Physical SciencePhysical ScienceNewton’s LawsNewton’s Laws

And ForcesAnd Forces

GALILEO:GALILEO:

Since he Since he experimented to get experimented to get EVIDENCEEVIDENCE for his for his conclusions, conclusions, he is considered to he is considered to be the be the

father of father of experimental experimental physics.physics.

He also He also figured figured out that…out that…

Things in motion tend to Things in motion tend to STAY IN MOTION; they STAY IN MOTION; they do NOT tend to SLOW do NOT tend to SLOW DOWN OR STOP ON DOWN OR STOP ON THEIR OWN.THEIR OWN.

In fact, Galileo figured In fact, Galileo figured out that it TAKES a force out that it TAKES a force to make something slow to make something slow down or stop. The usual down or stop. The usual force is (he discovered): force is (he discovered): FRICTION.FRICTION.

So in fact, GALILEO figured out So in fact, GALILEO figured out the principle behind what we callthe principle behind what we call

Newton’s 1Newton’s 1stst Law Law

NEWTON’S 1NEWTON’S 1STST LAW LAW An object at rest An object at rest

tends to stay at tends to stay at rest, and an object rest, and an object in motion tends to in motion tends to stay in motion in a stay in motion in a straight line at straight line at constant velocity constant velocity unless acted on by unless acted on by an outside net an outside net force.force.

INERTIAINERTIA refers to refers to Newton’s 1Newton’s 1stst Law Law

Inertia is the TENDENCY of ANY MASS to Inertia is the TENDENCY of ANY MASS to KEEP DOING what it is ALREADY DOING KEEP DOING what it is ALREADY DOING (resting, or going in a straight line at a (resting, or going in a straight line at a constant speed).constant speed).

A FORCE is a push or a pull, and A FORCE is a push or a pull, and

INERTIA IS INERTIA IS NOT A FORCENOT A FORCE!!! !!! IT IS A IT IS A TENDENCEY OR TENDENCEY OR

CHARACTERISTICCHARACTERISTIC!!

……INERTIA…INERTIA… Inertia is a measure of an object’s Inertia is a measure of an object’s

RESISTANCE RESISTANCE toto CHANGE CHANGE in state of motion. in state of motion. (Things—like people—don’t want to change!)(Things—like people—don’t want to change!)

The word inertia comes from the Italian word The word inertia comes from the Italian word for “for “lazinesslaziness.”.”

Inertia is DIRECTLY PROPORTIONAL TO Inertia is DIRECTLY PROPORTIONAL TO MASS MASS ONLYONLY. . (More mass means more inertia.) (More mass means more inertia.)

Inertia has nothing to do with size, Inertia has nothing to do with size, speed, or anything else.speed, or anything else.

What is the difference between What is the difference between mass and weightmass and weight??

WEIGHTWEIGHT is the is the pull due to pull due to gravity.gravity.

– Weight Weight CHANGES due CHANGES due to location!to location!

– Weight is a Weight is a FORCE!FORCE!

MASSMASS is the is the amount of amount of MATTER in an MATTER in an object.object.Mass does NOT Mass does NOT change, change, regardless of regardless of location.location.Mass is measured Mass is measured in kg.in kg.

inertiainertia…… If you are floating If you are floating

around in space outside around in space outside the space shuttle, and the space shuttle, and go up to it and give it a go up to it and give it a kick, what will be the kick, what will be the results?results?

a)The space shuttle will go flying off, of course, since it has no weight!

b) You will go flying off in the opposite direction with a hurt foot, of course, since the space shuttle has the same amount of mass (and inertia) in space as it does here on earth!

WHAT IS VOLUME?WHAT IS VOLUME?– The amount of space something takes upThe amount of space something takes up

WHAT IS DENSITY?WHAT IS DENSITY?– The amount of mass per unit of volumeThe amount of mass per unit of volume

D = m / VD = m / V

WHAT HAS MORE VOLUME: A WHAT HAS MORE VOLUME: A kilogram of lead, or a kilogram of kilogram of lead, or a kilogram of bread?bread?– THE BREAD TAKES UP MORE SPACE, OF THE BREAD TAKES UP MORE SPACE, OF

COURSE!COURSE!

WHAT HAS A HIGHER DENSITY: A WHAT HAS A HIGHER DENSITY: A kilogram of lead, or a kilogram of kilogram of lead, or a kilogram of bread?bread?– THE LEAD DOES—IT IS MORE COMPACTTHE LEAD DOES—IT IS MORE COMPACT

—it has more matter packed into a —it has more matter packed into a smaller space!smaller space!

What is force?What is force? A A push or a pullpush or a pull..

A A NETNET force is the leftover (or force is the leftover (or unbalanced) amount of force after you unbalanced) amount of force after you add up all the forces acting in a certain add up all the forces acting in a certain direction. direction.

What is ACCELERATION?What is ACCELERATION? A CHANGE in motion!A CHANGE in motion!

– That is ANY change in SPEED AND/OR That is ANY change in SPEED AND/OR DIRECTION!!!DIRECTION!!!

Only NET forces cause Only NET forces cause accelerationsaccelerations!!

No net forceNo net force means means NO CHANGE in NO CHANGE in speed or direction!speed or direction!

Newton’s 2Newton’s 2ndnd Law: Law: The BIGGER the amount of the force, The BIGGER the amount of the force, the BIGGER the acceleration!the BIGGER the acceleration!

The BIGGER the mass, The BIGGER the mass, the LESS the acceleration!the LESS the acceleration!

In fancy termsIn fancy termsThe acceleration of a body is directly proportional The acceleration of a body is directly proportional

to the net force on it, and inversely proportional to the net force on it, and inversely proportional to the mass.to the mass.

FORCE UNITSFORCE UNITSThink of Newton’s 2Think of Newton’s 2ndnd law: F law: F

= m a= m aN = kg m/sN = kg m/s22

Since mass is in kg, and a is in m/s/s, Since mass is in kg, and a is in m/s/s, or m/sor m/s22, F or Forces are measured in , F or Forces are measured in kg m/skg m/s22. .

That is the SAME as a Newton, or NThat is the SAME as a Newton, or N..

F = ma

100N = 100kg X 1.00m/s2

a = F/m

1.00m/s2 = 100N / 100kg

……Newton’s 2Newton’s 2ndnd Law… Law… This makes sense…the This makes sense…the

more massive something more massive something is, the is, the harderharder it is to it is to accelerate. And the accelerate. And the more force there is, the more force there is, the easiereasier something will something will accelerate.accelerate.

Zero net force Zero net force means zero means zero acceleration.acceleration.

F = maF = ma

A 2 kg mass accelerating at 3 m/sA 2 kg mass accelerating at 3 m/s22 has how much has how much net force on it?net force on it?

SOLUTIONSOLUTION

A 2 kg mass accelerating at 3 m/sA 2 kg mass accelerating at 3 m/s22 has how much net force on it?has how much net force on it?

F = ma = 2kg X 3 m/sF = ma = 2kg X 3 m/s22 = 6 N = 6 N

F = ma W = mgF = ma W = mg

If you weigh 150 pounds, If you weigh 150 pounds, your mass is about 68 your mass is about 68 kg. (1 kg = 2.2 lbs.) kg. (1 kg = 2.2 lbs.) What is your What is your WEIGHT in WEIGHT in NewtonsNewtons here on Earth? here on Earth?

SOLUTIONSOLUTION

If you weigh 150 pounds, your mass is If you weigh 150 pounds, your mass is about 68 kg. (1 kg = 2.2 lbs.) What is about 68 kg. (1 kg = 2.2 lbs.) What is your WEIGHT in Newtons here on Earth?your WEIGHT in Newtons here on Earth?

W = mg = 68kg X 10m/sW = mg = 68kg X 10m/s22 = 680 N = 680 NF = maF = ma

F = ma a = F/mF = ma a = F/m A force of 16 N is pulling on a 2 kg A force of 16 N is pulling on a 2 kg

wagon. If there is no friction, how fast wagon. If there is no friction, how fast is the wagon accelerating?is the wagon accelerating?

SOLUTIONSOLUTION

A force of 16 N is pulling on a 2 kg A force of 16 N is pulling on a 2 kg wagon. If there is no friction, how wagon. If there is no friction, how fast is the wagon accelerating?fast is the wagon accelerating?

a = F/m = 16N / 2kg = 8 m/sa = F/m = 16N / 2kg = 8 m/s22

F = ma a = F/mF = ma a = F/m

If the wagon above is going at a If the wagon above is going at a CONSTANT VELOCITYCONSTANT VELOCITY with the 16N with the 16N pull, pull, how much frictionhow much friction is there? is there?

SOLUTIONSOLUTION

If the wagon above is going at a If the wagon above is going at a CONSTANT VELOCITYCONSTANT VELOCITY with the 16N pull, with the 16N pull, how much frictionhow much friction is there? is there?

16 N ! If going constant velocity, 16 N ! If going constant velocity, acceleration is 0, & net force is 0 so acceleration is 0, & net force is 0 so all forces are BALANCED (equal) all forces are BALANCED (equal)

in any 1 direction.in any 1 direction.

Newton’s 3Newton’s 3rdrd Law: Law:Every action has an Every action has an

equal and opposite equal and opposite reaction.reaction.

There are NO single There are NO single forces. They are forces. They are always an always an action/reaction pair.”action/reaction pair.”

For example, you touch For example, you touch the table with 1N of the table with 1N of Force, it touches you Force, it touches you back with 1N of Force back with 1N of Force SIMULTANEOUSLY (SIMULTANEOUSLY (notnot a a little later!).little later!).

Newton’s 3Newton’s 3rdrd Law says that an action Law says that an action reaction pair would be reaction pair would be hands pushing hands pushing on barbell, barbell pushing on handson barbell, barbell pushing on hands..

Then HOW can the barbell be Then HOW can the barbell be ACCELERATED upwardACCELERATED upward??

What What OTHEROTHER action reaction pairs are action reaction pairs are involved?involved?

More on Newton’s 3More on Newton’s 3rdrd……

If you smash the wall with your If you smash the wall with your fist with 80 N of force, the wall fist with 80 N of force, the wall will hit you right back with 80 N.will hit you right back with 80 N.

But what if you hold a piece of But what if you hold a piece of paper in the air and try to hit it paper in the air and try to hit it with 80 N?with 80 N?

If the paper …If the paper … cannot support or hit you back with 80 cannot support or hit you back with 80

N, can YOU really hit IT with 80 N? N, can YOU really hit IT with 80 N?

If the paper can only hit you back with If the paper can only hit you back with 5 N, what happens to the other 75 N 5 N, what happens to the other 75 N from your punch??!from your punch??!

HINT:

How do they accelerate?How do they accelerate?

Force of frictionForce of frictionOpposes motionOpposes motion

Occurs between Occurs between surfaces in surfaces in contactcontact

Gravitational force…Gravitational force… Is the WEAKEST Is the WEAKEST

of all the forces, of all the forces, but acts over but acts over the GREATEST the GREATEST distances!distances!

You are pulling and pulled by EVERY You are pulling and pulled by EVERY ATOM IN THE UNIVERSE!!!ATOM IN THE UNIVERSE!!!

Electromagnetic Electromagnetic forces…forces…

Are the forces that act due to Are the forces that act due to electron configuration in atomselectron configuration in atoms

These forces are responsible These forces are responsible for the PROPERTIES or for the PROPERTIES or characteristics of elements, characteristics of elements, compounds, and materialscompounds, and materials

This force This force makes paint makes paint stick to a wall, stick to a wall, makes makes diamonds diamonds extremely hard, extremely hard, makes gold makes gold malleable, malleable, rubber stretchy, rubber stretchy, water a bipolar water a bipolar molecule, etc.molecule, etc.