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For more information, please see full course syllabus of AP Physics 1 & 2
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Lecture Comments (14)

1 answer

Last reply by: Professor Dan Fullerton
Sat Mar 4, 2017 12:47 PM

Post by Woong Ryeol Yoo on March 4, 2017

Hi mr Fullerton.
I'm studying with your ap physics 1 essentials book.

One part of your explanation on newton's third law says
"Further, if two objects interact with each other by applying a force upon each other, and the objects are part of the same system, the velocity of the system's center of mass cannot change."

How can this be true for this instance?
If a girl exerts force on the ball at rest, the ball flies and has velocity.
So its velocity changed from 0 rest to something.

0 answers

Post by Elman Ahmed on July 7, 2016

Example number 4 is very interesting. My professor talked about it in the class!

0 answers

Post by Saadman Elman on June 19, 2016

Questino 4 was interesting. We can't apply Newton's 1st law in Example no. 4 because Newton's 1st law exerting force on the SAME object. For example, Earth pushing the book (downward) and the surface pushing the book upward. Here, it's the SAME object. But in Newtons' 3rd law, its two different objects...Interesting.  :)

1 answer

Last reply by: Professor Dan Fullerton
Thu Mar 31, 2016 3:36 PM

Post by Jayden Luis on March 30, 2016

Nice way to describe ex. 4! Nice lectures!

1 answer

Last reply by: Professor Dan Fullerton
Sun Jul 20, 2014 5:15 PM

Post by Him Tam on July 20, 2014

I understand that the force from a hand pushing an object along a table is met with a  reaction force back to the hand. But what about for friction?

Friction seems to push back against the object to cause it to stop, but where does the reaction force from the object go that pushes the friction back? Is the force from the object pushing the table or the air in front of it or something else?

5 answers

Last reply by: Professor Dan Fullerton
Tue Aug 27, 2013 10:18 AM

Post by Larry wang on August 26, 2013

Hello Professor, From what I understand, Newton's third law comes in pairs so you cannot say F (1 on 2) = - F (3 on 1). On the last example 4: what threw me off is action-reaction forces F(1 on 2)= - F (2 on 1),which is acting on different objects therefore don't cancel each other. Clearly there is only two object 1 and 2 acting upon each other. Thank you.

Related Articles:

Newton's 3rd Law of Motion

  • A force on an object is due to the interaction of that object with another object.
  • Forces come in pairs.
  • F(1 on 2) = -F(2 on 1)

Newton's 3rd Law of Motion

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Objectives 0:06
  • Newton's 3rd Law of Motion 0:20
    • All Forces Come in Pairs
  • Examples 1:22
  • Action-Reaction Pairs 2:07
    • Girl Kicking Soccer Ball
    • Rocket Ship in Space
    • Gravity on You
  • Example 1: Force of Gravity 3:34
  • Example 2: Sailboat 4:00
  • Example 3: Hammer and Nail 4:49
  • Example 4: Net Force 5:06

Transcription: Newton's 3rd Law of Motion

Hi and welcome back to

This lesson is on Newton's Third Law of Motion.0003

Our objectives are going to be to explain the meaning of Newton's Third Law of Motion.0006

To recognize and identify force pairs, and finally to utilize Newton's Third Law to solve dynamics problems.0010

Newton's Third Law of Motion. All forces come in pairs. You can't have a single force.0021

If object 1 exerts a force on the object 2, then object 2 must exert a force back on object 1.0029

That force that exerted back is exactly equal in magnitude and opposite in direction.0037

The force of object 1 on 2 is equal, but opposite in direction to the force of 2 on 1.0043

Oftentimes you might have heard this phrased as the Law of Action-Reaction.0049

For every action there is an equal and opposite reaction.0054

Have to be careful with that terminology.0058

What we are really talking about are forces with that law.0060

For example, if somebody comes and they punch you in the nose with a force of 100N with their fist, your nose exerts a force of 100N back on their fist.0064

The same force exerted on your nose is opposite in direction exerted back on to them.0073

Let us take a look at some examples. How does a cat run forward?0080

Well, if you want to run forward, don't you push back on the ground to move forward?0085

So it pushes backward on ground and the ground is what actually causes the cat to run forward.0090

Or if you want to swim forward, which way do you push the water?0098

Don't you push it behind you? You push back so that the water pushes you forward?0102

Or how do you jump in the air? When you want to jump, you push down on the ground and the ground pushes you back up.0108

Newton's Third Law, we use all the time, so much so that it is almost silly to talk about.0119

Let us talk about action-reaction pairs or force pairs.0127

If a girl is kicking a soccer ball, she has the force of the girl's foot on the ball, then there must also be a reactionary force.0131

You have the foot on the ball, the reactionary force must be the ball applying a force on the foot.0139

Or a rocketship in space. Hot gases are pushed out by the rocketship.0149

Then what is the force pair? The ship is pushed by the hot gases.0161

How about the force of gravity on you?0173

Earth, the force of gravity on you, is pulling you toward its center.0175

Guess what? With the exact same force, you are pulling Earth toward your center.0189

Granted you do not see the effect nearly as much because the Earth is so massive, you cause such a tiny, little acceleration.0194

Let us take a look at a couple of examples.0214

Earth's mass is approximately 81 times the mass of the moon.0216

If the Earth exerts a gravitational force of magnitude F from the moon, the magnitude of the gravitational force of the moon on the Earth is, well, Newton's Third Law.0220

If we exert a force of one on another, an exact same force but opposite in direction is exerted back, has to be 1F.0229

The sailboat example. A 400N girl standing on a dock exerts a force of 100N on a 10,000N sailboat as she pushes it away from the dock.0239

How much force does the sailboat exert on the girl?0249

Well, the 400N girl -- that is talking about the weight of the girl, that is describing the girl's weight -- exerts a force of 100N, that is the force that she applies to the boat.0253

On a 10,000N sailboat, that is the weight of the sailboat.0265

So the force that the girl exerts is the 100N, that must be the force that the boat exerts back on her, 100N.0274

Do not let these other details screw you up.0283

The hammer and nail. A carpenter hits a nail with the hammer.0289

Compared to the magnitude of the force the hammer exerts on the nail, the magnitude of the force the nail exerts on the hammer during contact has got to be the same, Newton's Third Law.0292

Let us look at one more here.0303

If forces only come in pairs, that are equal and opposite, why does all forces not cancel each other out?0306

Now, we have to think a little bit.0314

Remember the force of object 1 on 2 is equal in magnitude and opposite in direction of the force of object 2 on 1.0317

Why don't they cancel each other out?0330

They are acting on different objects.0333

If they were acting on the same object, of course they would cancel each other out, but since they are acting on different objects, no cancellation.0338

Newton's Third Law, we use it all the time every day.0347

Very simple concept but so easy to overlook.0351

Thanks for watching Make it a great day.0355