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Chapter 13 Newton’s Laws

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Inertia The property of matter that tends to resist any change in motion

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Newton’s 1 st Law of Motion a.k.a. - the “law of inertia” An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

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What does inertia mean? idle or lazy the more massive an object, the more difficult it is to change its motion therefore, the more massive the object, the more inertia is has

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Where does FRICTION fit in? remember that friction is the force that resists motion no friction = object continues to travel forever

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When do we feel inertia? when the car stops suddenly & the body keeps moving forward

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When do we see inertia? when the truck stops suddenly & the ladder keeps moving forward

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When forces are balanced & acceleration equals zero...

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What should happen... to the dishes if the person pulls the tablecloth from the table?

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What should happen... to the balls if the person spins around in a circle?

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What should happen... to the person in a car that is traveling along a curved path?

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Newton's Second Law of Motion: Force = mass * acceleration The greater the force acting on the object, the greater its acceleration (direct relationship) The smaller the mass of the object, the greater its acceleration (inverse relationship)

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Newton's Second Law of Motion: Force = mass * acceleration Think about a shopping cart – the greater the force you apply to it, the greater it accelerates If the shopping cart is full, it requires greater force applied to it to accelerate the cart

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Newton's Second Law of Motion: Units = Newtons 1 N = 1 kg * 1 m/s/s

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Newton's Third Law of Motion: For every action, there is an equal & opposite reaction All forces come in pairs

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Action Reaction

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Can you name the six forces shown in the picture below?

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Rockets are unable to accelerate in space because... a. there is no air in space for the rockets to push off of. b. there is no gravity is in space. c. there is no air resistance in space. d.... nonsense! Rockets do accelerate in space.

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Answer: D It is a common misconception that rockets are unable to accelerate in space. The fact is that rockets do accelerate. Rockets are able to accelerate due to the fact that they burn fuel and push the exhaust gases in a direction opposite the direction which they wish to accelerate.

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A student is pulling upon a rope which is attached to a wall. Then, the student is pulling upon a rope which is attached to an elephant. In each case, the force scale reads 500 Newtons. The student is pulling a. with more force when the rope is attached to the wall. b. with more force when the rope is attached to the elephant. c. the same force in each case.

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Answer: C The student is pulling with 500 N of force in each case. The rope transmits the force from the physics student to the wall (or to the elephant) and vice versa. Since the force of the student pulling on the wall and the wall pulling on the student are action- reaction force pairs, they must have equal magnitudes. Inanimate objects such as walls can push and pull.

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A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is... a. greater than the acceleration of the bullet. b. smaller than the acceleration of the bullet. c. the same size as the acceleration of the bullet.

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Answer: B The force on the gun equals the force on the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration & mass are inversely proportional.

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An unfortunate bug strikes the windshield of a bus moving down the road. Quite obviously, this is a case of Newton's third law of motion. The bug hit the bus and the windshield hit the bus. Which of the two forces is greater: the force on the bug or the force on the bus?

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Trick Question! Each force is the same size. For every action, there is an equal... (equal!). The fact that the bug splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.

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Planetary motion is determined by the Sun's inward pull of gravity competing with the planet's tendency to continue moving in a straight line.

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