More on Newton's third law | Forces and Newton's laws of motion | Physics | Khan Academy

Summary

This script delves into Newton's Third Law, clarifying common misconceptions. It emphasizes that the law involves equal and opposite forces acting on different objects, which cannot cancel each other out. The video explains why these forces, despite being equal, result in different accelerations due to mass differences. It also dispels the myth of a delay in the creation of reaction forces, illustrating that they are instantaneous. The script uses examples, such as a box on a table and gravitational forces, to demonstrate how to correctly identify Newton's Third Law partner forces.

Takeaways

• 🔄 Newton's Third Law is often misunderstood; it's more complex than the simple 'action-reaction' phrase suggests.
• 📏 The law states that for every force, there is an equal and opposite force, emphasizing that these are vector quantities with magnitude and direction.
• 📍 The forces described by Newton's Third Law act on different objects, which is crucial for understanding why they don't cancel each other out.
• 🌌 Forces in the universe do not cancel out because Newton's Third Law forces are exerted on different objects, not the same object.
• 👥 The law applies to interactions between two objects, such as object A exerting a force on object B, and vice versa.
• 🌍 Even vastly different-sized objects, like a planet and a star, exert forces of equal magnitude on each other according to Newton's Third Law.
• 🔄 The misconception that smaller objects exert less force on larger ones is incorrect; the forces are equal regardless of size.
• ⏱️ Newton's Third Law forces are instantaneous; there is no delay in the creation of the partner force when one force is applied.
• 📉 Forces being equal does not mean the results are equal; acceleration depends on the net force divided by mass, leading to different outcomes.
• 🧩 It's important to correctly identify third law partner forces by considering both interacting objects and not confusing them with other balanced forces.
• 🚫 Forces that are equal and opposite due to other reasons, like the second law (no acceleration, net force is zero), are not third law partner forces.

Q & A

What is the common misconception about Newton's Third Law of Motion?

The common misconception is that for every action, there's an equal and opposite reaction, which is often interpreted as forces canceling each other out universally.

What is a more accurate way to phrase Newton's Third Law?

A more accurate phrasing is that for every force, there's an equal and opposite force, emphasizing that these forces are equal in magnitude but opposite in direction.

Why doesn't Newton's Third Law imply that all forces in the universe cancel each other out?

It doesn't imply that because the forces are exerted on different objects; hence, they do not cancel out.

How are Newton's Third Law forces related to the objects they act upon?

The forces are exerted on different objects, with one being the action force on one object and the other being the reaction force on the second object.

Why do the forces exerted by two different objects on each other not cancel out?

They don't cancel out because they act on different objects, and Newton's Third Law ensures they are equal in magnitude and opposite in direction but do not negate each other due to acting on separate entities.

Can the magnitude of the forces in Newton's Third Law be different if the objects have different masses?

No, the forces must be of equal magnitude regardless of the masses of the objects involved.

Why does a planet orbit a star instead of the star moving around the planet, even though the forces they exert on each other are equal according to Newton's Third Law?

The reason is that while the forces are equal, the resulting accelerations are not, due to the difference in masses. The larger mass of the star results in a smaller acceleration compared to the planet.

Is there a delay in the creation of the reaction force in Newton's Third Law?

No, the reaction force is created instantaneously as soon as the action force is applied.

How can one identify the partner forces in Newton's Third Law?

One can identify the partner forces by listing both objects involved and reversing the labels to find the force on the second object exerted by the first.

Why are some forces equal and opposite but not considered Newton's Third Law partner forces?

Some forces may be equal and opposite due to other reasons, such as the second law of motion (net force being zero for no acceleration), and not because they are acting on different objects as required by the third law.

Can the force exerted by a table on a box be considered a Newton's Third Law partner force to the gravitational force acting on the box?

No, the force exerted by the table is not a Newton's Third Law partner force to the gravitational force. The partner force to gravity would be the force exerted by the box on the Earth.

Outlines

00:00

🔍 Clarifying Newton's Third Law Misconceptions

The paragraph clarifies common misunderstandings about Newton's Third Law, which is often summarized as 'for every action, there's an equal and opposite reaction.' The voiceover emphasizes that this is too vague and suggests a more precise version: 'for every force, there's an equal and opposite force.' It explains that these forces are equal in magnitude but opposite in direction. The paragraph also addresses the misconception that forces should cancel each other out, highlighting that Newton's Third Law applies to forces exerted on different objects, not the same object. It uses the example of object A exerting a force on object B, and object B exerting an equal and opposite force on object A, to illustrate that these forces do not cancel out because they act on different objects.

05:00

📚 Newton's Third Law: Forces on Different Objects

This paragraph delves deeper into Newton's Third Law, explaining that the law pertains to forces acting on two different objects, ensuring that these forces cannot cancel each other out. It refutes the idea that there might be a delay in the creation of the 'partner force' and asserts that these forces are always equal and opposite at every given moment in time. The paragraph uses the example of a person exerting force on a wall and the wall exerting an equal and opposite force back to demonstrate the instantaneous nature of this law. It also discusses the challenge of identifying third law partner forces and suggests listing both objects involved as a method to determine the partner force by reversing the labels.

10:02

🚀 Understanding the Implications of Newton's Third Law

The final paragraph discusses the implications of Newton's Third Law, focusing on the fact that equal forces do not necessarily result in equal accelerations due to differences in mass. It explains that while the forces between two interacting objects are always equal and opposite, the outcomes can differ significantly because of the mass of the objects involved. The paragraph also addresses the misconception that third law partner forces might cancel each other out in certain situations, such as when an object is sitting on a table or accelerating. It concludes by emphasizing the universality of Newton's Third Law and the importance of correctly identifying third law partner forces, which are always exerted on different objects and are equal in magnitude regardless of the circumstances.

Keywords

💡Newton's Third Law

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. In the video, it is explained that this law is often misinterpreted as simply 'for every force, there's an equal and opposite force.' However, the law is more nuanced, emphasizing that these forces are not only equal in magnitude but also opposite in direction and act on different objects. The video uses the example of a person exerting force on a wall and the wall exerting an equal and opposite force back on the person to illustrate this concept.

💡Action and Reaction

The terms 'action' and 'reaction' in the context of Newton's Third Law refer to the forces that two interacting objects exert on each other. The video clarifies that these forces are equal in magnitude and opposite in direction but act on different objects. This is exemplified by the force a person applies to a wall and the force the wall applies back to the person, which are action and reaction forces, respectively.

💡Magnitude

In physics, 'magnitude' refers to the size or quantity of a physical property, such as force. The video emphasizes that the forces described in Newton's Third Law are equal in magnitude, meaning they have the same strength but opposite directions. This is demonstrated through the example of a planet and a star exerting gravitational forces on each other, which are equal in magnitude despite the difference in their sizes.

💡Direction

The 'direction' of a force is a critical aspect of Newton's Third Law, as it dictates that the action and reaction forces are opposite in direction. The video script uses the analogy of a pink vector representing an upward force and a green vector representing the downward force of equal magnitude to visually represent this concept.

💡Vectors

Vectors are quantities that have both magnitude and direction, which is fundamental in understanding forces in physics. The video script explains that forces are vectors, using the example of a pink vector representing an upward force and a green vector representing a downward force of equal magnitude but opposite directions to illustrate Newton's Third Law.

💡Acceleration

Acceleration is the rate of change of velocity of an object and is related to the net force acting on it, according to Newton's Second Law. The video clarifies that even though the forces exerted by two objects on each other are equal in magnitude, the resulting accelerations can be different due to differing masses of the objects. This is exemplified by the Earth and the Moon, where both exert equal gravitational forces on each other, but the Moon accelerates more due to its smaller mass.

💡Mass

Mass is a measure of the amount of matter in an object and plays a crucial role in determining the acceleration of an object under the influence of a force, as per Newton's Second Law. The video script points out that the magnitude of the forces between two objects is independent of their masses, as demonstrated by the Earth and the Moon exerting equal gravitational forces on each other, regardless of their vastly different masses.

💡Instantaneous

The term 'instantaneous' in the context of the video refers to the immediate response of the reaction force when an action force is applied. The video emphasizes that there is no delay in the creation of the reaction force as per Newton's Third Law, using the example of a foot exerting force on a wall and the wall exerting an equal and opposite force back on the foot immediately.

💡Misconceptions

The video script addresses several 'misconceptions' about Newton's Third Law, such as the belief that forces might cancel each other out or that there could be a delay in the reaction force. It aims to clarify these misunderstandings by explaining the correct interpretation of the law and providing examples to illustrate the points.

💡Partner Forces

In the script, 'partner forces' refers to the action and reaction forces described by Newton's Third Law. The video explains that these forces are always equal in magnitude and opposite in direction, and they act on different objects. It also provides a method to identify partner forces by reversing the labels of the interacting objects, as seen in the examples of the Earth and a box on a table.

Highlights

Newton's Third Law is often misunderstood, with a common misconception that every action has an equal and opposite reaction.

A more accurate phrasing is that for every force, there's an equal and opposite force, emphasizing the vector nature of forces.

The 'equal' signifies forces of the same magnitude, while 'opposite' indicates they have opposite directions.

A common confusion is the belief that forces in the universe should cancel each other out, but they don't because they act on different objects.

Forces described by Newton's Third Law are always exerted on different objects, which is crucial for understanding the law.

The law implies that the force on object A exerted by object B has a corresponding force on object B exerted by object A.

Misunderstandings include the belief that larger objects exert greater forces, which is incorrect according to Newton's Third Law.

Even if one object is significantly smaller or less massive, it exerts the same magnitude of force on a larger object.

The acceleration resulting from a force depends on the mass of the object, not the size or mass of the exerting object.

There is no delay in the creation of the partner force in Newton's Third Law; it occurs instantaneously.

The identification of third law partner forces can be facilitated by listing both interacting objects and reversing the labels.

An example of a common misunderstanding is the force exerted by a table on a box and vice versa, which are not third law partner forces.

The forces between the Earth and an object, such as a planet, are third law partner forces and are always equal in magnitude regardless of the object's motion.

The gravitational force exerted by the Earth on an object and the object's force on the Earth are always equal, illustrating Newton's Third Law.

Forces that seem equal and opposite might not be third law partner forces if they act on the same object and can cancel each other out.

In a scenario of acceleration, such as in an elevator or rocket, the forces that were previously equal may no longer be, but third law partner forces remain equal.

Newton's Third Law is a fundamental principle that applies universally, regardless of the situation or the objects involved.

Transcripts

- [Voiceover] We should talk a little more

about Newtons's Third Law,

because there are some deep misconceptions

that many people have about this law.

It seems simple, but it's not nearly

as simple as you might think.

So people often phrase it as,

for every action there's an equal and opposite reaction.

But that's just way too vague to be useful.

So a version that's a little better,

says that for every force, there's an equal

and opposite force.

So this is a little better.

The equal sign means that these forces

are equal in magnitude.

And this negative sign means they're just different

by the direction of the vector.

So these are vectors, so this says that this pink

vector F, has the opposite direction,

but equal in magnitude to this green vector F.

But to show you why this is still a little bit too vague,

consider this, if this is all you knew about

Newtons's Third Law, that for every force,

there's an equal and opposite force,

you might wonder, if you were clever,

you might be like, wait a minute,

if for every force F, right, there's got to be

a force that's equal and opposite.

Well why doesn't that just mean

that every force in the universe cancels?

Shouldn't every force just cancel then, at that point?

Doesn't that just mean that there's

no acceleration that's even possible?

Because if I go and exert a force F on something,

if there's gonna be a force negative F,

doesn't that mean that no matter what force I put forward,

it's just gonna get cancelled?

And the answer no, and the reason it's no

is because these two forces

are exerted on different objects.

So you have to be careful.

So the reason I say that this statement

of Newtons's Third Law is still a little bit too vague,

is because this is really on different objects.

So if this is the force on object A,

exerted by object B, then this force over here

has to be the force on object B, exerted by object A.

In other words, these forces down here are exerted

on different objects.

I'm gonna move this over to this side.

I'm gonna move this over to here.

Let's draw two different objects

to show explicitly what I mean.

So if there was some object A,

so I put some object A in here.

Just wanna make sure there's an object A.

Let's say this is object A, and it had

this green force exerted on it, F.

So this object right here is A.

Well, there's gonna be another object, object B.

We'll just make it another circle.

So we'll make it look like this.

So here's object B.

And it's gonna have this pink force,

F, negative F exerted on it.

So I'm gonna call this object B.

Now we're okay, now we know these forces can't cancel,

and the reason these forces can't cancel,

is cause they're on two different objects.

But when you just say that Newtons's Third Law,

is that every force has an equal and opposite force,

it's not clear that it has to be on different objects.

But it does have to be on different objects.

So these Newtons force law pairs,

often times is called force pairs,

or Newton's third law partner forces,

are always on different objects.

So the convention I'm using is that the first letter

represents the object that the force is on.

So this A represents that this green force F

this green force F, is on A and it's exerted by B.

And this shows that it's exerted on B,

because the first letter's on the first one,

and it's exerted by the second object, A.

So this pink force is exerted on B.

This green force is exerted on A.

They're equal and opposite, they do not cancel,

they cannot cancel because they're not on the same object.

So that's why these don't cancel.

And they are the same magnitude,

even if the two objects are not the same size.

This is another misconception,

if object A is a planet, a big planet.

Or maybe a star, this is yellow, it looks like a star.

Let's say this is some big star,

and this is some smaller planet orbiting that star.

This is not to scale, unless this planet was enormous.

So this is some planet, but this planet could be

hundreds, thousands of times, millions of times

less massive than this star but it would still

exert the same force.

So if this star is pulling on the planet

with this pink force negative F,

then this planet has to be pulling on the star

with this green force F and they have to have

the same magnitude, even if they are different sizes.

So people quote Newtons's Third Law,

but sometimes they don't really believe it.

If I told you this planet was a million times

less massive than this star, people would want to say

that well, then the star obviously pulls more on the planet,

than the planet pulls on the star.

But that's not true according to Newtons's Third Law.

And Newtons's Third Law says that they have to be the same,

even if they're different sizes.

So if this was the earth and this was the moon,

the earth pulls on the moon, just as much

as the moon pulls on the earth.

And you might still object, you might say,

wait that makes no sense, I know the star just basically

sits there and the planet gets whipped around in a circle.

How come this planet's getting whipped around

and the star's just staying put?

That's because, just because the forces are equal,

that doesn't mean that the result is equal.

In other words, the forces could be equal,

but the accelerations don't have to be equal.

Acceleration is gonna be the net force divided by the mass.

So even if the force is the same,

you divide by that mass, you'll get

a different acceleration and that's why the result of

the force does not have to be the same,

even though the forces do have to be the same,

because of Newtons's Third Law.

Another misconception people sometimes make,

is they think there might be a delay in the creation

of this Newtons's Third Law partner force.

And people think, maybe if I exert this first force

fast enough, I can catch the universe sleeping,

and there might be some sort of delay

in the creation of this other force.

But that's not true, Newtons's Third Law is universal.

No matter what the situation,

no matter what the acceleration or non acceleration,

or motion or no motion, whether one object

is bigger or smaller, if their Newtons's Third Law

partner forces, they are equal they are opposite

and they are always equal and opposite,

at every given moment in time.

So even if I came in all guns a blazing,

Chuck Norris style, trying to dropkick some wall.

That does not look like the correct form for a drop kick.

But even if I came in, flying at this wall,

as soon as I start to make contact with the wall,

I'm gonna exert a force on the wall,

and the wall has to exert a force back.

So I'd exert a force on the wall to the right.

And this would be the force on the wall, by my foot.

There'd have to be an equal and opposite force

instantly transmitted backwards, on my foot.

So this would be the force on my foot, by the wall.

This happens instantaneously, there is no delay.

You can't kick this wall fast enough,

for this other force to not be generated instantaneously.

As soon as your foot starts to exert any force on the wall

what so ever, the wall is gonna start exerting

that same force back on your foot.

So Newtons's Third Law is universal,

but people still have trouble identifying

these third law partner forces.

So one of the best ways to do it,

is by listing both objects,

as soon as you list both objects,

well to figure out where the partner force is,

you can just reverse these labels.

So I know over here, if one of my forces

is the force on the wall by my foot,

to find the partner force to this force,

I can just reverse the labels and say it's gotta be

the force on my foot, by the wall, which I drew over here.

So this is a great way to identify

the third law partner forces,

cause it's not always obvious

what force is the partner force.

So to show you how this can be tricky,

consider this example.

Say we got the ground and a table.

So this example drives people crazy for some reason.

If I've got a box sitting on a table,

we'll call it box A.

Box A is gonna have forces exerted on it.

One of those forces is gonna be the gravitational force.

So the force of gravity is gonna pull straight down

on box A, and if I were to ask you,

what force is the third law partner force to this force

of gravity, I'm willing to bet a lot of people might say,

well there's an upwards force on box A,

exerted by the table.

And that's true.

And if this box A is just sitting here,

not accelerating, these two forces are going

to be equal and opposite.

So it's even more tempting to say that these two forces

are equal and opposite because of the third law,

but that's not true.

These two forces are equal and opposite

because of the second law.

The second law says if there's no acceleration,

then the net force has to be zero,

the forces have to cancel.

And that's what's happening here.

These forces are equal and opposite,

they're canceling on box A.

Which is a way to know that they are not third law

partner forces, cause third law partner forces

are always exerted on different objects.

They can never cancel if they're third law partner forces.

So what's going on over here?

We've got two forces that are canceling,

that are equal and opposite, but they're not third law

partner forces, they're partner forces are somewhere else.

I haven't drawn their partner forces yet.

So let's try to figure out what they're partner forces are.

So let's get rid of this, let's come back to here,

let's slow it down to figure out what the partner force is,

name the two objects interacting.

So this force of gravity, I shouldn't be vague,

I should call it the force on object A,

our box A exerted by, well you can't just say gravity.

Gravity is not an object.

So the object that is exerting

this gravitational force on A, is the earth.

So this force really, this gravitational force,

if I wanna be careful, is the force on object A

exerted by the earth.

Now it's easy to figure out where the partner force is.

The partner force can be found just

by reversing these labels.

So instead of the force on A by the earth,

there's gotta be an equal and opposite force,

which is the force on the earth, by box A.

So opposite means it has to point up.

So it has to be an upward force.

And that upward force has to be exerted on the earth,

by box A, and this is kind of weird,

because you may not have realized it,

but if the earth is pulling down on a box, or you,

that means you are pulling up on the earth.

And this might seem ridiculous,

I mean if you jump up, you jump up, you fall back down,

you move around, but the earth just sits there.

If your forces are equal, how come the earth doesn't

move around like you do.

And again, it's because just because

the forces are the same, the acceleration

doesn't have to be the same.

The mass of the earth is so big,

compared to your mass, there's basically no acceleration.

Even though the forces on you

and the forces on the earth are the same.

So these two are third law partner forces.

These two are joined together forever.

They have to be equal, no matter what happens,

these two forces will always be equal.

I don't care if this box is accelerating

or not accelerating, or that there's motion or no motion.

Whether it's hitting a wall, sitting on a table,

falling through space, these two forces must

always be equal and opposite, because of the third law.

So how about this other force,

this force that the table was exerting.

So this is, the force on A by the table.

So if I wanna label it correctly,

I'd call it the force on box A, exerted by the table.

Now finding the third law partner force is easy,

I can just reverse these labels,

and I'd get that there must be,

instead of an upwards force, a downwards

force on the table, by A.

So I'm gonna have another force here on the table.

It's gonna be a downward force.

Downward force on the table by A,

that's the third law partner force to this upward

force that the table is exerting.

These two forces are also third law partner forces.

these forces are going to be equal

and opposite no matter what happens.

This force on box A by the table.

And this force on the table by box A

must be equal no matter what happens,

but the force on box A by the table,

does not have to be equal and opposite

to the force on A by the earth.

It happens to be equal and opposite,

in a case where there's no acceleration.

If we stuck this whole situation into an elevator,

or a rocket that had some huge acceleration upwards,

even if there's acceleration upwards,

these partner forces have to be equal.

So the force on A by the table,

and the force on the table by A will have to be equal.

Similarly the force on the earth by A,

and the force on A by the earth have to be equal.

But no longer will these two forces have to be equal,

cause they're not partner forces.

They might be equal and opposite in some circumstances,

but they don't always have to be equal and opposite.

If we're accelerating upwards,

this upward force on the box,

must be bigger than the downwards force on the box.

So these won't be equal.

Recapping quickly, Newtons's Third Law is a statement

about the forces on two different objects.

And because it's about two different objects,

those forces can never cancel.

To find the Newtons's Third Law partner force,

just reverse the label after you've identified

the two objects that are interacting.

The third law partner forces have to be equal in magnitude,

even if one object is larger than the other,

or has more charge or any property that might

seem like it would convey more force, than another object.

If those are the two objects interacting,

their forces must be of equal magnitude

and opposite directions, the forces instantaneously

generated this partner forces.

And be careful, some forces might seem like partner forces,

and might be equal and opposite,

but they're not necessarily third law partner forces.

They made just be equal and opposite for other reasons.