Using

the

FORCE

Sir Isaac

Newton

January 4th, 1643 – March 31st, 1727

Newton was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian and one of the most influential men in human history.

Among many other things, he is credited with describing the Universal Law of Gravitation and the Three Laws of Motion.

A 2005 survey of the British Royal Society ranked Newton ahead of even Einstein as having a greater influence on the history of science.

Philosophiæ Naturalis Principia MathematicaLatin for "mathematical principles of natural philosophy"

A three-volume work by Isaac Newton

published on 5 July 1687 that contains the

statement of Newton's laws of motion forming

the foundation of classical mechanics, as well

as his law of universal gravitation and a

derivation of Kepler's laws for the motion of

the planets (which were first obtained

empirically).

The Principia is widely regarded

as one of the most important

scientific works ever written.

The physical law describing the gravitational attraction

between bodies with mass

It states the following:

• F is the magnitude of the gravitational force between the two point masses,• G is the gravitational constant,• m1 is the mass of the first point mass,

• m2 is the mass of the second point mass,• r is the distance between the two point masses.

Every point mass attracts every other point mass by a force pointing along the

line intersecting both points. The force is proportional to the product of the two

masses and inversely proportional to the square of the distance between the

point masses:                    

where:

Newton's law of universal gravitation

F1 = F2 = G m1m2

r2

Got it?

Force? A force is a push or pull

upon an object resulting

from the object's

interaction with another

object.

Forces only exist as a

result of an interaction. Force has both

direction and

magnitude

Inertia “The vis insita, or innate force of matter is a power of resisting, by which every body, as much as in it lies, endeavors to preserve in

its present state, whether it be of rest, or of moving uniformly forward in a straight line.”

In English, please!

Inertia is the resistance of an object to a change in its state of motion. A bowling ball has more inertia

than a soccer ball.Don’t believe me?

Kick the bowling ball, then kick the soccer ball.

More mass = More inertiaMore inertia Less inertia

Balanced vs. Unbalanced

forcesA soccer ball sits alone on the field, quiet and still…

Gravity pulling down

Ground pushing up

Forces are balanced so

ball does not move

Balanced vs. Unbalanced

forcesSuddenly….

An unbalanced force is exerted on the ball in the direction of the arrow, causing it to move in that direction until…

Balanced vs. Unbalanced

forces…its motion is stopped by an interaction with another force!

Once again, the forces are

balanced and all is well in the

universe.

Balanced

ForcesIf the forces

acting on an

object are

balanced, the

object is at rest.

We say the

“Net Force” = 0

Net force = sum of all forces acting on an object

If the forces acting

on an object become

unbalanced,

the object moves in

the direction of the

greatest net force.

Unbalanced

Forces

In this case, the “Net Force” = 5 Newtons to the left

Force applied in this direction

The newton is a unit of force that is defined as the amount of force required to accelerate a mass of one

kilogram at a rate of one meter per second per second.

Algebraically:

Mmm

m,

Newtons!

Oh, sorry, wrong

Newtons

1 N = 1kg ● m

s2

Diagramming

ForcesIf you were paying attention, you would have

noticed that forces were illustrated using arrows.

The size and direction of the arrow represents the

relative strength and direction of the force.

Balance forces - arrows equal in lengthUnbalanced forces - arrows not equal,

Movement is to the right

Calculating Net ForcesTo calculate net forces, add all forces exerted

on the object.

In this example, the net force up the ramp is greater than gravity,

and the piano is moved forward and up the

ramp into the truck. A

B

C

For this example Net force = (force A + force B) – force C

Calculating Net Forces

21

For this example Net force = force 1 - force 2

The force due to gravity in this example is negligibleWhat other

forces can you identify in this

picture?

Calculating Net Forces

21

The dogs exert a force on the toy

as they bite down on it

Feeling the Force

Pow!Pow!

Which ram seems to have the advantage?

Why?

Newton’s First Law of Motion

“An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction

unless acted upon by an unbalanced force.”

AKA - The Law of Inertia

“Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by

force impressed.” Not again!One more time…

English, please!

Newton’s First Law of Motion

Said another way, Newton’s First Law of Motion states

that a moving object moves in a straight line with

constant speed unless a force acts on it.

An object will not start moving unless a force acts on it

An object will not stop moving unless a force acts on it

An object will not change speed unless a force acts on it

An object will not change direction unless a force acts on it

FrictionA force that opposes motion between two

surfaces that are touching

Even surfaces that seem to be extremely smooth have microscopic hills and valleys, and when two

surfaces are in contact, the hills and valleys of one stick to the hills and valleys of the other, causing friction to resist the force of motion

Static friction

The friction that exists between two objects in contact

Friction

Sliding friction

When force is applied that is strong enough to break

the bonds of static friction and movement starts,

sliding friction acts to slow that object down

Friction

Rolling frictionThe resistance

that occurs when a round object

such as a ball or tire rolls on a flat

surface

Friction

Fluid friction

Fluid friction

occurs when a

solid object

travels through

a liquid or gas.

Friction

Wake turbulence and wingtip vortices from jet airliner

passing through a layer of clouds, showing the fluid

nature of air

Friction

Friction always acts in the opposite

direction of movement, and

always acts to slow object down.

Static friction

Sliding friction

Rolling friction

Fluid friction

Movement starts

Force required to overcome friction

More

Less

Newton’s Second Law of

MotionForce equals mass times acceleration (F = ma)

The net force on an object is equal to the mass (m) of

the object multiplied by its acceleration (a)

Units of Force

Mass = kilograms (kg)

Acceleration = (m/s2)

Therefore….

F = kg ● m/s2

Force is measured in (Newtons, that is)

What are the forces acting on this bicycle and rider, coasting along at 25 km/h on this flat, wet, Alaskan road?

B

A

D

C

E

A = force of gravityB = force of the roadC = rolling frictional forceD = force of momentum*E = fluid frictional force (air or wind resistance)

*Momentum = mass (kg) ● velocity (m/s)

Is the net force balanced?

Write out the formula…

B

A

D

C

E

A = force of gravityB = force of the roadC = rolling frictional forceD = force of momentum*E = fluid frictional force (air or wind resistance)

Is the net force balanced?

Write out the formula…

Net force = (A-B) + D - (E+C)

GravityGravity is a force that always attracts or pulls objects toward each other without direct contact or impact.

Gravitational attraction depends on the mass of the two objects and the distance they are apart.

Objects on Earth are pulled toward the center of Earth.

The force of gravity, like all other forces, can cause changes in the speed of objects. As an object falls, its speed will continually increase as Earth’s gravity continually pulls it downward. When air resistance is ignored, all objects will speed up at the same rate as they fall.

Gravity can also cause an object that is thrown into the air to change its upward motion, slow down, and fall back toward Earth’s surface.

The pull of Earth’s gravity keeps the Moon in orbit; the moon is constantly changing direction because of gravity.

The acceleration due to gravity is 9.8 m/sec2

When air resistance is ignored, all objects will speed up at the same rate as they fall.

When the ball is dropped off the cliff, the ball will accelerate by 9.8 meters/second each second.

At the end of 1 second, the ball is traveling at 9.8 m/sec

At the end of 2 seconds, the ball’s velocity is 19.6 m/sec (9.8 m/sec X 2)

Etc…..

GravityFactoring in air resistance (fluid friction) will cause an object to reach a limit to its acceleration. This concept is known as terminal velocity. For an average skydiver, terminal velocity is approximately 195 km/sec (55 m/sec).

Terminal velocity is reached when the net force between gravity ( Fg ) and fluid friction,

or drag ( Fd ) reaches zero.

Gravity

Acceleration due to gravity9.8 m/s2

F = mass X (9.8 m/s2)

This dude is nuts!

F = ma

Newton’s Third Law of Motion

Whenever a particle (A) exerts a force on another particle (B),

(B) simultaneously exerts a force on (A) with the same

magnitude in the opposite direction.

This law is often simplified into the sentence,

"To every action

there is an equal and opposite

reaction."

Lex III: Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se

mutuo semper esse æquales et in partes contrarias dirigi.

Third Law in Action

Third Law in Action?Third Law in Action?

Think about Sabine’s actions

in this photo.

Do they represent

Newton’s Third Law of Motion?

If yes, explain.

If no, which law(s) are illustrated?

Sabine Lisinki, 2009 Family Circle Cup

Champion

Quick ReviewA paratrooper has a mass of 100 kg. He jumps from his C-17 at an altitude of 10,000 feet and

accelerates toward the ground at 9.8 m/sec2. What is the force on the paratrooper? Explain

how you got your answer, and which of Newton’s laws of motion is involved.

A roller coaster reaches the top of the big hill traveling at a speed of 15 m/sec. When it reaches the bottom of the hill 3 seconds later it is traveling at 27 m/sec. Calculate the average acceleration. Explain how you got your answer.

Force and Newton’s Laws note-taking sheet

Section 1A. Force

1. net2. balanced3. unbalanced

B. first lawC. Friction

1. slows down2. Static3. Sliding4. Rolling

Section 2A. second lawB. Gravity; weightC. calculateD. centripetalE. terminal velocityF. unbalanced

Force and Newton’s Laws note-taking sheet

Section 2A. second lawB. Gravity; weightC. calculateD. centripetalE. terminal velocityF. unbalanced

Self-check questions1) Think about our happy cyclist from earlier in the

discussion

2) Your weight will decrease as you get farther from Earth because the gravitational pull of Earth is decreasing

3) Greater the speed, greater the air resistance, up to the point of terminal velocity

4) Net force will push diagonally at some angle on the car and to the right

5) The motion of the box will not change until the forces become unbalanced

Self-check questions

6)F = maF = 1500 kg X 2.0 m/s2 = 3,000 N

7) F = mam = F/a = 300 N / 1500 m/s2 = 0.2 kg

or (300 kg ● m/s2) / 1500 m/s2 = 0.2 kg

Quick ReviewA paratrooper has a mass of 100 kg. He jumps from his

C-17 at an altitude of 10,000 feet and accelerates toward the ground at 9.8 m/sec2. What is the force on

the paratrooper? Explain how you got your answer, and which of Newton’s laws of motion is involved.

Quick ReviewA roller coaster reaches the top of the big hill traveling at a speed of 15 m/sec. When it reaches the bottom of the hill 3 seconds later it is traveling at 27 m/sec. Calculate the average acceleration. Explain how you got your answer.

Quick ReviewA paratrooper has a mass of 100 kg. He jumps from his C-17

at an altitude of 10,000 feet and accelerates toward the ground at 9.8 m/sec2. What is the force on the paratrooper? Explain how you got your answer, and which of Newton’s

laws of motion is involved.

F = maF = 100 kg ● 9.8 m/sec2

F = 980 kg ● m/sec2

(1 Newton = 1 kg ● m/sec2)Answer = 980 Newtons (or 980 N)This is Newton’s Second Law of Motion

Quick ReviewA roller coaster reaches the top of the big hill traveling at a

speed of 15 m/sec. When it reaches the bottom of the hill 3 seconds later it is traveling at 27 m/sec. Calculate the average

acceleration. Explain how you got your answer.

Average acceleration = (Final velocity – Initial velocity) / Time

= (27 m/sec – 15 m/sec) / 3 seconds

= (12 m/sec) / 3 seconds

= 4 m/sec2

Look carefully at these illustrations. Decide which of Newton’s laws is illustrated in each

example. Explain how the situation illustrates

the law you chose.

1 2

34

5 6

First Law – the football remains at rest b/c net force = 0.

1 2

34

5 6

Third Law – the Boat moves back when the boy moves forward.

First Law – the car stops but passengersInertia keeps them moving forward.

Second Law – the golf ball moves with more force with larger acceleration.

Second Law– same force on large ball produces less acceleration than small ball.

First Law – bike continues to coast at constant speed w/o unbalanced force.