NEWTON’S LAWSUNIT 2

Notes, examples, and graphics from Physicsclassrom.com

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Newton’s 1st Law

Questions What is Newton’s first law? What is inertia? How is inertia measured? Explain how you would know if forces

are balanced or unbalanced.

Law of inertia In the 1600’s Isaac Newton explained

why objects move or do not move.

The first law is often called the law of inertia, and is often stated:

An object at rest stays at rest and an object

in motion stays in motion with the same speed and direction unless acted upon

byan unbalanced force

Two parts There are two parts to Newton’s first law

All objects resist changes to their current states.

Suppose you fill a shallow dish with water and try to race around a track. The water will tend to spill at specific times during your race. What are these times, and why does the water behave this way?

The water spills whenever the state of the container is changed:Rest to movingChange in directionMoving to rest

When have you experienced Newton’s first law?

Catsup bottles Elevators Car headrests Seatbelts + many more!

Inertia Inertia is the resistance an object has to

a change in its state of motion.

Galileo was the first to think about inertia and how it relates to friction. He hypothesized that a ball would roll forever if friction were absent.

Newton built upon Galileo’s thoughts.

Newton’s was first to say that a force was NOT needed to keep an object in motion, but the presence of a force prevented it from staying in motion.

A book pushed across the floor comes to a stop due to the force of friction. If there was no friction the book would continue forever, or until stopped by another force

Mass and Inertia All objects resist a change in their state

of motion, they all have inertia.

But do some objects have more inertia than other?

Yes!

An object’s resistance to change varies with mass.

Objects with more inertia have more mass, thus it has a greater resistance to changes in it’s state of motion.

Newton’s 1st law demos https://youtu.be/T1ux9D7-O38

Inertia Club membership

Check Your Understanding Imagine a place in space away from

gravity and friction. If you throw a rock, what will the rock do?

Mac and Tosh are arguing in the cafeteria. Mac says that if he throws Jell-0 with a greater speed it will have more inertia. Tosh disagrees. Who do you agree with?

Fred spends most Sunday afternoons on the sofa watching football and eating. What affect (if any) does this have on his inertia? Why?

Ben Tooclose is being chased by a bull moose through the woods. Ben runs in a zigzag pattern to try to outrun the moose. Explain this advantage using the terms inertia and Newton’s first law.

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States of Motion

Questions How is the state of motion defined? Provide 2 alternate definitions of inertia. Give an example of balanced and

unbalanced forces. Explain why force is a vector quantity.

What is meant by the phrase “state of motion?”

State of motion is defined as the velocity of an object. That is, the speed and direction of the object.

This gives us one alternate definition for inertia:The tendency for an object to resist changes in

velocity.An object with a velocity of 0, with keep a 0

velocity unless an unbalanced force acts on it. If the velocity is not changing, then the

acceleration is zero, giving us another definition: The tendency for an object to resist changes in

acceleration.

Check Your Understanding A group of physics teachers goes mini

golfing. The 15th hole has a large metal rim that guides balls toward the hole. When one of the teachers hits their ball, which path does it follow, and explain why.

A 4-kg object is moving across a friction-free surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion?0.5 N2 N8 N0 NIt depends on the speed.

Balanced and Unbalanced Forces Consider a physics book on a tabletop. Two forces are acting upon the book

GravityNormal force – desk pushing up on the book

When two forces of equal magnitude and opposite directions are on the same object, the object is said to be in equilibrium.

When an object is in equilibrium it keeps its state of motion and does not accelerate.

Give another example of an object in equilibrium. ___________________

What is the book is moving across the counter?

The force of gravity and the normal force are still in equilibrium.

However, there is friction acting to the left since the book is moving right, the forces are unbalanced.

Unbalanced forces cause accelerations due to not being in equilibrium.

So, an object can be moving and have balanced forces, but it cannot accelerate.

To determine if the forces are balanced or unbalanced, first determine what forces are acting upon an object.

Next, determine if any are balanced. Find any unbalanced forces, and solve

as a vector problem.

Check Your Understanding If the forces acting upon an object are

balanced, then the object, Must not be movingMust be moving with a constant velocityMust not be acceleratingNone of these

Questions How is the state of motion defined? Provide 2 alternate definitions of inertia. Give an example of balanced and

unbalanced forces. Explain why force is a vector quantity.

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The Meaning of Force

Questions Define force. Give examples of the two types of

forces. How did one of each force affect you

today? Forces are measured in Newtons which

is how much force?

Contact vs. Noncontact A force is a push or a pull on an object.

When there is interaction between two objects, there is force.

Forces ONLY exist as a result of interactions.

There are two categories of forces:ContactNoncontact

Contact forces occur when two objects physically interact

Noncontact forces happen when two objects do not touch, yet interact.

Contact Noncontact Friction Tension Normal Air Resistance Applied Spring

Gravitational Electrical Magnetic

Forces are measured in units called Newtons.

A Newton is the amount of force needed to give a 1-kg mass an acceleration of 1m/s/s

So, 1Newton = 1-kg x m/s2

Types of ForcesType of Force DescriptionApplied Force applied by a person or object. Ex. A person pushing

a deskGravity The weight of an object. NOT mass. Found by mass x

9.8m/s/sNormal Support force exerted on one object by another. Ex. Book

on tableFriction Force exerted when an object moves against another

surfaceAir Resistance Special frictional force that acts as objects move through

the airTension The force transmitted through a rope, cable or string pulled

tight by forces on opposite ends. Ex. ClotheslineSpring Force exerted by a stretched or compressed spring on an

object attached to it. Ex. Elastic

Mass vs. Weight Weight: how much gravity pulls on an object.

If you weigh a lot, you are rich in gravity!

Mass: how much matter an object contains.You have a lot of atoms!

Weight and mass are directly related, but are not the same thing. You can go to space and you will have the same mass, but not the same weight.

Questions Define force. Give examples of the two types of

forces. How did one of each force affect you

today? Forces are measured in Newtons which

is how much force?

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Finding and Showing Forces

Questions What is net force? How do we calculate net force? Identify situations in which there is an

unbalanced force. Draw free-body diagrams to show the

net force on objects in different scenarios.

Net Force Unbalanced forces are the forces that

are not balanced, or cancelled out, by other forces.

Each of the examples below has an unbalanced force.

The net force is the vector sum of all the forces that act upon an object.

So, to figure net force you have to “add” the force vectors together to determine the force.

“Adding” Vectors Review

Calculate the net force.

When a net force is present, there is acceleration.

Is there a net force below?

Is there a net force here?

Check Your Understanding Determine the net forces in the

situations below.

Check Your Understanding Given the net force and some of the

magnitudes, find the missing forces.

Draw Your Own Free Body A book is at rest on a tabletop. Diagram the forces acting on the book.

A girl is suspended motionless from the ceiling by two ropes. Diagram the forces acting on the combination of girl and bar.

An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it is falling.

A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel.

A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.

Questions What is net force? How do we calculate net force? Identify situations in which there is an

unbalanced force. Draw free-body diagrams to show the

net force on objects in different scenarios.

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Newton’s Second Law

Questions How are the mass and acceleration

related to the force upon an object?

Using Newton’s 2nd law, explain how you can weigh something different on another planet.

Predict the change in motion of objects given changes in mass or force.

Newton’s 1st law deals with objects in equilibrium.

Newton’s 2nd law discusses objects that are not balanced, and are accelerating.

The second law can formally be stated:The acceleration of an object as produced

by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Or, as an equation: Fnet = m • a Forces cause accelerations!

Your Equation

Force

Mass Acceleration

Thinking about the 2nd Law An empty shopping cart vs. a full

shopping cart

Pulling a rope that breaks

Pushing something so hard your feet slip instead of moving the object.

Mathematical Relationship If you double the force, the acceleration

doubles. If you halve the force, the acceleration

halves. If you halve the mass, the acceleration

doubles If you double the mass, the acceleration

halves.It is easy to see the relationship if you use one of the previous examples as a guide.

Using the equation, how does your weight change when you visit other planets?Your mass stays the sameDoes the force change?Does the acceleration change?

Remember the acceleration of gravity on Earth is 10m/s/s (9.8m/s/s).

Forces cause acceleration. What force is causing the acceleration on

planets? Would different planets’ masses matter?

Due to the different planets’ masses, the force of gravity is different, causing you to have different amounts of acceleration on your body.

The force of gravity causes your weight which varies with the masses of the planets.

Check Your Understanding A net force of 15 N is exerted on an

encyclopedia to cause it to accelerate at a rate of 5 m/s2. Determine the mass of the encyclopedia.

Determine the accelerations that result when a 12-N net force is applied to a 3-kg object and then to a 6-kg object.

Anna Litical suggests to Noah Formula that the object under discussion could be moving. In fact, Anna suggests that if friction and air resistance could be ignored (because of their negligible size), the object could be moving in a horizontal direction. According to Anna, an object experiencing forces as described at the right could be experiencing a horizontal motion as described below.

Noah Formula objects, arguing that the object could not have any horizontal motion if there are only vertical forces acting upon it. Noah claims that the object must be at rest, perhaps on a table or floor. After all, says Noah, an object experiencing a balance of forces will be at rest. Who do you agree with?

Questions How are the mass and acceleration

related to the force upon an object?

Using Newton’s 2nd law, explain how you can weigh something different on another planet.

Predict the change in motion of objects given changes in mass or force.

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Newton’s 3rd Law

Questions Explain which force is the action force

and which is the reaction force for any given situation.

How do forces always occur? What is Newton’s 3rd law?

Force interactions Forces are caused by pushes or pulls,

and are either in contact or noncontact.

When two objects interact, they exert forces on each other.

You on your chair, your chair on you.

Newton’s 3rd law states:For every action, there is an equal and

opposite reaction.

The sizes of the forces are equal, but opposite in direction.

Forces always come in pairs!

Force Pairs

Identifying Force Pairs First identify the two objects in the pair

Baseball and bat Describe the pushing from each object

Baseball forces the bat left, bat forces baseball right

Sometimes, they are labeled “A” on “B” and “B” on “A”

Complete the force pairs

Check Your Understanding Identify 6 pairs of forces.

Questions Explain which force is the action force

and which is the reaction force for any given situation.

How do forces always occur? What is Newton’s 3rd law?

THE END OF UNIT 2