Newton's Third Law of Motion: Action and Reaction

Are you tired of Newton's laws of motion yet? Well, lucky you, this is the last one. And it's the most misunderstood as well! People love making inappropriate metaphors with scientific principles, unfortunately. Watch this to make sure you don't do the same!

Check out "Is This Wi-Fi Organic?", my book on disarming pseudoscience!

[DarthSammoth]

This guy is amazing!! How did he know I was sitting on a chair?? Brilliant.

[The_Gill]

I learn physics in my school just for exams....but I come on this channel just for my self....I understand concept nice sir

[WarrenMiller-timu]

There is an error at 1:21. The example given is not an action/reaction pair. (Gravity and Normal Force). The normal force can often be more or less then the gravitational pull on an object. For example, if you lean on a desk while sitting in your chair, the normal force from the chair will be less then your weight since some of the weight is held up by the desk. Action/reaction pairs are always equal in magnitude. The other clue here is action reaction pairs never act on the same system. The normal force from the chair is paired with the normal force from your body onto the chair. Two separate systems. Please either edit or have a floating comment as this is one of the most common misconceptions for students. Thanks.

[redbaron]

Uh oh, the normal force on you from the chair is NOT the Newton's 3rd law "equal and opposite reaction" (counterforce) to your weight, even if it happens to be equal in magnitude and opposite in direction. Instead, the other member of the action-reaction pair is your gravitational pull up on the Earth. Action-reaction pairs (1) act on different objects and (2) must be of the same type!

[Poop-nuso]

"Professor Dave Explains!" *toooot* gets me every time.

[rajank]

I have a different view on this. As you said when a body falls to the ground, it accelerates to ground and earth also accelerates a minuscule amount towards the falling object. Similiarly an action like a person jumping on the ground causes the earth to go down a minuscule amount since mass of earth is very high compared to the mass of the person. This is as per Newton’s III law of motion. In this example when you strike the hammer on nail, there are two different types of force on the nail. One is the normal gravitational force on the hammer. The other one is the downward force the striker exerts on the hammer . The normal gravitational force on the hammer has an effect on earth same as a body falls to the ground. When the striker forces the hammer down he is applying a force more than the gravitational force on hammer. In this case the force opposing the striking force as per the III law will be on the body of the striker . The body will go up a bit. If the striker stands on a weighing scale you will notice the scale dipping a bit.

[aseul_]

so clear and comprehensive, dave clearly knows the common misconceptions and misunderstandings of the third law. helped me clarify when the topic seemed so fuzzy i didn't know where to start or what to fix. thanks sm dave!

[pedropasquini]

Hi! I wound like to point out that in min 1:30 for me the video seems to imply that the normal force is the reaction of gravity .

That is not true. The reaction of gravity is gravity itself. And the reaction of the normal force is another normal force.

What happens is that while you are being pulled by earth ('s gravity), you encounter the earth's surface and your accelerated body pulls the ground down.

The ground returns the favour by pushing you up. That is the normal force.

To see that clearer, notice later in the video: the reaction never acts on the same body as the "original" force.

Therefore, normal force is not the reaction of gravity.

[langstonrichardson]

The equal and opposite force is not between the hammer and the nail but rather, the source of the force, your arm, against the source of the opposite force which stops the force of your arm, the wood. Hope this helped!

[spino]

Thanks for the help Mr. Dave, your videos are fun and entertaining to watch, also, they teach me very well my lessons!
I hope you are doing well at this difficult times, and I wish you luck.

[naderkhaled]

hello appreciate the video. But I have one thing to clarify. The example you mentioned with the chair has a wrong part I guess. The third law states that every action force has an equal and opposite reaction force, BUT with the same TYPE. So the real reaction force to the gravitational force acting on the guy sitting on the chair is the gravitational force of the guy himself not the NORMAL force of the chair. Since the NORMAL force is not the same type of the GRAVITAIONAL Force. thanks for reading this and please correct me if I am wrong.

[rljxbsj]

when hammering a nail into a piece of wood the hammer imparts force on to the nail and the nail imparts force in to the hammer but the nail is drivin because there is a net force acting upon the nail from the motion of the hammer likewise the hammer has two forces upperating upon it the downwards force from your arm and the upward force from the nail that's why when you strike the nail, the hammers stops moving at roughly the location where the impact to curse so if we looking for the nail we only consider the forces acting at the nail if looking at the hammer we only consider the forces acting at the hammer

[skeeterburke]

i always thought the 3 laws are best understood when learned in reverse order ... when students get to the first and final law, they are relieved that there is nothing happening to the object and no more equations to learn

[kellygotell]

So does the nail experience more acceleration than the hammer?
A=F/m, (the nail has more net force than the hammer). Like bowling ball and pin, if they both have equal and opposite forces, what net force moves pin away from the ball? Acceleration? A=F/m if F is equal on both objects, one with smaller mass will accelerate more than larger mass object.

[cazyang]

1:18 this is NOT an example of a Newton's Third Law pair. both the Force of Gravity and the Normal Force are acting on the person sitting in the chair. Newton's Third Law requires two objects.

[cjbartoz]

Light can break Newton's third law:

Wimmer, M., Regensburger, A., Bersch, C. et al. Optical diametric drive acceleration through action–reaction symmetry breaking. Nature Phys 9, 780–784 (2013). DOI: 10.1038/NPHYS2777

[Aditya-zvtj]

Really sir you have cleared my all doubt 🤩, I have watched 10-11 videos but my doubts had not got cleared from them. Even my teacher didn't cleared my doubt😔
But you are awesome 🤩🤩🤩🤩

[elisarbongcales]

why is the bowling ball and bowling pin imparts equal force when the bowling ball has more mass and acceleration than the bowling pin?

[midnightspotlight]

my dad and i we always watch this every day

[rebobong]

I believe you. You really are the chemistry Jesus. Thank you <3