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Chapter 4 Dynamics: Newton’s Laws of Motion 2013 - 2014

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Definition of Force A force is a push or pull. Forces can be : Contact Forces Field Forces (Forces at a Distance) 2013 - 2014

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Newton’s First Law (Often called the “Law of Inertia”): Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it. Inertia: Tendency of an object to resist a change in its motion Mass: Proportional to the inertia of an object 2013 - 2014

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Newton’s Second Law Newton’s second law is the relationship between acceleration and force: Acceleration is proportional to net force on an object and inversely proportional to its mass. 2013 - 2014

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Equilibrium A condition in which the net force on an object is equal to zero. According to Newton’s 2 nd Law, if F Net = 0, then a = 0 m/s. Object is: a) at rest (v = 0 m/s) b) moving with constant velocity 2013 - 2014

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Newton’s Third Law Any time a force is exerted on an object, that force is caused by another object: Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. 2013 - 2014

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Weight – the Force of Gravity Weight is the force exerted on an object by gravitational attraction. Close to the surface of the Earth, where the gravitational force is caused by the Earth and is nearly constant, the weight of an object is: 2013 - 2014

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The Normal Force The force exerted perpendicular to a surface is called the normal force. 2013 - 2014

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Friction On a microscopic scale, most surfaces are rough. For kinetic (sliding) friction: is the coefficient of kinetic friction, and is different for every pair of surfaces. 2013 - 2014

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Static Friction Static friction is the frictional force between two surfaces that are not moving along each other. Static friction keeps objects on inclines from sliding, and keeps objects from moving when a force is first applied. 2013 - 2014

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Hooke’s Law F s = kx k: spring constant x: stretch distance 2013 - 2014

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Tension and Pulleys Tension is uniform within a cord/string, pulleys are frictionless. 2013 - 2014

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“Apparent Weight”: Normal Force Scales/Normal Force - Measure the normal force - Scale reading is equal to weight on a flat, stationary surface - Scale reading will change if surface is tilted or accelerating 2013 - 2014

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Terminal Velocity For an object in free-fall, when frictional force of air resistance (“drag force”) is equal to object’s weight, object is in equilibrium and will maintain constant velocity. 2013 - 2014

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Forces at Angles 2013 - 2014

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Solving Problems with Newton’s Laws: Free-Body Diagrams 1. Draw a sketch. 2. For one object, draw a free-body diagram, showing all the forces acting on the object. Make the magnitudes and directions as accurate as you can. Label each force. If there are multiple objects, draw a separate diagram for each one. 3. Resolve vectors into components. 4. Apply Newton’s second law to each component. 5. Solve. 2013 - 2014

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Example 1 (g = 9.8 m/s 2 ) 2013 - 2014

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Example 2 2013 - 2014

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