Newton's Third Law

Summary

In this AP Physics essentials video, Mr. Andersen explores Newton's third law, the law of action and reaction. He explains how forces are reciprocal, using examples like a cannon firing and a person pushing a wall or a bowling ball. The video emphasizes the importance of free-body diagrams to visualize and understand action-reaction pairs and calculate overall acceleration. It illustrates scenarios both on Earth and in space, highlighting how the absence of a reaction force, like the ground's normal force, affects motion. The lesson aims to help viewers grasp Newton's third law and apply it to solve physics problems.

Takeaways

• 🔵 Newton's third law, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction.
• 🎯 When a force is applied to an object, such as launching a cannonball, the object exerts an equal and opposite force on the source of the action, causing recoil.
• 🧩 Action-reaction pairs are always equal in magnitude and opposite in direction, and can be identified using free body diagrams.
• 🚫 Free body diagrams should not include internal forces within the body or forces exerted by the body on other objects.
• 👤 Pushing against a wall results in an equal and opposite force from the wall, but since there's friction with the ground, there's no net acceleration.
• 🌌 In space, without a ground to push against, applying a force to an object results in both the object and the person applying the force accelerating away from each other.
• 🏋️‍♂️ The mass of an object affects how it accelerates in response to a force; a person with a greater mass will accelerate less than a lighter object like a bowling ball.
• 🚶‍♂️ When walking, the force applied to the ground is met with an equal and opposite force, allowing for movement due to the interaction with the ground.
• 📊 Free body diagrams are essential for visualizing and calculating the net force and acceleration on an object by considering only external forces.
• 📚 Understanding Newton's third law and constructing free body diagrams are crucial for analyzing the dynamics of physical systems and solving physics problems.

Q & A

What is Newton's third law of motion?

Newton's third law of motion, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Why does a cannon recoil when it fires a cannonball?

A cannon recoils when it fires a cannonball because of Newton's third law. The force exerted by the cannon on the cannonball is met with an equal and opposite force exerted by the cannonball on the cannon, causing it to move backward.

What is the significance of action-reaction pairs in physics?

Action-reaction pairs are significant in physics because they illustrate the mutual nature of forces between objects. These pairs are always equal in magnitude and opposite in direction, and they help in understanding the dynamics of how objects interact.

How can a free-body diagram help in identifying forces and acceleration?

A free-body diagram is a visual tool used in physics to represent all the external forces acting on an object. It helps in identifying where these forces are applied and calculating the overall acceleration of the object by considering the net force.

What should be included in a free-body diagram according to the script?

In a free-body diagram, one should include the body itself and all the external forces acting on it. It's important to note that internal forces, forces exerted by the body, and other bodies should not be included.

Why doesn't a person move when they push against a wall?

A person doesn't move when they push against a wall because the wall exerts an equal and opposite reaction force. Additionally, the person's feet apply a force on the ground, and the ground provides an equal and opposite force, resulting in no net force and no acceleration.

What happens when a person pushes a bowling ball in space, where there is no ground?

In space, where there is no ground, when a person pushes a bowling ball, both the person and the ball will accelerate away from each other due to the action-reaction forces. The person's acceleration will be less than the ball's because of their greater mass.

How does walking or running across the floor relate to Newton's third law?

When walking or running, a person applies a force into the floor, and the floor applies an equal and opposite force back. This interaction allows the person to move forward, demonstrating the action-reaction principle in everyday motion.

Why is it important to consider the mass of objects when analyzing action-reaction pairs?

The mass of objects is important when analyzing action-reaction pairs because it affects the acceleration of the objects. According to Newton's second law, F = ma, where F is the force, m is the mass, and a is the acceleration. A greater mass results in less acceleration for the same force.

What is the role of friction in the scenarios described in the script?

Friction plays a crucial role in scenarios by providing a resistive force that opposes the motion of objects. In the script, friction between the person's feet and the ground prevents them from moving when pushing against a wall, demonstrating the balance of forces.

Outlines

00:00

🚀 Newton's Third Law: Action and Reaction

This paragraph introduces Newton's third law, also known as the law of action and reaction. It explains that for every action, there is an equal and opposite reaction. The concept is illustrated with examples such as firing a cannon, pushing against a wall, and pushing a bowling ball. The paragraph emphasizes the importance of identifying action-reaction pairs and using free-body diagrams to understand the overall acceleration of objects. It also highlights the difference in scenarios on Earth, where there is a ground to push against, and in space, where the absence of a ground leads to different outcomes.

05:04

🌌 Applying Newton's Third Law in Different Scenarios

The second paragraph delves deeper into applying Newton's third law in various situations, including pushing a bowling ball and walking on the floor. It contrasts the effects of applying force in the absence of external resistance, such as in space, versus on Earth where the ground provides a reaction force. The paragraph also discusses the construction of free-body diagrams, emphasizing what to include (external forces on the body) and what to exclude (internal forces and forces exerted by the body). Examples are provided to illustrate how to create these diagrams and analyze the forces acting on different objects.

Keywords

💡Newton's Third Law

Newton's Third Law, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction. This law is fundamental to understanding forces and their effects in physics. In the video, Mr. Andersen uses the example of a cannon firing a ball to illustrate this concept, explaining that the cannonball exerts an equal and opposite force on the cannon, causing it to recoil. This law is central to the video's theme of explaining how forces interact.

💡Action and Reaction Pairs

Action and reaction pairs are the forces that are equal in magnitude and opposite in direction, as described by Newton's Third Law. These pairs are crucial for analyzing the motion of objects when forces are applied. In the video, Mr. Andersen clarifies that these pairs are always present when one object exerts a force on another, and they are the basis for understanding the scenarios presented, such as pushing a wall or a bowling ball.

💡Free Body Diagram

A free body diagram is a tool used in physics to visualize all the forces acting on an object. It helps to determine the net force and acceleration of the object by breaking down the forces into their components. In the video, Mr. Andersen emphasizes the importance of free body diagrams in identifying action-reaction pairs and calculating overall acceleration, especially in complex scenarios like those in space where gravity and normal forces may not be present.

💡Recoil

Recoil is the backward movement of an object as a reaction to the force exerted on another object. It is a direct consequence of Newton's Third Law. The video uses the example of a cannon firing to explain recoil, where the cannonball exerts a force on the cannon, causing it to move backward. Recoil is a key concept in understanding the mutual forces between objects.

💡Magnitude

In the context of physics, magnitude refers to the size or strength of a physical quantity, such as force. The video script mentions that action and reaction forces are equal in magnitude, meaning they have the same strength but opposite directions. This concept is essential for understanding the balance of forces and the resulting motion of objects.

💡Direction

Direction is a fundamental aspect of vector quantities in physics, such as force, velocity, and acceleration. The video script explains that action and reaction forces are opposite in direction, which means they act along the same line but in reverse. Understanding direction is crucial for analyzing the motion of objects and the effects of forces.

💡Acceleration

Acceleration is the rate of change of velocity of an object with respect to time. It is a key concept in Newton's laws of motion. In the video, Mr. Andersen discusses how the application of forces results in acceleration, using examples like pushing a bowling ball or a person pushing against a wall. The concept of acceleration is integral to understanding the effects of action-reaction forces.

💡Net Force

Net force is the vector sum of all the forces acting on an object. It determines the object's overall acceleration according to Newton's Second Law (F = ma). The video script uses the concept of net force to explain scenarios where action-reaction pairs cancel each other out, resulting in no net force and no acceleration, such as when a person pushes against a wall.

💡Frictional Force

Frictional force is the force that resists the relative motion or tendency of such motion between two surfaces in contact. In the video, Mr. Andersen mentions frictional force as a reaction to the force applied by a person on the ground while walking or running. This force is essential for understanding how objects move on surfaces and is a part of the action-reaction concept.

💡Mass

Mass is a measure of the amount of matter in an object and is an intrinsic property that determines the object's resistance to acceleration. The video script discusses how mass affects the acceleration of objects when forces are applied, using the example of a person and a bowling ball in space. The concept of mass is crucial for understanding the differences in acceleration between objects of different sizes and weights.

Highlights

Newton's third law, also known as the law of action and reaction, states that forces between objects are equal and opposite.

The recoil of a cannon is explained by the action-reaction force pair between the cannon and the cannonball.

Action-reaction pairs are always equal in magnitude and opposite in direction.

Free body diagrams are essential tools for visualizing and analyzing action-reaction forces.

When pushing a wall, the wall exerts an equal and opposite force, but you don't move due to the ground's reaction force.

In space, without a ground to push against, applying a force to a wall would cause both you and the wall to accelerate.

The difference in acceleration between you and a bowling ball when pushed is due to mass differences.

Without a floor, walking or running would not be possible as there is no action-reaction force to propel you forward.

Free-body diagrams should include only the body of interest and external forces acting on it, excluding internal forces and forces exerted by the body.

A person standing on the ground experiences gravity and an equal normal force from the ground.

When a person pushes a wall, the wall's reaction force is included in the free-body diagram, but not the person's force on the wall.

Frictional force on the ground opposes the person's force, resulting in no net force or acceleration.

In space, without gravity, the normal force is absent, and the wall's reaction force becomes the only external force acting on a person pushing it.

For the bowling ball, gravity is the only external force acting on it before being pushed, as there is no normal force.

The acceleration of the bowling ball is directed down and to the right due to the person's push.

Understanding and identifying action-reaction pairs is crucial for applying Newton's third law.

Free-body diagrams help in identifying all external forces acting on an object, which is vital for analyzing motion.

Transcripts

Hi. It's Mr. Andersen and this AP Physics essentials video 41. It is on Newton's third

law which is sometimes referred to as the law of action and reaction. In other words

when you launch a canon ball out this side of the canon you are applying a force to that

canon ball. But the canon ball is exerting an opposite and equal force on the canon.

And that is going to cause recoil. And that is why these giant ropes are on this canon

so it does not just go shooting off across the deck. And so according to Newton's third

law if an object exerts a force on another object, then that other object is going to

exert an opposite and equal force on the first object. We call these action reaction pairs.

This is the action and this is the reaction. And those action-reaction pairs remember will

always be equal in magnitude and opposite in their direction. Now these are sometimes

hard to identify. So you can use a free body diagram to identify where those forces are

and figure out the overall acceleration. So you can use a free body diagram to identify

where those forces are and figure out the overall acceleration. So imagine pushing into

a wall, applying a force to a wall. You are applying an action into the wall and the wall

is actually applying an opposite and equal reaction on you. The wall is pushing on you.

Now you do not go anywhere and the reason why is you apply a force into the ground,

an action, and there is a reaction force from the ground pushing back on you. And so the

net force on you is the same. You do not accelerate anywhere. Now let's put you in space and do

this same scenario. And let's just make sure that that wall does not move anywhere. So

now you apply a force into the wall. It applies an opposite and equal force back on you. And

so now since there is no ground to push against you are simply going to quickly accelerate

and then coast away from the wall. Let's look at another scenario. You are pushing a bowling

ball. And we have kind of frozen it at the point at which you are going to push the bowling

ball. So you apply a force into the bowling ball and it is going to apply an opposite

and equal force back on you. Now the ball is going to move, but you are not going to

move. And the reason why is you are applying a force against the floor and it is applying

a force back on you. So you do not move but the ball is going to quickly move away like

that. Let's do this scenario again in space. What happens? You apply a force into the bowling

ball. It applies an opposite and equal force back on you. What is going to happen now?

There is no ground. You are both going to accelerate away from each other. Why do you

not move as fast as the ball? Because you have a greater mass. Let's look at just walking

or running across the floor. You apply a force into the floor and it applies an opposite

and equal force back on you. That does not make sense. But if you think about it in space,

what happens if there is no floor? There is no action. You apply an action, nothing happens

and there is nothing to push against. You are simply stranded there. And so these action-reaction

pairs make sense but it is sometimes hard to figure out what is pushing on what. And

so we use a free-body diagram to do that. And this is the first time we have used free-body

diagrams. And so what do you include in a free-body diagram? The body. So you are going

to include the body. And you are going to include any external forces on that body.

Now it is more important what you do not include in a free-body diagram. You do not include

any other bodies. No internal forces, you for example pushing on yourself or parts of

an object pushing on other parts of the object. And also you do not include any of the forces

exerted by that body. That is the one mistake that students tend to make. And so let's do

a free-body diagram, a few examples. And so if you have a person standing right here on

the ground, let's do a free-body diagram of their body. So we start, I usually draw a

square like this. This represents the body. What are the forces that are acting on that

body? Well gravity is clearly acting on it in the vertical direction. But the person,

since they do not accelerate down into the earth, there must be an opposite and equal

reaction. That is the ground pushing back on the person. And we call that the normal

force. And so those are going to be the two forces in our free body diagram. Let's change

it up. Now we are looking at this person pushing on the wall. So what did we start with? It

is always easiest to start with the gravity. So gravity is pushing down. That is an external

force. What else do we have? We have that normal force, it is not moving down into the

ground. What else do we have? Let's start identifying those action-reaction pairs. So

you are pushing against the wall. But that is a force exerted by the bodies. So we do

not include it. We do include the wall pushing back on you. The force of the wall. If we

look at the ground, remember, you are applying a force on the ground, but there is a frictional

force in the opposite direction. And so there is no net force there. No net acceleration.

If we put this scenario in space what do we have? Now we have the body again. But there

is no gravity. And therefore there is no normal force. We are applying a force into the wall

and it is applying a force back on us. So that is going to be your free-body diagram.

Where is the net force? It is to the left. And so where is the acceleration? It is to

the left as well. Let's breakdown these two objects. So we have object one which is the

person. And so we have gravity and normal force. Those are easy to do right away. You

are applying a force to the bowling ball and so it is going to apply an opposite and equal

force back on you. And then again you are applying a force in the ground and it is applying

an opposite force on you. And so we have that frictional force. Now let's shift to the bowling

ball itself. So we again have the object. Is gravity acting on the bowling ball? For

sure. But you are quickly going to let it go. There is no normal force. You have just

kind of frozen this picture in time so there is no normal force. What is the only other

external force? It is going to be the person pushing on it. And so where is the acceleration

going to be? Down and to the right. And so did you learn to construct explanations using

Newton's third law or action-reaction? Can you identify those action-reaction pairs?

And then finally, can you use a free-body diagram to identify not only the object but

all the external forces acting on it? I hope so. And I hope that was helpful.