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Newton’s Laws of Motion

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

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Newton’s Laws of Motion 1 st Law – Law of Inertia: An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 1 st Law – Law of Inertia: An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 2 nd Law – Law of Acceleration, or F=ma 2 nd Law – Law of Acceleration, or F=ma 3 rd Law – Law of Interaction: For every action there is an equal and opposite reaction. 3 rd Law – Law of Interaction: For every action there is an equal and opposite reaction.

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1 st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

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1 st Law Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force.

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

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

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Check for Understanding If you hold a coin above your head while in a bus that is not moving, the coin will land at your feet when you drop it. Where will it land if the bus is moving in a straight line at constant speed? If you hold a coin above your head while in a bus that is not moving, the coin will land at your feet when you drop it. Where will it land if the bus is moving in a straight line at constant speed? It will fall in the same spot as if the bus wasn't moving. This is because once the bus is moving at a constant velocity, the coin is also moving at the velocity, and so are you. So in reality, when you dropped the coin, it moved forward in addition to falling down. However since you are moving the same speed as the coin, it appears to fall straight down. Motion is relative. It will fall in the same spot as if the bus wasn't moving. This is because once the bus is moving at a constant velocity, the coin is also moving at the velocity, and so are you. So in reality, when you dropped the coin, it moved forward in addition to falling down. However since you are moving the same speed as the coin, it appears to fall straight down. Motion is relative.

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Check for Understanding In the cabin of a jetliner that cruises at 600 km/hr, a pillow drops from an overhead compartment into your lap below. Since the jetliner is going so fast, why doesn’t the pillow slam into the rear of the plane when it falls out? What is the speed of the pillow relative to the jetliner? To the ground? In the cabin of a jetliner that cruises at 600 km/hr, a pillow drops from an overhead compartment into your lap below. Since the jetliner is going so fast, why doesn’t the pillow slam into the rear of the plane when it falls out? What is the speed of the pillow relative to the jetliner? To the ground?

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Net Force The sum of all forces on an object. The net force changes an object’s state of motion. The sum of all forces on an object. The net force changes an object’s state of motion. Ex: pushing a book across a table. Ex: pushing a book across a table. Gravity, air friction, & your pushing muscles = the net force Gravity, air friction, & your pushing muscles = the net force Newton (N) is the scientific unit of force/weight Newton (N) is the scientific unit of force/weight

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Net Force

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Equilibrium for Objects at Rest (Demo spring scale & weight) (Demo spring scale & weight) Tension – the state of being stretched (the spring scale experiences a “stretching” force) Tension – the state of being stretched (the spring scale experiences a “stretching” force) Tension force acts upwards & weight acts downwards. 2 forces equal & opposite = 0, so it’s resting Tension force acts upwards & weight acts downwards. 2 forces equal & opposite = 0, so it’s resting

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Equilibrium Rule When the net force on an object is 0, its state of motion doesn’t change, & the object is in mechanical equilibrium When the net force on an object is 0, its state of motion doesn’t change, & the object is in mechanical equilibrium Σ = vector (direction) Σ = vector (direction) SUM OF SUM OF F = forces F = forces Forces acting upwards (+) Forces acting upwards (+) balance w/forces acting downwards (-) balance w/forces acting downwards (-) (+) and (-) = 0 (+) and (-) = 0

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Review (Speed & Equilib) 1) Rosa’s sports car takes 60.0 seconds to cover 1200 meters. What’s the average speed in meters per second? 2) Marisela walks at an average speed of 0.5 m/s from the Bay Farm bridge to the Hornet (aircraft carrier). The trip takes her 1300 seconds. How far did she walk in meters? 3) A snail travels at an average speed of m/s across a 3.0 meter walkway. How long does the snail take to cross the walkway? 4) Mario weighs 400N & stands in the middle of a board that weighs 100N that is hanging from 2 scales (one 1 on each side). Find the upward force of each scale. 5) Mario moves to the left, and the reading on the left scale (the one closest to him) is 350. What is the reading on the scale on the right? (DRAW DIAGRAM)

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Review - Equilibrium 1) Mario weighs 400N & stands in the middle of a board that weighs 100N that is hanging from 2 scales (1 on each side). Find the upward force of each scale. 2) Mario moves to the left, and the reading on the left scale (the one closest to him) is 350. What is the reading on the scale on the right? DRAW A PICTURE! DRAW A PICTURE!

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??? Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – FRICTION. It’s a force we sometimes cannot see – FRICTION.

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

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A force that arises when an object rubs against something else A force that arises when an object rubs against something else There are four main types of friction: There are four main types of friction: Sliding friction: Sliding friction: ice skating Rolling friction: Rolling friction: bowling Fluid friction (air or liquid): Fluid friction (air or liquid): air or water resistance Static friction: Static friction: initial friction when moving an object What is this unbalanced force that acts on an object in motion?

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

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

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Newtons’s 1 st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

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Support Force The force that supports an object against gravity The force that supports an object against gravity Ex: book lying on table Ex: book lying on table Support force is the upward force from the table Support force is the upward force from the table Support force is equal to weight of book Support force is equal to weight of book Support force + gravity = 0 Support force + gravity = 0 Ex: bathroom scale (2 forces) Ex: bathroom scale (2 forces) Downward pull of gravity (your weight) Downward pull of gravity (your weight) upward support of floor upward support of floor Scale shows support force Scale shows support force

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Equilibrium for Moving Objects A moving object is in equilibrium if it moves steadily, without changing its state of motion A moving object is in equilibrium if it moves steadily, without changing its state of motion A rolling ball is in equilibrium if it moves at constant velocity A rolling ball is in equilibrium if it moves at constant velocity ΣF=0 ΣF=0

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2 nd Law The acceleration produced by a net force acting on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.

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2 nd Law When mass is in kilograms and acceleration is in m/s/s (m/s 2 ), the unit of force is in newtons (N). When mass is in kilograms and acceleration is in m/s/s (m/s 2 ), the unit of force is in newtons (N). One Newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second. One Newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

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2 nd Law (F = m x a) How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second? Write the formula Write the formula F = m x a Fill in given numbers and units Fill in given numbers and units F = 1400 kg x 2 meters per second/second Solve for the unknown Solve for the unknown 2800 kg-meters/second/second or 2800 N

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

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Newton’s 2 nd 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 2 nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s

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Check Your Understanding 1. What acceleration will result when a 24 N net force applied to a 4 kg object? A 8 kg object? 1. What acceleration will result when a 24 N net force applied to a 4 kg object? A 8 kg object? 2. A net force of 36 N causes a mass to accelerate at a rate of 4 m/s 2. Determine the mass. 2. A net force of 36 N causes a mass to accelerate at a rate of 4 m/s 2. Determine the mass. 3. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec? 3. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec? 4. A frictional force of 10 N acts against a forward force of 25 N. If the forward force is applied to a 12-kg object, what is the acceleration of the object? 4. A frictional force of 10 N acts against a forward force of 25 N. If the forward force is applied to a 12-kg object, what is the acceleration of the object?

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Check Your Understanding 1. What acceleration will result when a 24 N net force applied to a 4 kg object? 8 kg? 1. What acceleration will result when a 24 N net force applied to a 4 kg object? 8 kg? 24 N = 4 kg x 6 m/s 2 24 N = 4 kg x 6 m/s 2 24 N = 8 kg x 3 m/s 2 24 N = 8 kg x 3 m/s 2 2. A net force of 36 N causes a mass to accelerate at a rate of 4 m/s 2. Determine the mass. 2. A net force of 36 N causes a mass to accelerate at a rate of 4 m/s 2. Determine the mass. 36 N = 9 kg x 4 m/s 2 36 N = 9 kg x 4 m/s 2 3. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s 2 ? 3. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s 2 ? 9800 kg-m/s 2 or 9800 N 4. A frictional force of 10 N acts against a forward force of 25 N. If the forward force is applied to a 12-kg object, what is the acceleration of the object? 4. A frictional force of 10 N acts against a forward force of 25 N. If the forward force is applied to a 12-kg object, what is the acceleration of the object? 1.25 m/s 2

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Mass vs Weight MASS IS NOT WEIGHT! Mass – the amount of matter in an object. Mass – the amount of matter in an object. Weight – the force due to gravity that acts on an object’s mass Weight – the force due to gravity that acts on an object’s mass The 2 are directly proportional: object with large mass has large weight, object with little mass has little weight. The 2 are directly proportional: object with large mass has large weight, object with little mass has little weight.

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

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

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3 rd Law There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

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Newton’s 3rd Law in Nature Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

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

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Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards. Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards. 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 (downwards) is opposite the direction of the force on the bird (upwards). 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 (downwards) is opposite the direction of the force on the bird (upwards). Action-reaction force pairs make it possible for birds to fly. Action-reaction force pairs make it possible for birds to fly.

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Other examples of Newton’s Third Law The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction). The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

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

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3 rd Law The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

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Check for Understanding A speeding bus makes contact with a bug that splatters onto the windshield. A speeding bus makes contact with a bug that splatters onto the windshield. Compare the force of the bug on the windshield to the force of the windshield on the bug. Compare the force of the bug on the windshield to the force of the windshield on the bug. Compare the acceleration of the bug and the acceleration of the bus. Compare the acceleration of the bug and the acceleration of the bus.

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Check for Understanding How does a rocket in outer space move when there is no air to “push” against? How does a rocket in outer space move when there is no air to “push” against?

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