# Action-Reaction Forces

In the last unit, students explored how shelters help animals survive in their environment. In this unit, students investigate the relationship between forces and motion, building propeller cars to observe action-reaction forces, the relationship between the distance their car travels and the amount of force applied, and how friction affects motion. This page highlights the first lesson in this unit which has students exploring the science phenomena of action and reaction forces.

## Science Background for Teachers:

The science background section gives teachers in-depth information about the science phenomena being studied (forces and motion).

Forces play a major role in running. A force is a push or a pull that acts on an object, changing its speed, direction, or shape.

Forces are needed to change the motion of any object because of the law of inertia, which is also called Newton’s first law of motion. This law is named in honor of the English scientist Isaac Newton, who defined the three laws of motion. According to the law of inertia, an object at rest will remain at rest and an object in motion will remain in motion unless an outside force acts on it.

For example, propulsion is the force applied by the runner’s muscles to move the runner forward. To propel means to push forward. How much the runner’s speed increases, also called acceleration, depends on how much force the runner exerts on the ground. This is because of Newton’s second law of motion, which states that force equals mass times acceleration. A greater force applied to an object will cause that object to accelerate more.

An action is a change caused by a force. A reaction is an action that occurs as a result of another action. We can see this when a balloon is released and flies around the room. The action of the air going backwards out of the balloon causes the reaction of the balloon moving forward.

Then, every time the runner’s foot pushes down on the ground, the ground pushes back with an equal and opposite force. This is because of the action-reaction law, which is also Newton’s third law. According to this law, for every action there is an equal and opposite reaction.

## Science Lesson: Discovering Action-Reaction Forces

In this lesson, students explore the relationship between forces and motion. They build a propeller car to observe action-reaction forces and then investigate how the car’s motion is affected by the magnitude of the force applied to it.

## Science Big Ideas

• A force is a push or pull that acts on an object, changing its speed, direction, or shape.
• An action is a change caused by force, and a reaction is an action that occurs as a result of another action.

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## Science Essential Questions

• What are different examples of forces?
• How do forces relate to motion, which is the movement of an object from one place to another?
• What has to happen to make a stationary object move?
• How much force do you use when you push something harder compared to when you push it softly?
• What action-reaction forces occur when you walk from one place to another?
• What action-reaction forces occur when you are standing still?

## Common Science Misconceptions

Misconception: Objects at rest have no forces acting on them.
Fact: There are forces acting on everything on Earth at all times. For example, when you stand on the ground, gravity pulls you toward the center of Earth. In reaction, the ground has its own force that pushes back with an equal and opposite force.
Misconception: Forces do not always come in pairs.
Fact: Forces always come in pairs. These are called action-reaction forces—for every action, there is an equal and opposite reaction.

## Science Vocabulary

Action : a change caused by force; moving to a new place and going faster are two actions

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

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

Energy :  the ability to do work

Motion :  the movement of an object from one place to another

Propel :  to push forward

Reaction :  an action that occurs as a result of another action

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

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

Action-Reaction Forces

Forces always come in pairs. This is known as “action-reaction.” An action is a change caused by force. Moving to a new position is an action. Going faster is also an action.

A reaction is an action that occurs as a result of another action. Gravity pulls you toward the center of Earth. In reaction, the ground has its own force that pushes back. This keeps you from sinking into the ground.

All motion is caused by action-reaction forces. When Noah pushes his foot against the ground, the ground pushes back. This propels him forward. To propel means to push forward.

The more Noah pushes down, the more the ground pushes back. This propels him farther than when he pushes down with less force.

Propellers

Boats and planes also use action-reaction forces to move. They use propellers. Propellers make boats and planes move by spinning around very quickly.

A propeller pushes or pulls an object forward. It does this by pushing a mass of air or water backwards.

## Hands-on Science Activity

large force?” Students record and compare the distance their propeller car traveled when a small force (15 propeller turns) and a large force (30 propeller turns) are applied. Students use the data they gather from their investigation to construct an explanation that either supports or rejects their prediction about whether propeller cars travel further with a large or small force. After students reach consensus and complete the investigation, they come together as a class, presenting their results.

## Science Assessments

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• Vocabulary Check
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• Concept Check Assessment
• Concept Map Assessment
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