Forces and Newton’s LawsForces and Newton’s Laws

PhysicsPhysics

Classical or Newtonian Classical or Newtonian mechanicsmechanics

Describes motion of macroscopic objects.Describes motion of macroscopic objects.Stays away from objects approaching the Stays away from objects approaching the

speed of light speed of light special relativity. special relativity.Stays away from teeny, tiny objects Stays away from teeny, tiny objects

quantum mechanics.quantum mechanics.

What is a Force?What is a Force? A Push or a Pull!A Push or a Pull! Standard Unit is the NEWTON (N)Standard Unit is the NEWTON (N) Types:Types:

Contact Forces (macroscopic)Contact Forces (macroscopic) Normal contact stuff (support forces, pushes, pulls,..)Normal contact stuff (support forces, pushes, pulls,..) Friction (always in the opposite direction of motion)Friction (always in the opposite direction of motion) Air Resistance (again always opposite in direction)Air Resistance (again always opposite in direction) Tension (Ropes, Wires, Cables)Tension (Ropes, Wires, Cables)

Field ForcesField Forces Electric, Magnetic, NuclearElectric, Magnetic, Nuclear Gravity (exists between any two objects)Gravity (exists between any two objects)

Requires lots of mass to be significant!Requires lots of mass to be significant! On earth: 1 kg is pulled on with a force of 9.8 NewtonsOn earth: 1 kg is pulled on with a force of 9.8 Newtons Practice ….Practice ….

Using Gravitational Field to Using Gravitational Field to Determine Weight / MassDetermine Weight / Mass

12 kg = __________ N12 kg = __________ N100 grams = ________ N100 grams = ________ N10 N = __________ kg10 N = __________ kg I weigh 175 pounds.I weigh 175 pounds.

How much mass is that in kg (earth)?How much mass is that in kg (earth)?What is that weight in N?What is that weight in N?

Overview of Newton’s LawsOverview of Newton’s Laws

First Law:First Law: What happens to the motion What happens to the motion of an object if there is no total, net force of an object if there is no total, net force acting on it?acting on it?

Second Law:Second Law: What happens to the What happens to the motion of an object if there are forces motion of an object if there are forces acting on it?acting on it?

Third Law:Third Law: What happens to the What happens to the motion of several objects when they motion of several objects when they interact? interact?

Newton’s First Law of InertiaNewton’s First Law of Inertia

Every object continues in its state of rest, or Every object continues in its state of rest, or of uniform motion in a straight line, unless it is of uniform motion in a straight line, unless it is compelled to change that state by forces compelled to change that state by forces impressed upon it.impressed upon it.

In other words; if there is no NET FORCE In other words; if there is no NET FORCE acting on the object, it’s motion should remain acting on the object, it’s motion should remain unchanged!unchanged!

The amount of Inertia an object has depends The amount of Inertia an object has depends on its masson its mass Ex’sEx’s Inertial BalanceInertial Balance

How does one weigh things in How does one weigh things in space?space? You can’t You can’t

WEIGH WEIGH things in things in space, but space, but one can one can mass mass object.object.

Inertia is WAY better than a Inertia is WAY better than a towel. Why?towel. Why?

More inertiaMore inertia

EggEggWhy does the blood rush to your feet Why does the blood rush to your feet

when a descending elevator stop?when a descending elevator stop?How do you get the last bit of ketchup out How do you get the last bit of ketchup out

of the bottle?of the bottle?Do head rests help prevent whiplash from Do head rests help prevent whiplash from

front-end or rear-end collisions?front-end or rear-end collisions?Shot putShot put

Review…Review…

How many Newtons does a 6 kg object How many Newtons does a 6 kg object experience due to the gravitational pull of experience due to the gravitational pull of the Earth?the Earth?

25N = _____kg25N = _____kgWhich of the following has the most Which of the following has the most

inertia? A submarine, a tricycle or an inertia? A submarine, a tricycle or an SUV?SUV?

What is inertia?What is inertia?

NET FORCENET FORCE

The sum of all forces acting on an object is the The sum of all forces acting on an object is the object’s NET FORCE.object’s NET FORCE.

When the NET FORCE is zero, the object is said When the NET FORCE is zero, the object is said to be at equilibrium.to be at equilibrium. Sometimes Equilibrium occurs and the object is at Sometimes Equilibrium occurs and the object is at

restrest Sometimes Equilibrium occurs and the object is Sometimes Equilibrium occurs and the object is

moving AT CONSTANT VELOCITY!moving AT CONSTANT VELOCITY! Ex’s – Support forces and scalesEx’s – Support forces and scales

Practice problemsPractice problems

80 kg = ___________ Newtons80 kg = ___________ Newtons 15 kg = ___________ Newtons15 kg = ___________ Newtons 1000 N = __________ Kg1000 N = __________ Kg 50 grams = ________ N50 grams = ________ N 20 pounds = _______ Newtons20 pounds = _______ Newtons Will a scale reading increase or decrease as you Will a scale reading increase or decrease as you

accelerate up in an elevator?accelerate up in an elevator? Will a scale reading increase or decrease as you Will a scale reading increase or decrease as you

move downward at constant velocity in an move downward at constant velocity in an elevator?elevator?

Diagrams and VectorsDiagrams and Vectors Force DiagramsForce Diagrams

Should consist of a small picture of the objectShould consist of a small picture of the object Also consist of scaled arrows that represent force acting Also consist of scaled arrows that represent force acting

on the object. Longer the arrow, greater the force. on the object. Longer the arrow, greater the force. Each “Force Arrow” should be labeled as “Object B on Each “Force Arrow” should be labeled as “Object B on

Object A”, where Object A is the object your concerned Object A”, where Object A is the object your concerned with.with.

Ex’s…. Practice …. Practice …Ex’s…. Practice …. Practice … Vectors: Force is a VECTOR – therefore we Vectors: Force is a VECTOR – therefore we

should follow the rules for adding vectors.should follow the rules for adding vectors. If in the same direction they just add up.If in the same direction they just add up. If in the opposite direction they just subtract.If in the opposite direction they just subtract. If at an angle, line the tails up together and create a If at an angle, line the tails up together and create a

parallelogram. The diagonal of the parallelogram parallelogram. The diagonal of the parallelogram (starting from the “tails”) is the Resultant. (starting from the “tails”) is the Resultant.

Ex’s … Practice … PracticeEx’s … Practice … Practice HONORS

Check for Understanding…Check for Understanding… Examine the diagram below of 3 books of Examine the diagram below of 3 books of

different masses atop a scale.different masses atop a scale. What is the mass of the middle book?What is the mass of the middle book? With what force does Book C exert on Book B?With what force does Book C exert on Book B? Draw a complete force diagram for each book !!!Draw a complete force diagram for each book !!!

Book A

Book B

Book C

Scale = 65 Newtons

Mass = 1 kg

Mass = 2.5 kg

Mass = ?

Check for Understanding… Again..Check for Understanding… Again.. Examine the diagram below of 3 planters of Examine the diagram below of 3 planters of

different masses hanging from each other.different masses hanging from each other. Plant B exerts 120 Newtons of Force on Plant A.Plant B exerts 120 Newtons of Force on Plant A. What is the mass of Plant B?What is the mass of Plant B? Draw a complete force diagram for each plant !!!Draw a complete force diagram for each plant !!!

Mass = 4 kg

Mass = 7 kg

Mass = ?

Plant A

Plant B

Plant C

Ceiling

Warm-upWarm-up

1.1. A student drags a sled across loosely packed A student drags a sled across loosely packed snow with a rightward acceleration. Draw a snow with a rightward acceleration. Draw a force diagram of the student and another force diagram of the student and another diagram of the sled.diagram of the sled.

2.2. A car is traveling at constant velocity up a a A car is traveling at constant velocity up a a hill to the right. Draw a force diagram for the hill to the right. Draw a force diagram for the car.car.

3.3. An airplane, traveling to the right, is An airplane, traveling to the right, is accelerating up and to the right. Draw a force accelerating up and to the right. Draw a force diagram for the plane.diagram for the plane.

Jar of FliesJar of Flies• A bunch of flies are in a capped jar. You place the jar on a

scale. The scale will register the most weight when the flies are

a) Sitting on the bottom of the jar.b) Flying around inside the jar.c) …. Weight of the jar is the

same in both cases.

Draw the force diagram for the block on the table. The mass of the little block is 10 kg. and the mass of the larger block is 5 kg. It is in static equilibrium.

Pulley PracticePulley Practice

The mass of the block is 4 The mass of the block is 4 kg. Find all five of the kg. Find all five of the tensions labeled in the tensions labeled in the diagram. (hint: draw force diagram. (hint: draw force diagrams for the pulleys)diagrams for the pulleys)

Draw force diagrams for A and B.Draw force diagrams for A and B.

Block A in the figure is Block A in the figure is heavier than block B heavier than block B and is sliding down the and is sliding down the incline at constant incline at constant velocity. All surfaces velocity. All surfaces have friction. The rope have friction. The rope is massless, and the is massless, and the massless pulley turns massless pulley turns on frictionless bearings.on frictionless bearings.

KinkKinkWater is shooting out the end of a pipe. The Water is shooting out the end of a pipe. The

end of the pipe is bent into a figure 6. end of the pipe is bent into a figure 6. Ignoring the effects of gravity, which of the Ignoring the effects of gravity, which of the following is true?following is true?

a) The water shoots outin a curved arc.

b) The water shoots outin a straight line.

FrictionFriction Two types, static and kineticTwo types, static and kinetic

Static Friction describes the resistive forces between Static Friction describes the resistive forces between two objects at rest.two objects at rest.

Kinetic Friction describes the resistive forces between Kinetic Friction describes the resistive forces between two objects that slide past each other.two objects that slide past each other.

Kinetic Friction is always less than static.Kinetic Friction is always less than static. Ex’s Tires and roadEx’s Tires and road

Friction depends only on two things; Normal Force Friction depends only on two things; Normal Force (support force) and the coefficient of friction(support force) and the coefficient of friction

Coefficient of friction describes how “sticky” the two objects Coefficient of friction describes how “sticky” the two objects are. These are listed in tables somewhere….are. These are listed in tables somewhere….

FFfrictionfriction = = μμ FFnormalnormal Ex…Ex…

Newton’s Second LawNewton’s Second Law

If there is a net force on an object – it will If there is a net force on an object – it will accelerate. (Understand Motion first!)accelerate. (Understand Motion first!)The acceleration will be proportional to the net The acceleration will be proportional to the net

force on the objectforce on the objectBut the acceleration will be inversely But the acceleration will be inversely

proportional to the mass (inertia) of the objectproportional to the mass (inertia) of the objectAcceleration = Net Force / MassAcceleration = Net Force / MassEx’sEx’s

Free Fall (a = 9.8 m/sFree Fall (a = 9.8 m/s22))

In the absence of air resistance and In the absence of air resistance and friction, all objects should fall at the same friction, all objects should fall at the same rate regardless of mass/weight.rate regardless of mass/weight.

Known as the “law of falling bodies”Known as the “law of falling bodies”While heavier objects have more force While heavier objects have more force

acting on them, they also have more acting on them, they also have more inertia, which keeps the ratio of F/M the inertia, which keeps the ratio of F/M the same.same.

Feather and Hammer Drop on the moon.Feather and Hammer Drop on the moon.

Non Free-Fall (a < 9.8 m/sNon Free-Fall (a < 9.8 m/s2)2)

Objects falling through the air will experience a Objects falling through the air will experience a drag force (air resistance)drag force (air resistance)

The amount of Air drag depends on …The amount of Air drag depends on … The cross-sectional area of the object (size)The cross-sectional area of the object (size) The speed of the objectThe speed of the object The density of the airThe density of the air The “texture” of the surface of the object (coefficient The “texture” of the surface of the object (coefficient

of drag, Cof drag, CDD)) ExEx

At one moment a skydiver (50 kg) is falling with an At one moment a skydiver (50 kg) is falling with an acceleration of 5 m/s/s, what is the force of air drag?acceleration of 5 m/s/s, what is the force of air drag?

Friction Frictional Force only depends on the surface

features (roughness) and the support force on the object.

Contrary to what many people think, Friction has very little to do with Area of contact.

Two types: Static Friction: Contacting surfaces lock together like

puzzle pieces when at rest. Kinetic Friction: Surfaces ride along the edge of each

other when in motion. Static Friction is always grater than Kinetic. Ex’s: Phone book friction, simple experiment

Newtons Third LawNewtons Third Law

For every action force there exists a reaction For every action force there exists a reaction force that is the same in magnitude but opposite force that is the same in magnitude but opposite in direction.in direction.

Ex’sEx’s Action:Action: My feet push on the floor with 650 N of force My feet push on the floor with 650 N of force Reaction:Reaction: The floor pushes on my feet with 650 N of The floor pushes on my feet with 650 N of

forceforce Action:Action: My cars’ tires push backwards on the My cars’ tires push backwards on the

pavement with 10,000 N of force.pavement with 10,000 N of force. Reaction:Reaction:

Vectors (Honors)Vectors (Honors)

To resolve multiple vectors at various To resolve multiple vectors at various angles:angles:Setup a x-y coordinate systemSetup a x-y coordinate systemBreak down each vector into x and y Break down each vector into x and y

componentscomponentsX component = hyp x Cos X component = hyp x Cos θθY component = hyp x Sin Y component = hyp x Sin θθ

Add up the x’s and y’s and follow Add up the x’s and y’s and follow Pythagorean to get resultant. XPythagorean to get resultant. X22 + y + y22 = R = R22