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FORCES Write 5 examples of an object accelerating Write 5 examples of an object accelerating In each example, what causes the object to accelerate? In each example, what causes the object to accelerate?

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FORCES Newtons Three Laws

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Newtons First Law of Motion Law of Inertia Law of Inertia Inertia is a property of matter.Inertia is a property of matter. Inertia is an objects tendency to resist a change in motion.Inertia is an objects tendency to resist a change in motion. The law states: The law states: An object at rest will stay at rest unless a net force acts upon it.An object at rest will stay at rest unless a net force acts upon it. An object in motion will stay in motion unless a net force acts upon it.An object in motion will stay in motion unless a net force acts upon it.

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Egg Drop Experiment What will happen to the egg when the pie tin is hit? What will happen to the egg when the pie tin is hit? Why would it do that? Why would it do that?

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Newtons Second Law of Motion Force = mass x acceleration Force = mass x acceleration F=maF=ma Force and acceleration are directly proportional. More Force means more acceleration. Force and acceleration are directly proportional. More Force means more acceleration. Mass and acceleration are inversely related. With the same amount of force, an object with greater mass will have less acceleration Mass and acceleration are inversely related. With the same amount of force, an object with greater mass will have less acceleration

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Ball Bearing Race Which ball moved farther? Which ball moved farther? Why did it move farther? Why did it move farther?

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Newtons Third Law of Motion States that: States that: For every force there is an equal and opposite reaction force.For every force there is an equal and opposite reaction force. When you push against something, it pushes back! When you push against something, it pushes back!

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Third Law Examples Space Shuttle Launches Space Shuttle Launches Jumping, Swimming, Running Jumping, Swimming, Running Gravity Gravity Swings Swings

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TYPES OF FORCES F g = Gravity! F g = Gravity! F a = Applied Force F a = Applied Force F f = Friction F f = Friction F N = Normal Force F N = Normal Force F t = Tension Force F t = Tension Force F s = Force of a spring F s = Force of a spring F air = Air Resistance F air = Air Resistance F net = The sum of all forces acting on an object. F net = The sum of all forces acting on an object.

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About Forces All forces: All forces: Obey the three lawsObey the three laws Are measured in Newtons (N)Are measured in Newtons (N) 1 Newton = 1 kg * m/s 2 1 Newton = 1 kg * m/s 2 Note: mass is in kilograms Note: mass is in kilograms Have a magnitude and directionHave a magnitude and direction Theyre vectors! Theyre vectors! Are represented with arrows. Are represented with arrows.

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Force of Gravity Always present in a downward direction. Always present in a downward direction. Also known as Weight Also known as Weight Can be calculated using Newtons Second Law: F=ma Can be calculated using Newtons Second Law: F=ma F g =m*a g (a g =9.8m/s 2 )F g =m*a g (a g =9.8m/s 2 )

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Gravity Example Your cat has a mass of 1.6 kg. What is its weight? Your cat has a mass of 1.6 kg. What is its weight? F g = ma F g = ma F g = 1.6kg * 9.8m/s2 F g = 1.6kg * 9.8m/s2 F g = kg*m/s2 = N F g = kg*m/s2 = N

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Normal Force Always perpendicular to the applied surface. Always perpendicular to the applied surface. Equal to the total force applied to that surface Equal to the total force applied to that surface

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Normal Force Calculation You headbutt someone with an applied force of 10 N. What is the normal force applied to your head? You headbutt someone with an applied force of 10 N. What is the normal force applied to your head? F N = 10N F N = 10N

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Tension Force Always in the direction of an object being pulled. Always in the direction of an object being pulled. Used with ropes, strings, cables, etc. Used with ropes, strings, cables, etc. Can either be used when pulling or hanging an object. Can either be used when pulling or hanging an object. When hanging, F t is equal to F gWhen hanging, F t is equal to F g F t has an upper limit, if that is exceeded, the cable/string will snap!F t has an upper limit, if that is exceeded, the cable/string will snap!

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Tension Force Calculation A 4000N block is hanging from a crane. What is the Tension Force applied by the cable? A 4000N block is hanging from a crane. What is the Tension Force applied by the cable? F t = 4000N F t = 4000N

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Friction Always acts in the opposite direction of the intended motion. Always acts in the opposite direction of the intended motion. Friction must be overcome for an object to accelerate across a surface. Friction must be overcome for an object to accelerate across a surface. Two types of Friction: Two types of Friction: Static Friction – opposes the start of motion (before an object is moving)Static Friction – opposes the start of motion (before an object is moving) Kinetic Friction – opposes the current motion of an objectKinetic Friction – opposes the current motion of an object Static Friction is larger than Kinetic Friction Static Friction is larger than Kinetic Friction

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Calculating Friction F f =μF N F f =μF N μ – (pronounced Mew) - Coefficient of Friction μ – (pronounced Mew) - Coefficient of Friction Doesnt have unitsDoesnt have units Value is different for different surfacesValue is different for different surfaces Rough surfaces have a higher μ Rough surfaces have a higher μ

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Friction Calculation The coefficient of friction for a object sliding on a horizontal concrete floor is.62, if the object weighs 20N, what is F f ? The coefficient of friction for a object sliding on a horizontal concrete floor is.62, if the object weighs 20N, what is F f ? F f =μF N F f =μF N F f =.62 * 20N F f =.62 * 20N F f = 12.4N F f = 12.4N

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Air Resistance When an object moves through air, the air pushes back on the object. When an object moves through air, the air pushes back on the object. Acts in the direction opposite the objects motionActs in the direction opposite the objects motion F air is normally very small, so we can ignore it, except when an object is falling. F air is normally very small, so we can ignore it, except when an object is falling.

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Free Body Diagrams Used to show the forces acting on a body. Used to show the forces acting on a body. Use a box (or dot) to represent an object. Use a box (or dot) to represent an object. Draw the forces that effect the objectDraw the forces that effect the object Remember that if the object isnt accelerating, F net =0.Remember that if the object isnt accelerating, F net =0.

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Free Body Diagram Practice A book is sitting on a desk A book is sitting on a desk

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Free Body Diagram Practice A book is being pushed along a desk. A book is being pushed along a desk.

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Free Body Diagram Practice A ball is hanging from a string A ball is hanging from a string

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Free Body Diagram Practice The same ball, but the string has been cut. The same ball, but the string has been cut.

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