Forces and MotionForces and MotionBook Chapter 12Book Chapter 12

12.1 - Forces12.1 - Forces

12.2 - 12.2 - Newton’s 1Newton’s 1stst and 2 and 2ndnd Law Lawss

12.3 - 12.3 - Newton’s 3Newton’s 3rdrd Law & Momentum Law & Momentum

12.4 - 12.4 - Universal ForcesUniversal Forces

ForcesForces

A force is a push or pull that acts on A force is a push or pull that acts on an object.an object.

A force can cause a resting object to A force can cause a resting object to move, or it can accelerate a move, or it can accelerate a moving object by changing the moving object by changing the object’s speed or direction.object’s speed or direction.

Measuring ForceMeasuring Force

Forces are measured in Forces are measured in NewtonsNewtons (named for Sir Issac Newton).(named for Sir Issac Newton).

One Newton is the force that causes One Newton is the force that causes a 1 kilogram mass to accelerate at a 1 kilogram mass to accelerate at 1 m/s1 m/s22..

Combining ForcesCombining Forces

Forces are vectors, so you can use an arrow Forces are vectors, so you can use an arrow to represent it. The arrow has to be to represent it. The arrow has to be proportional to the force and in the same proportional to the force and in the same direction.direction.

When you combine forces by adding or When you combine forces by adding or subtracting, you are finding the subtracting, you are finding the Net Force.Net Force.

The net force is the overall force acting on an The net force is the overall force acting on an object after all forces have been combined.object after all forces have been combined.

Balanced and Unbalanced Balanced and Unbalanced ForcesForces

When the forces on an object are When the forces on an object are balanced, the net force is zero and balanced, the net force is zero and there is no change in the object’s there is no change in the object’s motion.motion.

When an unbalanced force acts on When an unbalanced force acts on an object, the object accelerates.an object, the object accelerates.

FrictionFriction

Friction is a force that affects motion by slowing an Friction is a force that affects motion by slowing an object down.object down.

It is always opposite to the direction of motion and It is always opposite to the direction of motion and affects how much an object can accelerate.affects how much an object can accelerate.

Friction is present because of irregularities in the Friction is present because of irregularities in the surfaces of objects that are in contact with one surfaces of objects that are in contact with one another.another.

Fluids like water and air also exert a friction force on Fluids like water and air also exert a friction force on a moving object.a moving object.

Four Types of FrictionFour Types of Friction

1.1. Static Friction – friction force that acts on Static Friction – friction force that acts on objects that are not objects that are not

movingmoving2.2. Sliding Friction – Force that opposes the Sliding Friction – Force that opposes the

motion of an object as it motion of an object as it slides over a surfaceslides over a surface

3.3. Rolling Friction-friction that acts on rolling Rolling Friction-friction that acts on rolling objectsobjects

4.4. Fluid Friction-opposes motion as an object Fluid Friction-opposes motion as an object moves through a moves through a

fluidfluid

FrictionFriction

Direction of motionDirection of motion

FRICTION BETWEEN TIRES AND ROAD

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GravityGravity

Gravity is an attractive force that Gravity is an attractive force that acts between any two masses.acts between any two masses.

Gravity causes objects to accelerate Gravity causes objects to accelerate downward towards the center of downward towards the center of the earth.the earth.

Falling and Air ResistanceFalling and Air Resistance

Falling and Air ResistanceFalling and Air Resistance

As an object falls faster, the force of As an object falls faster, the force of air resistance increases until it air resistance increases until it equals the weight of the object. At equals the weight of the object. At this time, the net force equals zero this time, the net force equals zero and the object stops accelerating.and the object stops accelerating.

This final speed is called This final speed is called terminal terminal velocityvelocity..

Falling and Air ResistanceFalling and Air Resistance

Discovering the Reason for Discovering the Reason for MotionMotion

Aristotle- Greek scientist who Aristotle- Greek scientist who thought that force was necessary thought that force was necessary to maintain motionto maintain motion

See Aristotle’s ideas on motion.See Aristotle’s ideas on motion.

Discovering the Reason for Discovering the Reason for MotionMotion

Galileo – Italian Galileo – Italian scientist who did scientist who did experiments that experiments that helped correct helped correct misconceptions misconceptions about force and about force and motionmotion

Discovering the Reason for Discovering the Reason for MotionMotion

Newton – scientist Newton – scientist who introduced who introduced several laws several laws describing force describing force and motion.and motion.

Newton’s 1st LawNewton’s 1st Law

Law of InertiaLaw of Inertia

Everything in motion stays in motion, Everything in motion stays in motion, and everything at rest stays at rest and everything at rest stays at rest unless an outside force acts upon it.unless an outside force acts upon it.

Everything in motion and at rest has Everything in motion and at rest has inertia.inertia.

Law of InertiaLaw of Inertia

Law of InertiaLaw of Inertia

Law of InertiaLaw of Inertia

MassMass

Mass determines the amount of Mass determines the amount of inertia possessed by an object.inertia possessed by an object.

Mass is the amount of matter in an Mass is the amount of matter in an object.object.

Mass is NOT VolumeMass is NOT Volume

Volume is the amount of space an Volume is the amount of space an object takes up.object takes up.

EXAMPLE:EXAMPLE:

A pillow will have a volume larger A pillow will have a volume larger than a car battery but less mass.than a car battery but less mass.

Mass is NOT weightMass is NOT weight

Weight is the force of gravity acting on a Weight is the force of gravity acting on a bodybody

Objects in space will still have mass but will Objects in space will still have mass but will not have weight.not have weight.

Metric Units:Metric Units: mass - kilogrammass - kilogram

weight - Newtonweight - Newton

On the surface of the earth, mass and On the surface of the earth, mass and weight are proportional.weight are proportional.

1 kg = 9.8 Newtons1 kg = 9.8 Newtons

Calculating the weight of an Calculating the weight of an objectobject

Since weight depends on the mass of an object Since weight depends on the mass of an object AND the gravitational pull of the earth, we AND the gravitational pull of the earth, we can say thatcan say that

Weight = mass x gravityWeight = mass x gravity

OrOr

Weight = mgWeight = mg

The weight of an object is measured in The weight of an object is measured in NewtonsNewtons..

Because weight depends on gravity, it is also a Because weight depends on gravity, it is also a force.force.

What is the weight of a 500 What is the weight of a 500 gram rock?gram rock?

1. Convert to standard units1. Convert to standard units

500 grams = .5 kg500 grams = .5 kg

2. Remember the constant for gravity is 9.8 m/s2. Remember the constant for gravity is 9.8 m/s22..

3. Plug into the equation and solve.3. Plug into the equation and solve.

Weight = mgWeight = mg

Weight = (.5 kg)(9.8 m/sWeight = (.5 kg)(9.8 m/s22))

Weight = 4.9 N (Newtons)Weight = 4.9 N (Newtons)

QuestionsQuestions

1. Your empty hand is not harmed if it bangs against the 1. Your empty hand is not harmed if it bangs against the wall, but it is harmed if you are carrying a heavy load. wall, but it is harmed if you are carrying a heavy load. Why?Why?

2. Does a person diet to lose 2. Does a person diet to lose massmass or to lose or to lose weightweight??

3. Can the force of gravity on a 1 kg mass ever be 3. Can the force of gravity on a 1 kg mass ever be greater than on a 2 kg mass? Explain how.greater than on a 2 kg mass? Explain how.

4. A car at a junk yard is compressed until its volume is 4. A car at a junk yard is compressed until its volume is less than 1 cubic meter. Has its mass changed? Has less than 1 cubic meter. Has its mass changed? Has its weight changed? Has its volume changed? its weight changed? Has its volume changed? Explain.Explain.

5. If you jump up in a bus that is moving at a constant 5. If you jump up in a bus that is moving at a constant velocity, will you land farther back in the bus? Explain.velocity, will you land farther back in the bus? Explain.

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Newtons 2nd Law of Newtons 2nd Law of MotionMotion

Law of AccelerationLaw of Acceleration

What causes an object to What causes an object to accelerate?accelerate?

-or change its state of motion?-or change its state of motion?

NET FORCENET FORCE

What is a Net Force?What is a Net Force?

A net force is the combination of all forces A net force is the combination of all forces acting on an objectacting on an object

Applied ForcesApplied Forces Net ForceNet Force5 N

10 N

15 N

10 N5 N5 N

EquilibriumEquilibrium

Applied ForcesApplied Forces Net ForceNet Force

5 N 5 N0 N

If the net force is zero, the motion of the object will not change.

This is a condition called EQUILIBRIUM.

If an object is in equilibrium, we have to go back to the Law of Inertia.

AccelerationAcceleration

Acceleration is directly related to the Acceleration is directly related to the net force.net force.

Acceleration ~ Net ForceAcceleration ~ Net Force

The larger the net force is acting on an The larger the net force is acting on an object, the greater the acceleration of object, the greater the acceleration of the object. the object.

AccelerationAcceleration

Acceleration is inversely proportional Acceleration is inversely proportional to the mass of an objectto the mass of an object

Acceleration ~ Acceleration ~

Try to push a 250 pound crate. Now try to Try to push a 250 pound crate. Now try to push a 25 pound crate. Which could push a 25 pound crate. Which could you move faster?you move faster?

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massmass

Newton’s 2nd Law of Newton’s 2nd Law of MotionMotion

The acceleration of an object is The acceleration of an object is directly proportional to the net directly proportional to the net force acting on it, is in the same force acting on it, is in the same direction as the net force, and direction as the net force, and inversely proportional to the mass inversely proportional to the mass of the object.of the object.

a = a =

F

mOR F = ma

F

m a

Newton’s 3rd Law of Newton’s 3rd Law of MotionMotion

Action / Reaction Action / Reaction

Law of InteractionLaw of Interaction

Newton realized that force is not a Newton realized that force is not a thing in itself, but part of an thing in itself, but part of an interaction between one thing and interaction between one thing and another.another.

Forces always occur in Forces always occur in pairspairs

One force is called the action force and One force is called the action force and the other the reaction force.the other the reaction force.

Action: Object A exerts a force on Action: Object A exerts a force on Object B.Object B.

Reaction: Object B exerts an equal and Reaction: Object B exerts an equal and opposite force on Object A.opposite force on Object A.

Newton’s 3rd LawNewton’s 3rd Law

For each action force, there is an For each action force, there is an equal and opposite reaction force.equal and opposite reaction force.

Whenever one object exerts a force Whenever one object exerts a force on a second object, the second on a second object, the second object exerts anobject exerts an

equal and opposite force on the equal and opposite force on the first object.first object.

Action / Reaction ForcesAction / Reaction Forces

Action: Tire pushes road

Reaction: Road pushes tire

Action: Rocket pushes gas

Reaction: Gas pushes rocket

Action: Earth pulls ball

Reaction: Ball pulls Earth

Action / Reaction ForcesAction / Reaction Forces

Consider the apple at rest on the table. If we call the gravitational force exerted on the apple action, what is the reaction force? Are there any other action/reaction forces present?

Action / Reaction ForcesAction / Reaction Forces

If a cement truck and a car have a head-on collision, which vehicle will experience the greater impact force?

A. The cement truck

B. The car

C. Both the same

D. ….it depends on other factors

MomentumMomentum

Momentum is inertia in motion.Momentum is inertia in motion.

Momentum = (mass)(velocity)Momentum = (mass)(velocity)

It is harder to stop a large truck than a It is harder to stop a large truck than a small car when both are moving at small car when both are moving at the same speed because the truck the same speed because the truck has a larger mass and larger has a larger mass and larger momentummomentum

Calculating MomentumCalculating Momentum

ExampleExample

Which would be harder to stop….a .25 kg Which would be harder to stop….a .25 kg ball thrown at you at 40 m/s or a 7 kg ball thrown at you at 40 m/s or a 7 kg bowling ball rolled at 1 m/s?bowling ball rolled at 1 m/s?

s

kgmsmkgmvp

s

kgmsmkgmvp

7)/1)(7(

10)/40)(25(.

It would be harder to stop the baseball; it has more momentum.

Law of Conservation of Law of Conservation of MomentumMomentum

In the absence of an external force, the In the absence of an external force, the total momentum of a system remains total momentum of a system remains unchanged.unchanged.

This is an extension of Newton’s 3rd LawThis is an extension of Newton’s 3rd Law

(Mass)(Velocity before) = (Mass)(Velocity (Mass)(Velocity before) = (Mass)(Velocity after)after)

See an example of the Law of Conservation of Momentum (and EnSee an example of the Law of Conservation of Momentum (and Energy)ergy)

Conservation of Conservation of MomentumMomentum

Conservation of Conservation of MomentumMomentum

Universal ForcesUniversal Forces

Electromagnetic forces, Nuclear forces, and Electromagnetic forces, Nuclear forces, and Gravitational forces are all universal forces.Gravitational forces are all universal forces.

Electromagnetic force is associated with Electromagnetic force is associated with charged particles.charged particles.

Electric force and magnetic force are the Electric force and magnetic force are the only forces that can both attract and repel.only forces that can both attract and repel.

Nuclear ForcesNuclear Forces

Two forces, the strong nuclear force and Two forces, the strong nuclear force and the weak nuclear force, act within the the weak nuclear force, act within the nucleus to hold it together.nucleus to hold it together.

The strong nuclear force acts only on the The strong nuclear force acts only on the neutrons and protons in the nucleus.neutrons and protons in the nucleus.

The weak nuclear force acts over a short The weak nuclear force acts over a short range and affects all particles, not just range and affects all particles, not just protons and neutrons.protons and neutrons.

Gravitational ForceGravitational Force

Gravitational force is an attractive force Gravitational force is an attractive force that acts between two masses.that acts between two masses.

The bigger the masses and the closer The bigger the masses and the closer they are, the more the attraction force.they are, the more the attraction force.

Newton’s Law of Universal Gravitation Newton’s Law of Universal Gravitation states that every object in the universe states that every object in the universe attracts every other object.attracts every other object.

Gravitational ForceGravitational Force

Gravity is the weakest universal force, Gravity is the weakest universal force, but it is the most effective over long but it is the most effective over long distances.distances.

Earth’s gravitational force keeps the Earth’s gravitational force keeps the moon in a nearly circular orbit.moon in a nearly circular orbit.

The gravitational pull of the moon on The gravitational pull of the moon on the Earth causes ocean tides.the Earth causes ocean tides.

SatellitesSatellites

When an artificial When an artificial satellite is put into satellite is put into orbit, its inertia and orbit, its inertia and the pull of the Earth the pull of the Earth keep it in orbit. If keep it in orbit. If the satellite is too the satellite is too close to the close to the atmosphere, friction atmosphere, friction slows it down and it slows it down and it crashes into the crashes into the Earth.Earth.

Satellite without GravitySatellite without Gravity

Launch speed less than 8000 m/s

Launch speed equals 8000 m/s