Newton’s Newton’s Laws of Laws of MotionMotion

I.I. Law of Inertia Law of Inertia

II.II. F=ma F=ma

III.III. Action-Reaction Action-Reaction

While most people While most people know what Newton's laws know what Newton's laws say, many people do not say, many people do not know what they mean (or know what they mean (or simply do not believe what simply do not believe what they mean). they mean).

Newton’s Laws of MotionNewton’s Laws of Motion

11stst Law Law – An object at rest will stay at – An object at rest will stay at rest, and an object in motion will stay rest, and an object in motion will stay in motion at constant velocity, unless in motion at constant velocity, unless acted upon by an unbalanced force.acted upon by an unbalanced force.

22ndnd Law Law – – Force equals mass times Force equals mass times acceleration.acceleration.

33rdrd Law Law – – For every action there is an For every action there is an equal and opposite reaction.equal and opposite reaction.

11stst Law of Motion Law of Motion (Law of Inertia) (Law of Inertia)

An object at rest will stay at An object at rest will stay at rest, and an object in motion rest, and an object in motion will stay in motion at will stay in motion at constant speed and in a constant speed and in a straight line, unless acted straight line, unless acted upon by an unbalanced force.upon by an unbalanced force.

11stst Law Law

Unless acted Unless acted upon by an upon by an unbalanced unbalanced force, this golf force, this golf ball would sit on ball would sit on the tee forever. the tee forever.

11stst Law Law Once airborne, Once airborne,

unless acted on unless acted on by an by an unbalanced force unbalanced force (gravity and air (gravity and air – fluid friction), – fluid friction), it would never it would never stop! stop!

11stst Law of Motion Law of Motion (Law of Inertia) (Law of Inertia)

The motion of an object that has a net force of The motion of an object that has a net force of 0 Newtons. No unbalanced force is acting on 0 Newtons. No unbalanced force is acting on them.them.

Objects at restObjects at rest: An object that is not moving.: An object that is not moving. They will remain at rest.They will remain at rest.

Objects in motionObjects in motion: An object that is in motion : An object that is in motion will remain in motion.will remain in motion. In motion in a straight path unless an unbalanced In motion in a straight path unless an unbalanced

force acts on them.force acts on them.

Why then, do we observe Why then, do we observe every day objects in motion every day objects in motion slowing down and becoming slowing down and becoming motionless seemingly without an motionless seemingly without an outside force?outside force?

It’s a force we sometimes cannot see – It’s a force we sometimes cannot see – friction.friction.

What is this unbalanced force that acts on an What is this unbalanced force that acts on an object in motion?object in motion?

11stst Law of Motion Law of Motion (Law of Inertia) (Law of Inertia)

What about friction?What about friction? This is an This is an unbalanced force unbalanced force that acts upon that acts upon

objects. Examples:objects. Examples: Chair and the floor.Chair and the floor. Car slowing down.Car slowing down.

11stst Law of Motion Law of Motion (Law of Inertia) (Law of Inertia)

Objects on earth, unlike the Objects on earth, unlike the frictionless space the moon frictionless space the moon travels through, are under the travels through, are under the influence of influence of frictionfriction..

11stst Law Law InertiaInertia is the is the

tendency of an tendency of an object to resist object to resist being moved or, if being moved or, if the object is the object is moving, to resist moving, to resist change in speed or change in speed or direction until an direction until an outside force acts outside force acts on the object.on the object.

These pumpkins will not move unless acted on by an unbalanced force.

11stst Law Law Mass is a measure of Mass is a measure of inertia.inertia.

An object that has a An object that has a small mass small mass has has lessless inertia inertia than an object that has a than an object that has a large masslarge mass..

Inertia Inertia makes it easier to accelerate the bicycle makes it easier to accelerate the bicycle than the car, but it also makes the car harder to than the car, but it also makes the car harder to stop than the bicycle.stop than the bicycle.

Slide a book Slide a book across a table and across a table and watch it slide to a watch it slide to a rest position. The rest position. The book comes to a book comes to a rest because of the rest because of the presencepresence of a force of a force - that force being - that force being the force of friction the force of friction - which brings the - which brings the book to a rest book to a rest position.position.

In the absence of a force of friction, the book In the absence of a force of friction, the book would continue in motion with the same speed would continue in motion with the same speed and direction - forever! (Or at least to the end and direction - forever! (Or at least to the end of the table top.) of the table top.)

Newton's 1Newton's 1stst Law and You Law and You

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

Because of inertia, objects (including you) Because of inertia, objects (including you) resist changes in their motion. When the resist changes in their motion. When the car going 80 km/hour is stopped by the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 brick wall, your body keeps moving at 80 m/hour.m/hour.

Skateboard & Coke can Skateboard & Coke can demo.demo.

Newton’s Second LawNewton’s Second Law

The acceleration of an object depends on The acceleration of an object depends on mass.mass.

If you push a skateboard that is If you push a skateboard that is emptyempty, you , you only need a only need a small force small force to accelerate it. But, to accelerate it. But, the same amount of will not accelerate a the same amount of will not accelerate a skateboard that is skateboard that is full of books full of books as much as the as much as the empty skateboard.empty skateboard.

Newton’s Second LawNewton’s Second Law

Acceleration depends on force.Acceleration depends on force.

Suppose you gave the skateboard a Suppose you gave the skateboard a hard push hard push as opposed to a as opposed to a soft pushsoft push? The skateboard ? The skateboard will start moving will start moving faster faster than if you gave it a than if you gave it a soft push.soft push.

Acceleration of an object is always in the same Acceleration of an object is always in the same direction as the force applied.direction as the force applied.

22ndnd Law Expressed Law Expressed MathematicalyMathematicaly

22ndnd Law Law

F = M x AF = M x A

F = Force (expressed in N)F = Force (expressed in N)

M = Mass (expressed in Kg.)M = Mass (expressed in Kg.)

A = Acceleration (m/sA = Acceleration (m/s22))

22ndnd Law Law

When mass is in kilograms and acceleration is When mass is in kilograms and acceleration is in m/sin m/s22, the unit of force is in newtons (N)., the unit of force is in newtons (N).

One newton is equal to the force required to One newton is equal to the force required to accelerate one kilogram of mass at one accelerate one kilogram of mass at one meter/second/second.meter/second/second.

22ndnd Law (F = m x a) Law (F = m x a)

How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?

Write the formulaWrite the formula F = m x a Fill in given numbers and unitsFill in given numbers and units F = 1400 kg x 2 meters per second/second Solve for the unknownSolve for the unknown 2800 kg-meters/second/second or 2800 N

22ndnd Law (F = m x a) Law (F = m x a)

If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

• We know that objects with different masses accelerate to the ground at the same rate.

• However, because of the 2nd Law we know that they don’t hit the ground with the same force.

F = maF = ma

98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s

F = maF = ma

9.8 N = 1 kg x 9.8 9.8 N = 1 kg x 9.8 m/s/sm/s/s

Check Your UnderstandingCheck Your Understanding

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

   2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s22. .

Determine the mass.Determine the mass.

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

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

Check Your UnderstandingCheck Your Understanding

1. What acceleration will result when a 12 N net force applied to a 3 kg object? 1. What acceleration will result when a 12 N net force applied to a 3 kg object? 12 N = 3 kg x 4 m/s/s12 N = 3 kg x 4 m/s/s

   2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s22. Determine the . Determine the

mass.mass. 16 N = 3.2 kg x 5 m/s/s16 N = 3.2 kg x 5 m/s/s

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

66 kg-m/sec/sec or 66 N66 kg-m/sec/sec or 66 N

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

 9800 kg-m/sec/sec or 9800 N9800 kg-m/sec/sec or 9800 N

33rdrd Law Law

For every action, there is an For every action, there is an equal and opposite reaction.equal and opposite reaction.

Whenever one object exerts a force on a second Whenever one object exerts a force on a second object, the second object exerts an equal and opposite object, the second object exerts an equal and opposite force on the first.force on the first.

33rdrd Law Law

According to Newton, According to Newton, whenever objects A and whenever objects A and B interact with each B interact with each other, they exert forces other, they exert forces upon each other. When upon each other. When you sit in your chair, you sit in your chair, your body exerts a your body exerts a downward force downward force on the on the chair and the chair chair and the chair exerts an exerts an upward force upward force on your body. on your body.

33rdrd Law Law

There are two forces There are two forces resulting from this resulting from this interaction - a force on interaction - a force on the chair and a force on the chair and a force on your body. These two your body. These two forces are called forces are called actionaction and and reactionreaction forces. forces.

Newton’s 3rd Law in NatureNewton’s 3rd Law in NatureForce Pairs Do Not Act On Force Pairs Do Not Act On

The Same ObjectThe Same Object Consider the propulsion of a Consider the propulsion of a fish through the water. A fish fish through the water. A fish uses its fins to push water uses its fins to push water backwards. In turn, the backwards. In turn, the water water reactsreacts by pushing the by pushing the fish forwards, propelling the fish forwards, propelling the fish through the water.fish through the water.

The size of the force on the The size of the force on the water equals the size of the water equals the size of the force on the fish; the force on the fish; the direction of the force on the direction of the force on the water water (backwards) (backwards) is is opposite the direction of the opposite the direction of the force on the fish force on the fish (forwards).(forwards).

33rdrd Law Law

Flying gracefully Flying gracefully through the air, through the air, birds depend on birds depend on Newton’s third Newton’s third law of motion. As law of motion. As the birds the birds push push down down on the air on the air with their wings, with their wings, the the air pushesair pushes their wings their wings upup and gives them and gives them lift.lift.

33rdrd Law Law

Consider the flying motion of birds. A bird flies by Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air use of its wings. The wings of a bird push air downwardsdownwards. In turn, the air reacts by pushing the bird . In turn, the air reacts by pushing the bird upwards. upwards.

The size of the force on the air equals the size of the The size of the force on the air equals the size of the force on the bird; the direction of the force on the air force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on (downwards) is opposite the direction of the force on the bird (upwards).the bird (upwards).

Action-reactionAction-reaction force pairs make it possible for birds force pairs make it possible for birds to fly.to fly.

Other examples of Newton’s Other examples of Newton’s Third LawThird Law

The baseball forces the The baseball forces the bat to the left (an bat to the left (an action); the bat forces action); the bat forces the ball to the right (the the ball to the right (the reaction). reaction).

33rdrd Law Law

Consider the motion of Consider the motion of a car on the way to a car on the way to school. A car is school. A car is equipped with wheels equipped with wheels which spin backwards. which spin backwards. As the wheels spin As the wheels spin backwards, they grip the backwards, they grip the road and push the road road and push the road backwards.backwards.

33rdrd Law LawThe reaction of a rocket The reaction of a rocket is an application of the is an application of the third law of motion. third law of motion. Various fuels are burned Various fuels are burned in the engine, producing in the engine, producing hot gases. hot gases.

The hot gases push The hot gases push against the inside tube of against the inside tube of the rocket and escape out the rocket and escape out the bottom of the tube. the bottom of the tube. As the gases move As the gases move downward, the rocket downward, the rocket moves in the opposite moves in the opposite direction.direction.

Bottle Rocket Demo.Bottle Rocket Demo.

MomentumMomentum

If a compact car and a large truck are moving If a compact car and a large truck are moving at the same speed and they apply the brakes at at the same speed and they apply the brakes at the same time, which vehicle will stop first?the same time, which vehicle will stop first?

The carThe car Why?Why? MomentumMomentum

MomentumMomentum

The momentum of an object depends on the The momentum of an object depends on the objects mass and velocity.objects mass and velocity.

So a truck with more mass than a compact car So a truck with more mass than a compact car will have more momentum and be harder to will have more momentum and be harder to stop.stop.

MomentumMomentum

MomentumMomentum is inertia in motion. is inertia in motion.

It is determined by how big an object is and It is determined by how big an object is and how fast it is moving.how fast it is moving.

p = m x vp = m x v (kg x m/s)(kg x m/s)

A stationary object has zero momentum.A stationary object has zero momentum.

Practice problem:Practice problem:

What is the momentum of an ostrich with a mass of What is the momentum of an ostrich with a mass of 120 kg. that runs with a velocity of 16 m/s north?120 kg. that runs with a velocity of 16 m/s north?

Write out the equation: Write out the equation: p = m x vp = m x v

Replace the values:Replace the values: 120 kg x 16 m/s north120 kg x 16 m/s north

Calculate:Calculate: 19,200 kg * m/s north19,200 kg * m/s north

Is it possible for a little Volkswagen to have Is it possible for a little Volkswagen to have more momentum than a Mack truck?more momentum than a Mack truck?

Explain.Explain.

The Law of Conservation of MomentumThe Law of Conservation of Momentum states states that the total amount of momentum of an that the total amount of momentum of an object or group of objects does not change object or group of objects does not change (unless acted on by a net force).(unless acted on by a net force).

Think of it like a class poker day in Physical Science. Think of it like a class poker day in Physical Science. Everybody brings their money, and some of that money Everybody brings their money, and some of that money will change hands throughout the course of the hour. But will change hands throughout the course of the hour. But from beginning to end, the total amount of money at the from beginning to end, the total amount of money at the beginning of the hour will be the same as at the end of the beginning of the hour will be the same as at the end of the hour.hour.

Examples:Examples:

Newton's CradleNewton's Cradle Dog pileDog pile Jar of marbles droppingJar of marbles dropping Pool breakPool break Car wreckCar wreck

MomentumMomentum

Interactive Computer Lab http://www.sciencejoywagon.com/physicszone

/06momentum/

Moving ManMoving Man http://phet.colorado.edu/en/simulation/moving

-man

Jump Off a CartJump Off a Cart

http://physics.weber.edu/amiri/director/http://physics.weber.edu/amiri/director/dcrfiles/momentum/carJumpS.dcrdcrfiles/momentum/carJumpS.dcr

Satellite MotionSatellite Motion SatellitesSatellites are objects in space which orbit a are objects in space which orbit a

larger body.larger body. Man-made satellites are used for Man-made satellites are used for

communications, weather, surveillance, communications, weather, surveillance, positioning systems, etc.positioning systems, etc.

1. Kenny drops an object out of a window. If gravity exerts a force of 147 N on the object, what is the mass of the object?

2. A baseball player hits a high fly ball to the outfield. If the ball is in the air for a total of 6 seconds, how fast was the ball traveling downward when the outfielder caught it?

3. In problem #3, how fast was the ball traveling upward when it left the bat? Explain.

4. Two bumper cars collide with each other. The first car has a mass of 124 kg (car and driver), while the second car has a total mass of 148 kg. When the cars collide, the first is knocked backwards with a rate of acceleration of 4.77 m/s/s. At what rate of acceleration was the other car knocked backwards?

5. Which will have more momentum, a softball (.42 kg) thrown at 45 m/s, or a baseball (.35 kg) thrown at 55 m/s? Show work.

In 1999, Krishna Gopal Shrivestava of India set an unverified world record by pulling a ship with a mass of 244,000 kg (536,800 pounds) with his teeth! His efforts can teach us about force, mass, and acceleration.

1. What would happen if the man were pulling a small rowboat instead of a large ship?

2. When he stops pulling, will the ship stop moving? Explain.

3. What direction is the ship moving? Why?

There is no up or down in orbit. If not secured, objects simply There is no up or down in orbit. If not secured, objects simply float here and there. Planes like NASA’s KC-135, the “Vomit float here and there. Planes like NASA’s KC-135, the “Vomit Comet,” are able to briefly simulate the conditions of being in Comet,” are able to briefly simulate the conditions of being in orbit.orbit.

1.On the plane, how can you tell up from down?

2. If a scale floated by and an astronaut stood on it, what would the scale say?

• It takes more than one firefighter to aim a fire hose. The force of the water shooting out of the nozzle causes a reaction that can be difficult to control.

1. Describe what happens if you step from a canoe or small boat onto a dock. How is this related to the fire hose?

2. Does a garden hose need more than one person to hold it? What’s the difference between a garden hose and a fire hose?

3. What would happen if the firefighters dropped the hose?

The dart is attached to the hoop by a string The dart is attached to the hoop by a string fed through an i-screw. So when the dart fed through an i-screw. So when the dart is thrown at the hoop, the string will is thrown at the hoop, the string will slacken and the hoop will drop downward slacken and the hoop will drop downward as the dart flies towards it. Where should as the dart flies towards it. Where should you aim in order to make the dart go you aim in order to make the dart go through the middle of the hoop as it is through the middle of the hoop as it is falling?falling?

Which one do you think Which one do you think moved further, him moved further, him forward or the canoe forward or the canoe backwards, and why?backwards, and why?

Eric (mass=70 kg) was canoeing at the lake and decided to Eric (mass=70 kg) was canoeing at the lake and decided to take a swim. When he dove off the front of the canoe, the take a swim. When he dove off the front of the canoe, the canoe moved backward at 4.5 m/s/s. If the canoe had a canoe moved backward at 4.5 m/s/s. If the canoe had a mass of 55 kg, at what rate did Eric accelerate forward mass of 55 kg, at what rate did Eric accelerate forward when he dove off the canoe?when he dove off the canoe?

Jim and Andy both threw a baseball through the air. Jim and Andy both threw a baseball through the air. Jim’s baseball landed 180 ft. away, while Andy’s Jim’s baseball landed 180 ft. away, while Andy’s landed 110 ft. away. landed 110 ft. away.

Which ball was in the air for a longer period of time?Which ball was in the air for a longer period of time? What are the forces which cause the baseballs to follow a What are the forces which cause the baseballs to follow a

curved path?curved path? How are these forces related?How are these forces related?

A mortar is fired at a target and it takes the shell 10 A mortar is fired at a target and it takes the shell 10 seconds to hit the target. (assume no effect of air seconds to hit the target. (assume no effect of air resistance)resistance)

What was the shells vertical velocity when it left the mortar?What was the shells vertical velocity when it left the mortar? If the shell travelled 800 m, what was it’s horizontal velocity?If the shell travelled 800 m, what was it’s horizontal velocity?

Four objects are dropped out of a window: a Four objects are dropped out of a window: a bowling ball, a feather, an iron weight, and a cat. bowling ball, a feather, an iron weight, and a cat. Which one will reach terminal velocity firstWhich one will reach terminal velocity first? ?

A dump truck (4,000 kg) is carrying a load of A dump truck (4,000 kg) is carrying a load of gravel (6,000 kg)gravel (6,000 kg)

A.A. How much momentum does it have if it’s traveling 60 mi/hr How much momentum does it have if it’s traveling 60 mi/hr (26.7 m/s)(26.7 m/s)

B. What are 2 ways that the momentum figured in A could be B. What are 2 ways that the momentum figured in A could be decreased by ½?decreased by ½?

A parachutist jumps from a A parachutist jumps from a plane, and due to air resistance plane, and due to air resistance she accelerates downward at ½ she accelerates downward at ½ the normal rate.the normal rate.

If her terminal velocity is 78.4 If her terminal velocity is 78.4 mi/hr, how long will it take for her mi/hr, how long will it take for her to reach that speed?to reach that speed?

What, in general would happen to What, in general would happen to her terminal velocity if she went her terminal velocity if she went head first instead of spread-eagle?head first instead of spread-eagle?

A sky-diving instructor is taking a tandem jump A sky-diving instructor is taking a tandem jump with one of his students. What effect, if any, with one of his students. What effect, if any, would this have on the instructors terminal would this have on the instructors terminal velocity if he had another person strapped to his velocity if he had another person strapped to his back? Hmmmm………back? Hmmmm………