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Newton’s Three Laws of Motion Physics We’ll discuss the following in this lesson: Newton’s First Law Newton’s Second Law Newton’s Third Law

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Inertia The INERTIA of an object is its resistance to CHANGE in motion. In other words, inertia is sluggishness.

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Inertia Galileo is given credit for first coming up with the concept of inertia. Galileo identified inertia by performing experiments with inclined planes.

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Mass MASS is the measure of inertia or the measure of how much matter an object contains. Quite often, the words “mass” and “inertia” are used interchangeably.

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Abbreviation for Mass Mass is abbreviated m.

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Mass is a Scalar Quantity Mass is a scalar quantity. In other words, when expressing mass, you only need to include a magnitude and a unit (NO DIRECTION IS NECESSARY).

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SI Unit for Mass The SI unit for mass is the kilogram (abbreviated kg). If the web page at the right does not open, click HERE.

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Mass An object’s mass is independent of its location. In other words, an object’s mass does not change if it is moved. Your mass is the same on the moon as it is on the earth. If the web page at the right does not open, click HERE.

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Newton’s First Law of Motion The sum of The outside forces acting on an object Equals Zero When this happens, an object will NOT accelerate.

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Newton’s First Law of Motion Newton’s First Law of Motion says that if zero net (total) outside force acts on an object (or system of objects), that object will not accelerate. Net means “the sum of.”

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Newton’s First Law of Motion In other words, if zero net outside force acts on an object, it will do one of two things: 1.it will stay at rest if it is already at rest, or 2.it will keep moving in a straight line at a constant speed if it is already moving. “Moving in a straight line at a constant speed” means the same thing as “moving at a constant velocity.”

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Newton’s First Law of Motion Newton’s First Law holds even when forces act on an object. The forces acting on the object just must ADD UP TO ZERO. If the web page at the left does not open, click HERE.

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Newton’s First Law of Motion Newton’s First Law of Motion is sometimes called the Law of Inertia because it is essentially a restatement of inertia. If the web page above does not open, click HERE.

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Newton’s Second Law of Motion Newton’s Second Law of Motion says that the net external force acting on an object (or system of objects) is equal to the mass of the object multiplied by the acceleration of the object. Watch the video below to find out what the symbols above mean.

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Newton’s Second Law of Motion

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Newton’s Second Law of Motion says that a larger net force results in a larger acceleration and that a smaller net force results in a smaller acceleration. If the web page at the right does not open, click HERE.

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Newton’s Second Law of Motion Newton’s Second Law of Motion also says that if an object has a larger mass, it will have a smaller acceleration for a given net force. If the web page at the right does not open, click HERE.

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Newton’s Second Law of Motion Remember: Newton’s Second Law deals with NET FORCE, not just force. Click HERE to use open a web page. Once the web page opens, click the “Run Now!” button to use a force simulation.

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Weight and Newton’s Second Law For right now, we will define weight as the force due to gravity acting on an object.

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Weight and Newton’s Second Law Weight is equal to mass x acceleration due to gravity. Weight = mg (direction is downward). Remember: g ≈ 10 m/s/s

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Weight and Newton’s Second Law An object’s weight depends on its location. The astronauts in the video to the right do not weigh the same on the moon as they do on the earth because the moon does not exert the same gravitational force on them as the earth does. Click HERE if the web page below does not open.

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Free-Fall Motion An object is in free fall when the only force exerted on it is the force due to gravity. Remember: an object is not really in free fall if wind resistance is acting on it.

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Free-Fall Motion An object is in free fall when the only force exerted on it is the force due to gravity. So, if we draw a force diagram for an object in free fall, the diagram will only have one force vector (for the force due to gravity). Let’s draw a force diagram for a basketball in free fall.

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Non-Free-Fall Motion When an object falls through the atmosphere, at least two forces act on that object: 1.gravity and 2.wind resistance. In this case, the object is not in free fall, and the object will NOT have an acceleration of g.

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Terminal Velocity/Speed

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Interaction If I exert a force on the wall, the wall exerts a force back on me. This is always the case. Any time Object A exerts a force on Object B, Object B exerts a force back on Object A. This pair of forces makes up ONE INTERACTION.

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Action/Reaction Pair of Forces If Object A exerts a force on Object B, we call that an ACTION force. When Object B exerts a force back on Object A, we call that a REACTION force. Remember: an action force + a reaction force makes a SINGLE INTERACTION. If the web page at the right does not open, click HERE.

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Newton’s Third Law of Motion Newton’s Third Law of Motion says the following: “For every action, there is an equal and opposite reaction.” If the web page at the right does not open, click HERE.

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Newton’s Third Law of Motion Newton’s Third Law says two things: 1.If Object A exerts a force on Object B, Object B exerts a force back on Object A, and 2.The two objects exert forces on one another that are equal in size and opposite in direction. If the web page below does not open, click HERE.

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Examples of Newton’s Third Law of Motion in Action A bird’s wings push down on the air. The air pushes up on the bird’s wings. In other words, air keeps a bird aloft. You pull down on a ladder. The ladder pushes you up. In other words, the ladder propels you upward. A car’s wheels push backward on the road. The road pushes forward on the car’s tires. In other words, the road propels the car forward.

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If the web page at the right does not open, click HERE.

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Newton’s Second and Third Laws for Objects of Different Masses If I hit a grasshopper with my PT Cruiser, which exerts the greater force on the other? Click on the correct choice below. The car exerts the greater force on the grasshopper. The grasshopper exerts the greater force on the car. The two do not exert forces on each other. The two exert forces with equal magnitudes on each other.

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Newton’s Second and Third Laws for Objects of Different Masses Although the PT Cruiser and the grasshopper exert equal sized forces on one another, they do not sustain the same amount of damage upon impact. According to Newton’s Second Law, because the grasshopper has a tiny mass, it will experience a huge acceleration compared to that of the PT Cruiser. It is the huge acceleration of the grasshopper that causes it to sustain such damage.

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If the game at the right does not open, click HERE.

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