8.5 - Third Law of Motion
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Understanding Action-Reaction Forces
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Today, we will discuss Newton's Third Law of Motion. Do any of you know what it states?
It says that for every action, there is an equal and opposite reaction!
Exactly! This means when one object applies a force to another, the second object exerts a force back. Can anyone give an example?
When I jump off a diving board, the board pushes back on me, causing me to launch into the air!
Great example! This is an action-reaction pair. Can someone explain why the diving board goes down when you jump?
Because my weight pushes it down, which is the action force, and the board pushes up against my feet with an equal reaction!
That's correct! Remember, the pair of action and reaction forces act on different objects. Now let’s recap what we learned: action and reaction forces are equal in size but opposite in direction.
Real-life Applications of the Third Law
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Now, let’s explore how this law influences our activities. For instance, how does a swimmer use action-reaction forces?
They push the water backwards to move forward in the pool!
That's right! The force they apply on the water generates a forward movement. How about in football?
When a player kicks the ball, the ball pushes back with the same force!
Exactly! The kick is the action, and the ball’s reaction is to exert an equal force back on the player’s foot. Let’s summarize this: in any sport or activity, when forces are applied, the reaction is always equally matched in the opposite direction.
Understanding Recoil
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Next, let's talk about recoil. Who can explain how it relates to Newton's Third Law?
When a gun is fired, the bullet moves forward, and the gun pushes back, which is the recoil!
Exactly! The bullet moves forward as an action force, while the corresponding reaction force is the backward motion of the gun. Can anyone think of other scenarios where we observe recoil?
What about a firework rocket? It pushes exhaust gases down and moves up!
Absolutely! The rocket demonstrates Newton’s Third Law perfectly. To conclude this session, remember that recoil is a tangible example of how action and reaction forces are always present.
Introduction & Overview
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Quick Overview
Standard
Newton's Third Law of Motion explains that when one object exerts a force on another object, the second object exerts a force back on the first that is equal in magnitude but opposite in direction. This law illustrates the interaction between objects and helps to understand phenomena such as why a gun recoils after firing or why a boat moves backward when a sailor jumps out.
Detailed
Third Law of Motion
Newton's Third Law of Motion articulates a foundational principle in mechanics: for every action, there is an equal and opposite reaction. This law indicates that forces always come in pairs; when one body exerts a force on another body, the second body consequently exerts an equal but oppositely directed force back on the first body. This is core to understanding interactions in physics, as it reveals the relationship and interplay between different objects under force.
Key Points:
- Action and Reaction Forces: These forces are equal in magnitude and opposite in direction, acting on different bodies simultaneously.
- Examples: Common examples include the recoil experienced when a gun is fired and the backward movement of a boat when a sailor jumps off.
- Applications in Sports and Everyday Life: The law of action and reaction is observed in various situations, such as a football player pushing the ground to kick a ball or a person swimming pushing the water backward to propel forward.
Understanding this law is critical for analyzing motion and understanding how forces interact in paired systems.
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Understanding Action and Reaction Forces
Chapter 1 of 4
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Chapter Content
The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first. These two forces are always equal in magnitude but opposite in direction.
Detailed Explanation
Newton's third law of motion tells us that every action has an equal and opposite reaction. This means if Object A pushes on Object B, then Object B pushes back on Object A with the same amount of force but in the opposite direction. Though both forces are equal, they act on different objects.
Examples & Analogies
Imagine you're standing on a skateboard and you push against a wall. As you push the wall, you will move backward on the skateboard due to the reaction force of the wall pushing back on you.
Different Masses, Different Accelerations
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Chapter Content
Even though action and reaction forces are equal in magnitude, they may not produce accelerations of equal magnitudes because each force acts on different objects that may have different masses.
Detailed Explanation
When two objects exert forces on one another, the effect of those forces can differ due to their masses. For instance, if a bullet is fired from a gun, the bullet accelerates quickly because it has a small mass, while the gun experiences a much smaller acceleration because it has a much greater mass, even though the forces are equal.
Examples & Analogies
Consider a basketball and a bowling ball. If you push them both with the same force, the basketball will roll away much farther and faster than the bowling ball. This illustrates how different masses affect the acceleration produced by equal forces.
Illustrating Action and Reaction with Examples
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Chapter Content
In the game of football, when a player kicks the ball, they exert a force on the ball, and the ball exerts an equal and opposite force back on the player’s foot, which is often felt as a kick back.
Detailed Explanation
The interaction between the player's foot and the ball demonstrates action and reaction: the force of the foot on the ball and the force of the ball back on the foot are equal and opposite. This principle governs many interactions in sports and daily activities.
Examples & Analogies
Think about how you feel when you jump off a small boat onto the shore. As you push yourself forward, the boat moves backward. Here, your jump is the action, and the boat’s movement backward is the reaction.
Real-World Applications of the Third Law
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Chapter Content
The third law of motion can also be illustrated when a sailor jumps out of a rowing boat. As the sailor jumps forward, the force on the boat moves it backward.
Detailed Explanation
When the sailor exerts a force against the boat to move forward, the boat pushes back in the opposite direction, which results in it moving backward. This is a direct application of Newton's third law, showcasing how forces interact between different bodies.
Examples & Analogies
Imagine you are standing on a skateboard or rollerblades, and you throw a heavy ball forward. As the ball goes one way, you roll backward. This happens because of the reaction force acting on you, displaying Newton's third law in a fun and relatable way.
Key Concepts
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Action-Reaction Forces: For every action, there is an equal and opposite reaction.
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Recoil: The backward motion experienced by an object when it exerts a force forward.
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Momentum: The quantity of motion that an object has, dependent on its mass and velocity.
Examples & Applications
When a swimmer pushes the water backward, he moves forward due to the reaction force.
A gun recoils backward when it fires a bullet forward, illustrating action-reaction pairs.
Memory Aids
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Rhymes
For every action taken, a reaction's in line; equal and opposite in force, every time.
Stories
Imagine a ball being kicked. As it rolls away, the feet that kicked it feel a push backward—a reminder that every action evokes an equal feeling in return.
Memory Tools
A - Action, R - Reaction: A reaction occurs for each action's direction!
Acronyms
EOR
Equal Opposite Reaction.
Flash Cards
Glossary
- Action Force
A force exerted by one object onto another.
- Reaction Force
The force exerted by the second object back onto the first object, equal in magnitude and opposite in direction.
- Recoil
The backward movement experienced by a gun or rocket when it exerts a force forward.
- Momentum
The quantity of motion an object possesses, calculated as the product of its mass and velocity.
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