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Chapter 3—Forces

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**Section 1: Newton’s Second Law**

For any object, the greater the force that’s applied, the greater its acceleration will be Force and Mass: Acceleration of an object depends on its mass as well as the force exerted on it Force, mass and acceleration are connected

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Newton’s Second Law Describes how force, mass and acceleration are connected Net force acting on an object causes the object to accelerate in the direction of the force OR: acceleration = net force / mass OR: Force = mass X acceleration

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**Friction Objects slow to a stop when moving freely—due to friction**

The force that opposes motion between two surfaces that are touching ea/other Amt. of friction depends on kinds of surfaces and the force pressing the surfaces together

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Types of Friction: Static Friction: friction between two surfaces that are not moving past ea/other Sliding friction: force that opposes the motion of two surfaces sliding past ea/other Rolling friction: friction between a rolling object and the surface it rolls on

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Air Resistance: The force on an object opposite the force of gravity when an object falls toward Earth Effects anything that moves in Earth’s atmosphere The amount of air resistance on an object depends on the speed, size and shape of the object

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Terminal Velocity The highest velocity that a falling object will reach As an object falls, it accelerates and its speed increases The force of air resistance increases until it becomes large enough to cancel the force of gravity (How sky divers can land softly)/video clip

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Section 2--Gravity Law of Gravitation: any two masses exert an attractive force on ea/other The attractive force depends on the mass of the two objects and the distance between them Four Basic Forces: *Gravity *Strong Nuclear *Electromagnetic *Weak Nuclear

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9.8 m/s/s Near Earth, the gravitational attraction of Earth causes all falling objects to have an acceleration of 9.8 m/s/s SO: F = Mass X 9.8 m/s/s The force of the Earth’s gravity is always downward When an object is influenced only by gravity—free fall

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(more on Gravity) All objects fall w/the same acceleration, no matter how large or small their mass Weight: actually the gravitational force exerted on an object Gravitational Force = Mass X Acceleration due to gravity

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**SO: Weight = mass X 9.8 m/s/s**

Greater mass means greater gravitational force, means greater weight Projectile Motion: anything thrown or shot through the air is said to be in projectile motion Projectiles follow a curved path-due to inertia

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**Horizontal and Vertical Motions**

Centripetal Acceleration: acceleration toward the center of a curved or circular path Centripetal force: an unbalanced force, accelerating an object toward the center, exerted by the outside wall pushing against it and keeping it from going straight

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**Section 3: Newton’s Third Law**

Describes action/reaction pairs When one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction OR: “to every action force there is an equal opposite reaction force”

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**Remember: Action/Reaction forces are acting on different objects**

Even though forces are equal, they are not balanced

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Rocket Propulsion Rockets use the principle that as the rocket exerts force on gases and causes them to escape out the back of the rocket, the gases exert an opposite but equal force on the rocket propelling it forward

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Momentum Related to how much force is needed to change an objects motion Momentum of an object is the product of its mass and velocity (abbrev is P) OR: P = Mass X Velocity Units used to report momentum are as follows: Kg m/s

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**Law of Conservation of Momentum**

Momentum of an object doesn’t change unless its mass, velocity or both change Momentum, however, can be transferred from one object to another Momentum is not lost or created, only transferred from one object to another

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