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For more information, please see full course syllabus of AP Physics 1 & 2
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Lecture Comments (16)

1 answer

Last reply by: Professor Dan Fullerton
Wed Oct 25, 2017 5:48 AM

Post by Carlins Almonor on October 24, 2017

The following question asks, "If, for every force there is an equal and opposite reaction force, then all forces in nature come in equal and opposite pairs, and are therefore balanced. Thus, since there can never be such a thing as an unbalanced force, how can any object ever accelerate?" Find the fault in this misconception.

I understand that the student's logic does not make sense, but I can't find a way to disprove why forces does not come in equal and opposite pairs.

2 answers

Last reply by: Professor Dan Fullerton
Fri Jul 24, 2015 6:56 AM

Post by Jim Tang on July 24, 2015

In the examples, we were only allowed to cross out inertia for mass bc they are directly proportional right? In all other cases, we would have to conceptualize inertia as an object's tendency to resist change in velocity...which would be pretty hard? Or is this method of mass = inertia always work?

1 answer

Last reply by: Professor Dan Fullerton
Fri Jul 24, 2015 6:52 AM

Post by Jim Tang on July 24, 2015

Hey Dan!

What's the difference between static and translational equilibrium?

1 answer

Last reply by: Professor Dan Fullerton
Sat Nov 29, 2014 7:17 AM

Post by MOGIN Daniloff on November 28, 2014

What if an object is acted upon by 3 different forces which do not cancel one another out? For example an object is acted upon by a net force of 2N to the north, 3N to the east, and 4N to the south west. What would be the net force in this case, and how would you calculate it?


1 answer

Last reply by: Professor Dan Fullerton
Wed Oct 30, 2013 8:44 AM

Post by Andrei Afilipoaei on October 30, 2013

Dear Mr. Fullerton,

In the lesson about Newton's first law, you stated that an object thrown into space will keep flying away for eternity if it doesn't hit anything.
However, a space ship has many stage rockets. I know that only the first one is needed to escape Earth's gravity. In space, it will never slow down. Then why do the space ships need all the other rockets, if it will never change is speed?
Thank you very much,


4 answers

Last reply by: Professor Dan Fullerton
Wed Aug 21, 2013 10:50 AM

Post by Gaurav Kumar on July 13, 2013

Thanks for the great lectures! One question though, what is the difference between static equilibrium, dynamic equilibrium, and translational equilibrium?

Related Articles:

Newton's 1st Law of Motion

  • Inertial mass is the property of an object that determines its acceleration when it interacts with other objects or systems.
  • Gravitational mass is the property of an object that determines the magnitude of its gravitational force when it interacts with other objects or systems.
  • Inertial mass and gravitational mass are experimentally the same.
  • The net force on an object is the vector sum of the individual forces.
  • Contact forces result from the interaction of one object touching another (ultimate due to interatomic electric forces).

Newton's 1st 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
  • Objective 0:05
  • Newton's 1st Law of Motion 0:16
    • An Object At Rest Will Remain At Rest
    • An Object In Motion Will Remain in Motion
    • Net Force
    • Also Known As the Law of Inertia
  • Force 1:02
    • Push or Pull
    • Newtons
    • Contact and Field Forces
    • Contact Forces
    • Field Forces
  • What is a Net Force? 2:30
    • Vector Sum of All the Forces Acting on an Object
    • Translational Equilibrium
    • Unbalanced Force Is a Net Force
  • What Does It Mean? 3:49
    • An Object Will Continue in Its Current State of Motion Unless an Unbalanced Force Acts Upon It
    • Example of Newton's First Law
  • Objects in Motion 5:05
    • Will Remain in Motion At Constant Velocity
    • Hard to Find a Frictionless Environment on Earth
  • Static Equilibrium 5:40
    • Net Force on an Object is 0
  • Inertia 6:21
    • Tendency of an Object to Resist a Change in Velocity
    • Inertial Mass
    • Gravitational Mass
  • Example 1: Inertia 7:10
  • Example 2: Inertia 7:37
  • Example 3: Translational Equilibrium 8:03
  • Example 4: Net Force 8:40

Transcription: Newton's 1st Law of Motion

Hi and welcome back to Educator. com.0000

This lesson is on Newton's First Law of Motion.0002

Our objects are going to be to define forces, to talk about mass and inertia, and most importantly to explain the meaning of Newton's First Law of Motion.0005

So, Newton's First Law of Motion, the technical definition.0016

An object at rest will remain at rest, part one.0021

And an object in motion will remain in motion at a constant velocity, and in a straight line, unless it is acted upon by a net force.0026

And what is a net force? A net force is just an unbalanced force.0038

Basically what we are saying here is that an object in motion is going to remain in its exact same form of motion or the velocity of an object remains unchanged, unless a net force acts upon it.0044

This is also known as the Law of Inertia.0057

So what is a force? A force is a push or a pull on an object.0062

The units of force are newtons, named of course after Sir Isaac Newton.0067

The abbreviation is capital N and one newton is equal to 1 kg times a meter divided by a second squared.0072

So how much really is a newton?0078

Well imagine in your hand you have a medium-sized apple.0081

The force of gravity on that medium-sized apple is in the ballpark of one newton.0084

Forces come in a couple of types.0089

We have what are known as contact forces -- forces in which the objects must be in contact with each other.0091

And we also have field forces -- forces that act at a distance.0098

Let us take a look at examples of each.0105

Contact forces are things like attention or what I call an applied force.0108

If anybody has ever applied a force to your nose with a fist, that would be an applied force.0117

Or the force of friction, is a contact force, whereas field force is forces enacted at distances -- are things like the gravitational force, or the electrical force, or the magnetic force.0123

The objects do not have to be in contact with each other in order for there to be a force exerted upon one by another.0139

So what really is a net force? A net force is the vector sum of all the forces acting on an object.0148

If all the forces are balanced, there is no net force.0157

That is known as translational equilibrium, constant velocity.0160

An unbalanced force is a net force.0166

So imagine for example, we have a candy bar. Lisa's pulling on it with 5N in one direction; Bart's pulling on it 5N the other direction.0169

5N one way, 5N the opposite direction, the net force is 0. The motion of that object is going to remain the same.0179

If it was moving at 2 meters per second (m/s), it is going to keep moving at 2 m/s.0188

If it was at rest, it will remain at rest.0191

On the other hand if we add a force of 5N to the left of an object and a force of 3N to the right of an object, the net force is the vector sum of those two, or 2N to the left.0195

And we know how to add up vectors already. For vectors like the 5N and the 3N addition, we have 5 to the left, we line them up tip to tail so then we have 3 to the right and what we are left with is from the starting point of the first to the ending point of the last, that 2N.0208

So what does this really mean?0228

An object will continue in its current state of motion unless an unbalanced force acts upon it.0230

That is really tough to see here on Earth because we have friction.0236

Have an object, put it on the surface, it stays there. No net force on it.0241

Very, very obvious. On the other hand, what about an object that we want to remain in motion with no net force.0247

Well to do that, I have to find a way to reduce the frictional load on an object.0254

So what I have here is a friction puck. If I blow up a balloon, put this in here, it is going to put a cushion of air to greatly reduce the amount of friction there and we will see what happens.0258

(Blows up balloon)0269

Okay, the balloons attached to the puck. If I put it on a surface and put it in motion, it continues in that state of motion until acted upon by a net force.0278

That is what Newton's First Law is saying, an object in motion will continue in motion in its same state of motion unless there is a net force to change it.0289

Now, an object at rest will remain at rest. We see that all the time.0300

The objects in motion, though, are so much less obvious because its hard to find a frictionless environment here on Earth.0307

Think of sliding a book across the floor. There is a net force on it at all times because of friction.0314

To really see this, you either need to have something like that low friction puck or you need to go out into space.0319

Throw a football through space. As it moves, it is going to keep moving forever and ever and ever in that straight line, constant velocity motion until its acted upon by a net force.0325

Or it eventually runs into something. Another net force.0335

All right. So static equilibrium. This is a special case where the net force on an object is 0 and its not moving.0338

The object is at rest. We will revisit this concept a little bit further when we explore Newton's Second Law.0348

We could also talk on the other hand about dynamic equilibrium.0357

In that case the net force on an object is still equal to 0, but now the object is moving at constant velocity or acceleration is equal to 0.0362

Again, in no net force, the object continues in its current state of motion. Very, very important concept.0374

Now, inertia is the tendency of an object to resist a change in velocity.0381

The measure of an objects inertia is its mass and we talked about this briefly previously, but mass has two aspects.0387

Inertial mass is how hard it is to change an objects velocity.0395

Gravitational mass is how strongly a gravitational field affects a mass, but for the purpose of physics and anywhere we have ever looked here, mass and inertia are going to be synonymous in this course.0399

Gravitational mass and inertial mass in any experiment ever done and documented, performed and documented, they have always been the same.0412

No theoretical reason why that is true, but experimentally, it always turns out that way.0422

Very convenient for us.0429

So let us take a look at a couple of sample problems. Which object here has the greatest inertia?0430

A fallen leaf, a softball in flight, a seated high school student, or a helium-filled toy balloon.0437

Well, the way I would take a look at this is say "Which one has the greatest mass?"0442

All of a sudden that becomes a much easier problem. It has to be the seated high school student.0448

Mass is a measure of an objects inertia. Which object here has the greatest inertia?0454

A 5 kg mass moving at 10 m/s? A 10 kg mass moving at 1 m/s? A 15 kg mass moving at 10 m/s? Or a 20 mg mass moving at 1 meter per second?0463

Again, designed to trick you, these types of questions get much easier when we just cross out inertia and write its measure, mass.0472

Must be for the 20 kg mass.0481

Draw the velocity time graph for an object in translational equilibrium.0485

Well, if its in translational equilibrium, then the net force on that object is 0.0490

The acceleration is 0, therefore it must be moving at constant velocity.0496

So a VT graph times velocity is going to be just a nice straight line. Where ever it is, it is going to be straight, unchanging, translational equilibrium.0503

All right. Let us try one more.0518

What is the net force on an object that is experiencing a pull of 5N to the North, a push of 3N to the South and a pull of 2N to the East?0522

Well the way I would do this is I would make a vector diagram to help me out.0531

We have an object feeling a pull of 5N to the North, a push of 3N to the South, 1, 2, 3 to the South and a pull of 2N to the East.0535

What is the net force? Well, all I have to do is add up the vector sum of all of those.0543

I have already lined them up tip to tail, so if I want the vector sum I draw a line from the starting point of the first to the ending point of the last.0548

If I broke this up into components, I would have 2N to the East and if I had 5 up and 3 back, I must have 2N North.0567

If I want the magnitude of that net force of that, that is going to be the square root of 2 2 + 2 2 or 2.83N.0580

And the angle of course, I do not even have to go to trig, if it is 2 to the right and 2 up, the angle is going to be 45 degrees.0597

Hopefully that gets you started with Newton's First Law. It is a very simple concept, but a very important one in physics.0606

Thank you for watching Educator.com. Make it a great day!0614