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Forces Standard: S8P3 b. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.

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Forces What are forces? InertiaGravity Newtons Laws Projectile motion

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What is a force? A push or a pull May be unbalanced or balanced

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What does a force do? Gives energy to an object Can set an object in motion, make it stop, or change its motion

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Unbalanced forces change motion

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What are forces represented by? Forces are represented by vectors

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Forces are added with vectors

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What is friction? Aforce which opposes motion, bringing objects to rest Types: Types:

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Inertia

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What is inertia? The tendency of an object to stay at rest or in motion. The tendency of an object to stay at rest or in motion. Objects with greater mass have greater inertia Objects with greater mass have greater inertia

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Inertia keeps astronauts in motion

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Seat belts protect against inertia

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Gravity

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Gravity The attractive force between all objects in the universe

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F = M 1 M 2 d 2 More mass means greater gravity More mass means greater gravity More distance means less gravity More distance means less gravity My gravitational attraction is small no matter what the distance! Massive objects have lots of gravity

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The earth exerts a force on the student and the student exerts a force on the earth because of GRAVITY!

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Newtons Laws of Motion 1. Inertia 2. F=ma 3. Action-reaction

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Newtons First Law of Motion The law of inertia: The law of inertia: An object at rest stays at rest an object in motion stays in motion unless acted upon by an outside force.

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Inertia keeps astronauts in motion

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Inertia keeps satellites in orbit

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Inertia keeps you at rest

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Newtons Second Law of Motion Force = Mass x acceleration Force = Mass x acceleration A force is needed for acceleration A force is needed for acceleration Acceleration will occur in the direction of the force. Acceleration will occur in the direction of the force.

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Which cars are accelerating? If you watch carefully, the green and blue cars are accelerating. The red car is traveling at constant speed. If you watch carefully, the green and blue cars are accelerating. The red car is traveling at constant speed.

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A force is needed for acceleration Will these accelerate? Will these accelerate? A skier going downhill? A skier going downhill? –What force is acting on the skier? An astronaut on a An astronaut on aspacewalk? –Are any forces acting on the astronaut?

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Which of the following will accelerate? Which of the following will accelerate? A bullet shot from A bullet shot from a gun? –What forces are acting on the acting on the bullet? bullet? A hammer and feather dropped on the moon? –What force is acting?

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Newtons 2 nd Law F_____ F_____ M A M A Force is measured in newtons. A newton is a kg m s 2 s 2 Example: A car with mass 1000kg hits a tree and decelerates at 50m/s/s. What force does it exert on the tree? Example: A car with mass 1000kg hits a tree and decelerates at 50m/s/s. What force does it exert on the tree? Force = mass x acceleration

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Answer: F_____ MAMAMAMA 1000kg * 50m/s/s = 50,000 kgm or 50,000N s2s2s2s2

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F = ma A car is traveling at constant speed. Is a force acting on it? A car is traveling at constant speed. Is a force acting on it? Answer: although there are forces acting on the car (gravity, friction, air resistance,etc…) since there is no acceleration, the net force in the direction of motion must be zero. Answer: although there are forces acting on the car (gravity, friction, air resistance,etc…) since there is no acceleration, the net force in the direction of motion must be zero.

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More 2nd Law Problems: 1. What is the acceleration on a mass of 50kg if a force of 10N is applied? 2. An object accelerates due to gravity at a rate of 10m/s/s. If its mass is 15kg, what force is acting on the mass?

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Answers: _F_ _F_ M A 1. _10N_ 1. _10N_ 0.2 n/kg= 0.2 kg m = 0.2 m/s 2 50kg A /kg 50kg A s 2 /kg 2. F______ 15kg 10m/s 2 15kg 10m/s kgm/s 2 or 150N 150 kgm/s 2 or 150N

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Gravity is a force that causes an acceleration On earth, ALL objects accelerate at 9.8m/s 2 (ignoring air resistance) because of gravity. On earth, ALL objects accelerate at 9.8m/s 2 (ignoring air resistance) because of gravity. No matter what the mass, ALL objects on earth accelerate at 9.8 m/s 2 No matter what the mass, ALL objects on earth accelerate at 9.8 m/s 2

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Newtons 2 nd Law F__ F__ m a m a 3. An object of mass 3kg falls off a cliff. How much force is acting on the object? Answer: F_______ 3kg 9.8 m/s 2 3kg 9.8 m/s 229.4N

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Free fall without air resistance Free fall without air resistance Both the elephant and the feather land at the same time!

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Free fall with air resistance Free fall with air resistance The feather takes lots longer to hit the ground!

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Newtons Third 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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For every action, there is an equal and opposite reaction In rocketry…… In rocketry…… The gases being expelled is the action. The rocket propelling forward is the reaction.

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How do Newtons laws describe the motorcycle rider? 1 st : Inertia keeps the rider moving forward. 1 st : Inertia keeps the rider moving forward. 2 nd : The force with which the rider hits the ground will be equal to his mass times his deceleration. 2 nd : The force with which the rider hits the ground will be equal to his mass times his deceleration. 3 rd : The action of hitting the ground will equal the reaction of the dirt. 3 rd : The action of hitting the ground will equal the reaction of the dirt.

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Projectile motion

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An object moves forward due to inertia and accelerates downward due to gravity. An object moves forward due to inertia and accelerates downward due to gravity.

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In this video, the ball is dropped straight down and moves forward due to inertia

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Projectile motion of a ball fired vertically The motion of the truck causes the ball to project forward. What would happen if the truck stopped?

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Package dropped from plane The forward motion of the plane causes the package to land in front of where its dropped.

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Satellites

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