or destroyed. But it can be changed from one form to another. It can also transfer into or out of an object or system.

For example, in a perfect system, a moving sled will have the same amount of energy as the energy that the dogs transferred to the sled. But in the real world, some of that energy transfers out of the system.

Friction is one way that energy is transferred out of a system. Friction is a force that slows motion when two objects rub against each other. Friction slows motion because it causes some of the energy of the moving object to change into heat. Friction is why your hands feel hot after you rub them together.

We know that energy is not destroyed as a result of friction. Instead, it transfers out of the system. The heat around the object tells us that energy has transferred out of the system.

The Speed of the Sled

Friction is the main reason that dog sleds work well in the snow but not on rougher surfaces like grass or gravel. Snow and ice are much smoother than other surfaces. This means there is less friction that occurs when objects like sleds move over the snow.

Because less energy transfers out of the system when a sled is pulled over snow or ice, less energy is needed to pull the sled than would be needed on grass or gravel.

Other factors also influence how fast the sled will move. For example, the more massive the sled is, the more force will be needed to move it.

And the more dogs there are attached to the sled, the more force they will produce to pull the sled. This means they will be able to transfer more energy to the sled and make it move faster. This is why the Iditarod has rules about how many dogs can pull a racer to make the race fair. A team of 30 dogs would be able to move the sled much faster than a team of 6 dogs.

Hands-on Science Activity

In this lesson, students configure out how the phenomena of surface friction affects the amount of force needed to pull a sled over surfaces with different textures. Students build a system involving moving objects to test and gather data as they try to explain how surfaces affect friction in systems, which affects the motion of different objects.

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Science Standards

Standards citation: NGSS Lead States. 2013. Next Generation Science Standards: For States, By States. Washington, DC: The National Academies Press. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.