In this section, we will explore some consequences of Newton's first law, which can be stated as, "**an object moving with a constant velocity will continue to move with the same speed and in same direction unless an interaction with another object(s) produces a net external force**".

Check out the OpenStax trailer about forces!

https://www.youtube.com/watch?v=lCxxH8nQtZQ

## Pre-lecture Study Resources

Read the BoxSand Introduction and watch the pre-lecture videos before doing the pre-lecture homework or attending class. If you have time, or would like more preparation, please read the OpenStax textbook and/or try the fundamental examples provided below.

### BoxSand Introduction

## Newton's Laws | Forces Introduction

Newton's first law introduces the force concept in relation to an object's speed and direction. If an object is in motion or at rest, the only way for the object's speed or direction to change is for an external force to act upon it. For example, a stationary ball in outer space will only begin to move when a force is applied to the ball. Since the ball begins to move we can say that its velocity has changed. Additionally, a ball in motion has no reason to change its velocity, speed or direction, unless an external force acts upon it.

Prior to this section, we did not ask any questions about what causes the motion of objects to begin or to change. In this section, we will use Newton's second law develop a cause and effect relationship between forces and motion. Newton's second law can be stated as follows:

**NOTE: Just like kinematics, the rest of PH201 works under the point particle approximation. Thus, we will not need to worry about the center of mass yet. But in PH202, will will start to consider the physical shape of objects, requiring us to determine center of masses.

## Key Equations and Infographics

Now, take a look at the pre-lecture reading and videos below.

### BoxSand Videos

## Required Videos

## Suggested Supplemental Videos

### OpenStax Reading

Here is an introduction to forces from the OpenStax textbook (Chapter 4)

OpenStax chapter 4.1 develops the force concept

Read about Newton's First Law of Motion in chapter 4.2

The most important law in forces is Newton's 2nd Law of Motion in chapter 4.3

Newton's 3rd law is the last (chapter 4.4), which talks about symmetry in forces.

Check out the normal force and tension in chapter 4.5

### Fundamental examples

1. An object is initially at rest on a smooth surface. In 10 seconds the object will be

a) 1 meter to the right

b) 1 meter to the left

c) in the same location

d) floating above the original location

2. At time t=0 an object is moving at a constant velocity of 5 m\s in the x-direction. What is the y component of the velocity at t=10 seconds.

a) y-component of velocity is 5 m/s

b) y-component of velocity is 0 m\s

c) y-component of velocity is -5 m\s

d) y-component of velocity is 14.8 m\s

Short foundation building questions, often used as clicker questions, can be found in the clicker questions repository for this subject.

## Post-Lecture Study Resources

Use the supplemental resources below to support your post-lecture study.

### Practice Problems

BoxSand's Quantitative Practice Problems

BoxSand's Multiple Select Problems

Recommended example practice problems

set 1: Newton's Laws of Motion (Part A)

set 2: Lots of worked examples.

For additional practice problems and worked examples, visit the link below. If you've found example problems that you've used please help us out and submit them to the student contributed content section.

## Additional Boxsand Study Resources

### Additional BoxSand Study Resources

### Learning Objectives

## Summary

The objective is introduce Newton's laws and apply the fundamentals of a force analysis. Specifically we focus on drawing free-body diagrams to facilitate translating the 2nd law to the mathematical representation. Connections to kinematics are also made.

## Atomistic Goals

Students will be able to...

- Demonstrate the fact that a force is inherently an interaction between two objects or two systems.
- Explain Newton's 1st law as it relates to inertia.
- Explain Newton's 1st law and how it relates to the 2nd law.
- Define static and dynamic equilibrium.
- Explain Newton's 2nd Law.
- Explain Newton's 3rd Law force pairs.
- Define the point particle model
- Define a system(s) boundary and adhere their analysis to that boundary(s).
- Identify the type of forces interacting with your system and the direction they are applied.
- Differentiate between contact and non-contact forces.
- Demonstrate that the normal force is always perpendicular to the surface
- Draw a free-body-diagram (FBD) for the system(s).
- Demonstrate the ability to draw a properly scaled FBD.
- Draw the coordinate system that reduces the complexity of the vector analysis next to the FBD.
- Apply geometry to determine appropriate angles for the given coordinate system.
- Find the components of a force in the chosen coordinate system.
- Differentiate between A force and a NET force.
- Apply Newton's 2nd law in the mathematical representation.
- Differentiate between static and dynamic equilibrium.
- Differentiate between weight and apparent weight.
- Synthesize a force and kinematics analysis via the acceleration.
- Demonstrate that the net force points in the same direction as the acceleration
- Demonstrate the fact that the net force can be in the opposite direction of the motion.

### YouTube Videos

Khan Academy's Newton's First Law Introduction 2 part video sequence

https://www.youtube.com/watch?v=5-ZFOhHQS68

https://www.youtube.com/watch?v=CQYELiTtUs8

This video from Veratasium explains inertia by discussing why the Earth spins;

https://www.youtube.com/watch?v=TQxeutcYP6I

Watch Doc Schuster's inertia video for further explanations about Newton's 1st Law:

https://www.youtube.com/watch?v=cliJbHYpNic

An Astronomer answers questions about Newton's 1st Law at the H.R MacMillan Space Center:

### Other Resources

This link will take you to the repository of other content related resources.

### Simulations

Try your hand at this sledding game after learning about Newton's 1st Law. Hint: It's a lot simpler than you would think!

This simulation discusses Newton's First Law and reference frames:

For additional simulations on this subject, visit the simulations repository.

### Demos

Pulling a table cloth out from under a stack of dishes,

https://www.youtube.com/watch?v=owOkomofYjU

This NASA video introduces Newton's First Law of motion with many examples.

https://www.youtube.com/watch?v=Q0Wz5P0JdeU

For additional demos involving this subject, visit the demo repository

### History

### Physics Fun

### Other Resources

This is a concise description of free body diagrams;

Follow along in this interactive tutorial by W.W. Norton & Company about FBD;

Here is a slide show demonstrating how to use free body diagrams with examples

## Resource Repository

This link will take you to the repository of other content on this topic.

## Problem Solving Guide

Use the Tips and Tricks below to support your post-lecture study.

### Assumptions

Newton’s First Law stated simply is: Objects in motion stay in motion unless acted upon by a net external force. Achieving this condition of having no net external forces can mean two things:

1) There are no forces on the object.

2) There are multiple forces on the object, but overall cancel out in every direction.

These situations are the conditions for equilibrium (ignoring rotation which we will cover in PH201). This does NOT mean:

1) The object is not moving, objects can move at a constant non-velocity and still have no net external forces in either of the above conditions. As such, the words “constant velocity” and “equilibrium” should be a trigger words to tell you there is no net force on the system.

### Checklist

### Misconceptions & Mistakes

- It is often thought that objects in motion will “naturally” come to rest. However, as stated in Newton's first law, an object in motion stays motion unless acted upon by an external force (i.e., A rolling ball comes to rest due to friction).
- Similarly an object at rest stays at rest until it interacts with another object(s) producing a net external force.
- Think about whether an object is in equilibrium (no net external forces); if it is it will continue with the same velocity, magnitude and direction.
- Here's a good Veritasium video that covers what confuses most people about Newtons laws:

### Pro Tips

- Keep everything in SI units as you go.
- The trickiest part of Newton's 1st law is defining the system that you are looking at and determining what an external force is. Drawing a dotted circle around your defined system will help identify internal and external forces.
- Check the units of your answer with the units you would expect for that quantity, for example a force should have units of Newtons (N) which is $\frac{kg*m}{s^2}$.
- The second law is consistent with the first law. If $\sum \vec{F}=0$ then $\vec{a}=0$ and there is no change in the movement of the object.

### Multiple Representations

Multiple Representations is the concept that a physical phenomena can be expressed in different ways.

### Physical

### Mathematical

The main mathematical representation for forces will be applying Newton's Second Law to a physical problem. You will need to analyze the motion of an object and create a free body diagram of the relative forces in order to derive the force equations. We learned in the kinematics section how to analyze motion without any concern about the forces that caused the motion. The infographic below describes the relationship between force and acceleration. Thus, we may now analyze the forces acting on an object to determine the relative acceleration.

### Graphical

.

### Descriptive

A spaceship, deep in space, has ran out of fuel. Since nothing is there to act on the ship, it will continue to move at the speed it is moving and in the direction it is moving. It will do this until something acts on it, something like the pull from a nearby star that becomes no longer negligible as the ship passes near the star.

### Experimental

Here is a great video of an experiment where a pen cap is dropped into a bottle using Newton's first law