Newton’s Laws of Motion

I. Law of Inertia

II. F=ma

III. Action-Reaction

Discovered the law of gravity

Discovered the three laws of motion.

Today these laws are known simply as Newton’s Laws

of Motion.

Published findings in the book Philosophiae Naturalis

Principia Mathematica (mathematic principles of

natural philosophy) in 1687.

Background

Sir Isaac Newton (1643-1727)

English scientist and mathematician

1st Law of Motion (Law of Inertia)

An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

What does this mean?

Basically, an object will “keep doing what it was doing” unless a force comes along to change it.

- If the object was sitting still, it will remain stationary.

- If it was moving at a constant velocity, it will keep moving.

- It takes force to change the motion of an object.

First we need to define the

word FORCE:

The cause of motion (what causes objects to move)

Two types of forces

.

.

Forces may be

balanced or unbalanced Balanced forces – all forces acting on an object are

equal

There is NO MOTION

Unbalanced forces – one or more forces acting on

an object are stronger than others

There is MOTION

A NET FORCE

Newton’s First Law is also called the

Law of Inertia

Inertia: the tendency of an object to resist changes in its state of motion

1st Law – all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).”

Ex. Semi Truck compared to motorcycle

Example

In Pawn Stars episode, Corey puts a chair in

the back of his pick up truck. Slams on gas to

leave parking lot because he is in a hurry. The

chair falls out the back of the truck

Other Video Clips

http://www.youtube.com/watch?v=HRq-v4Gmzxg

Eureka – Inertia

Fast forward to 3.57 min left

Examples Law of Inertia Ex. A powerful train engine begins to pull a long line of

boxcars that were sitting at rest. Since the boxcars are so

massive, they have a great deal of inertia and it takes a

large force to get them going. Once they are moving, it

takes a large force to stop them.

Ex. On your way to school, a bug

flies into your windshield. Since

the bug is so small, it has very

little inertia and exerts a very

small force on your car (so small

that you don’t even feel it).

Examples Law of Inertia

Unless acted upon by an

unbalanced force, this golf ball

would sit on the tee forever.

Once airborne, unless acted on

by an unbalanced force

(gravity and air resistance

(fluid friction), it would never

stop!

The truck is in motion.

What is the force that causes it to stop?

The push of the stopped car.

The car is at rest.

What is the force that causes it to move?

The push of the truck.

Why does a book that is sliding across

a table eventually slow down/stop?

Answer: There are forces we cannot see, such as

friction and gravity.

A book sliding across a table slows down and stops because of the force of friction.

Other Forces?

If you throw a ball

upwards it will

eventually slow down

and fall because of

the force of gravity.

In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction

would keep going in that same direction and at that same speed

forever.

Force of Gravity

Attraction between any two objects in the universe

Force of gravity increases as...

mass increases (gravitational force exerted by the moon is

less than that exerted by the earth)

distance decreases

There are four main types of friction:

Sliding friction: ice skating

Rolling friction: bowling

Fluid friction (air or liquid): air or water resistance

Static friction: initial friction just before an object begins to move

Newtons’s 1st Law and You

Don’t let this be you. Wear seat belts.

Because of inertia, objects (including you)

resist changes in their motion. When the car

going 80 km/hour is stopped by the brick

wall, your body keeps moving at 80 m/hour.

Car Safety and 1st Law of Motion

Car Safety features

http://www.youtube.com/watch?v=8zsE3mpZ6Hw

&feature=related

2nd Law

Net Force = mass times acceleration

The net force of an

object is equal to the

product of its mass and

acceleration, or Fnet=ma.

Unit of Force = Newton (N)

One newton (N) is equal to the force

required to accelerate one kilogram

of mass at a rate of one

meter/second/second.

N = kg·m/s2

What does F = ma mean? Force is directly proportional to mass for a given acceleration.

Greater mass requires greater force for the same acceleration.

If a heavy SUV and a small sports car accelerate at the same rate, the SUV requires greater force.

Force is directly proportional to acceleration for a given mass.

Greater force causes greater acceleration of a given object.

A tennis player with a strong (more forceful) serve will accelerate the ball more than a player with a weaker serve.

Mass and acceleration are inversely proportional for a given force.

Greater mass undergoes smaller acceleration with a given force.

If the same force is applied to a golf ball and a bowling ball, the golf ball will have a greater acceleration.

Problem Solving using F = m a

How much force is needed to accelerate a 1400 kilogram car 2 m/s2?

F = m x a

F = 1400 kg x 2 m/s2

F = 2800 kg-m/s2 or 2800 N

Check Your Understanding

1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object?

12 N = 3 kg x a = 4 m/s2

12 N = 6 kg x a = 2 m/s2

2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.

16 N = m x 5 m/s2 =3.2 kg

3. How much force is needed to accelerate a 66 kg skier 1 m/s2?

F = 66kg x 1 m/s2 =66 N

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2?

F = 1000kg x 9.8 m/s2 =9800 N

Mass, weight, and acceleration of gravity

Weight is a force, specifically the gravitational force

exerted on an object.

In this case, Fw = m g

A 10 kg mass has weight = 98 N.

A 1-kg mass has weight = 9.8 N.

What about the apple?

F = ma

Weight

the force of gravity on an object

MASS always the same

(kg)

WEIGHT depends on gravity

(N)

W = mg W: weight (N)

m: mass (kg)

g: acceleration due

to gravity (m/s2)

F = ma Mrs. M weighs 557 N. What is her

mass?

GIVEN:

W = 557 N

m = ?

g = 9.8 m/s2

WORK:

m = W ÷ g

m = (557 N) ÷ (9.8 m/s2)

m = 56.8 kg

m

W

g

Newton’s 3rd Law

For every action, there is an

equal and opposite reaction.

3rd Law Newton’s Law States: whenever 2 objects

interact, they exert forces upon each other.

Ex. When you sit in your chair,

your body exerts a downward

force on the chair and the chair

exerts an upward force on your

body.

This is called action –reaction

Everything occurs in pairs.

Newton’s 3rd Law in Nature

Ex. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.

Newton’s 3rd Law in Nature

Equal but opposite

Size of force on the water =size of the force on the fish

Direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

3rd Law

Ex. As the birds push

down on the air with

their wings, the air

pushes their wings up

and gives them lift.

Other examples of Newton’s Third Law

The baseball forces the

bat to the left (an

action); the bat forces

the ball to the right

(the reaction).

As the wheels turn, they “grip”

the road (think friction) and

push the road backwards as the

road pushes the car forward.

3rd Law

Ex. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube.

As the gases move downward, the rocket moves in the opposite direction.

Think about it . . .

What happens if you are standing on a

skateboard or a slippery floor and push

against a wall?

You slide in the opposite direction (away from the

wall), because you pushed on the wall but the wall

pushed back on you with equal and opposite force.

Why does it hurt so much when you

stub your toe?

When your toe exerts a force on a rock, the

rock exerts an equal force back on your toe.

The harder you hit your toe against it, the

more force the rock exerts back on your toe

(and the more your toe hurts).