# Forces and Newton's laws of motion

Contents

This is the meat of much of classical physics. We think about what a force is and how Newton changed the world's (and possibly your) view of how reality works.

## Newton's laws of motion

This tutorial will expose you to the foundation of classical mechanics--Newton's laws. On one level they are intuitive, on another lever they are completely counter-intuitive. Challenge your take on reality and watch this tutorial. The world will look very different after you're done.

5:26

Newton's first law of motion

Basic primer on Newton's First Law of Motion

7:14

Newton's first law of motion concepts

A little quiz on some of the ideas in Newton's first law

9:32

More on Newton's first law of motion

Newton's First Law (Galileo's Law of Inertia).

Article

What is Newton's first law?

Also called the law of inertia, this is the most important thing to realize about motion.

Exercise

Newton's first law

Testing your conceptual knowledge of Newton's First Law of Motion

7:14

Newton's second law of motion

Newton's Second Law of Motion: F=ma

16:03

More on Newton's second law

In this video David explains how to use Newton's second law when dealing with multiple forces, forces in two dimensions, and diagonal forces.

Article

What is Newton's second law?

Learn about the fact that forces cause acceleration.

8:00

Newton's third law of motion

Every action has an equal and opposite reaction

13:06

More on Newton's third law

In this video David explains some of the common misconceptions in dealing with Newton's Third Law. He also shows how to correctly and reliably identify Third Law force pairs.

Article

What is Newton's third law?

Learn about the fact that forces come in pairs.

Exercise

Newton's third law of motion

Conceptual question testing understanding of Newton's Third Law of Motion

Exercise

All of Newton's laws of motion

## Normal force and contact force

A dog is balancing on one arm on my head. Is my head applying a force to the dog's hand? If it weren't, wouldn't there be nothing to offset the pull of gravity causing the acrobatic dog to fall? What would we call this force? Can we have a general term from the component of a contact force that acts perpendicular to the plane of contact? These are absolutely normal questions to ask.

7:18

Normal force and contact force

The force that keeps a block of ice from falling towards the center of the earth

Article

What is weight?

Weight is another word for the force of gravity.

11:49

Normal force in an elevator

How the normal force changes when an elevator accelerates

10:45

More on Normal force (shoe on floor)

David explains how to determine the normal force for a variety of scenarios (extra forces, diagonal forces, acceleration) involving a shoe on the floor.

6:46

More on Normal force (shoe on wall)

David shows how to determine the normal force for a shoe shoved against a wall with a diagonal force.

Article

What is normal force?

When two objects touch, they exert a force on each other.

## Balanced and unbalanced forces

You will often hear physics professors be careful to say "net force" or "unbalanced force" rather than just "force". Why? This tutorial explains why and might give you more intuition about Newton's laws in the process.

8:11

Balanced and unbalanced forces

Primer on identifying balanced and unbalanced forces

6:37

Unbalanced forces and motion

Thinking about what is true about how unbalanced forces relate to motion and acceleration

## Slow sock on Lubricon VI

This short tutorial will have you dealing with orbiting frozen socks in order to understand whether you understand Newton's Laws. We also quiz you a bit during the videos just to make sure that you aren't daydreaming about what you would do with a frozen sock.

4:47

Slow sock on Lubricon VI

What would happen to a slowly moving frozen sock on a frictionless planet

7:12

Normal forces on Lubricon VI

Whether the normal force balances the force of gravity for a frozen sock or banana

## Inclined planes and friction

We've all slid down slides/snow-or-mud-covered-hills/railings at some point in our life (if not, you haven't really lived) and noticed that the smoother the surface the more we would accelerate (try to slide down a non-snow-or-mud-covered hill). This tutorial looks into this in some depth. We'll look at masses on inclined planes and think about static and kinetic friction.

12:42

Inclined plane force components

Figuring out the components of the force due to gravity that are parallel and perpendicular to the surface of an inclined plane

10:37

Ice accelerating down an incline

Figuring out the acceleration of ice down a plane made of ice

8:41

Force of friction keeping the block stationary

Block of wood kept stationary by the force of friction (Correction made in next video)

0:51

Correction to force of friction keeping the block stationary

Correction to Force of Friction Keeping the Block Stationary

9:09

Force of friction keeping velocity constant

Calculating the coefficient of kinetic friction (correction made in next video)

7:21

Intuition on static and kinetic friction comparisons

Why static friction is harder to overcome than kinetic friction

9:50

Static and kinetic friction example

Thinking about the coefficients of static and kinetic friction

Article

What is friction?

Until now in physics, you've probably been ignoring friction to make things simpler. Now, it's time to include this very real force and see what happens.

Article

What are inclines?

Surfaces usually aren't perfectly horizontal. Learn how to deal with slopes!

## Tension

Bad commute? Baby crying? Bills to pay? Looking to take a bath with some Calgon (do a search on YouTube for context) to ease your tension? This tutorial has nothing (actually little, not nothing) to do with that.
So far, most of the forces we've been dealing with are forces of "pushing"--contact forces at the macro level because of atoms not wanting to get to close at the micro level. Now we'll deal with "pulling" force or tension (at a micro level this is the force of attraction between bonded atoms).

14:09

The force of tension

David explains what the force of tension is, how to solve for it, and some common misconceptions involving the force of tension.

17:30

Mild and medium tension

David explains how to solve tension problems for hanging objects.

16:56

Super hot tension

David shows how to solve a super hot tension problem where a can of peppers hangs from two diagonal strings.

Article

What is tension?

Ropes pull on things! Learn how to handle that kind of force.

10:19

Introduction to tension

An introduction to tension. Solving for the tension(s) in a set of wires when a weight is hanging from them.

10:19

Introduction to tension (part 2)

A slightly more difficult tension problem.

9:28

Tension in an accelerating system and pie in the face

The second part to the complicated problem. We figure out the tension in the wire connecting the two masses. Then we figure our how much we need to accelerate a pie for it to safely reach a man's face.

## Treating systems

When two or more objects must move with the same magnitude of acceleration (like masses on strings, or boxes pushed into each other), we can treat the entire system as a single object when finding the acceleration.

15:27

Treating systems (the hard way)

10:26

Treating systems (the easy way)

David shows the easier way to find the acceleration of two masses connected by a rope.

8:50

Two masses hanging from a pulley

In this video David explains how to find the acceleration of two masses hanging from a pulley (using the easy method).

6:29

Three box system problem

In this video David explains how to easily find the acceleration of a three box system by treating it as a single mass.

9:58

Masses on incline system problem

In this video David explains how to find the acceleration and tension for a system of masses involving an incline.