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Drawing free-body diagrams: Air resistance, free fall, terminal velocity and friction Most of the information is from:

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1. According to Newton's second law, the acceleration of an object as produced by a force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object; or F net = ma 2. The weight of an object is a product of its mass and acceleration due to gravity or w = mg Or; Fw = mg

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3. Air resistance is due to the colliding of an object with molecules of air. A falling skydiver collides with air molecules during the downward fall. These air molecules create a force pushing upward which is opposite to the skydiver's direction of travel

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4. The amount of air resistance encountered by the skydiver depends on three factors: a: The speed of the skydiver. The faster he goes the more air particles he collides with per second therefore the bigger the air resistance will be. If he is not moving the air resistance will be zero.

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b: The area of intercept of the skydiver. In other words, how much area the air is pushing up against. The larger this is the more air particles he will bump into per second and the higher the air resistance will be. – Parachutes work!!!!

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c. The air density The greater the density of the air (it is less dense at higher altitudes) the more air particles the skydiver will bump into each second making the air resistance greater.

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5. Terminal Velocity: occurs when downward pull of gravity equals upward force of air resistance. a. The resultant force of gravity minus air resistance determines the acceleration. So,F W - F AR = ma where F W = weight (downward) and F AR = the upward force of the air

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b. As long as F w is greater than the F AR the skydiver will accelerate - get faster. If F w is smaller than F AR s/he will decelerate - undergo negative acceleration - get slower JMc

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c. When the upward force of air resistance equals the downward pull of gravity, the object will fall at a constant speed. F w = F AR ; Fnet = 0 JMc

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6. Forces that are affecting an object are shown in free body diagrams using vectors Terms: F net = Resultant Force: when forces are balanced, it is zero. F ar = Force of air resistance F app = The applied force: which direction is the object heading. F f = Force of friction: it’s resisting forward motion. It is always created by contact between two surfaces and is opposite in direction to applied force. JMc

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F n = The ‘normal’ force: for an object in contact with a surface, it is the force of the surface against the object. It is always perpendicular to the surface. F g or F w = the force straight downward due to Earth’s gravitational field. It is the mass of the object in ‘kg’ times 9.8 m/s 2. Or: Fn Fw Fn Fw JMc

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7. What happens if someone tilts the table? Fn Fw The force of gravity is still straight downward. The ‘normal’ force is still perpendicular to the table. But what else will act on the book now???? JMc

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Now we must consider the applied force pulling the object down the table, and the frictional force that resists that motion. FfFf F app F f = F app ; the book won’t start moving. Once F app overcomes frictional forces, it accelerates. Once moving, if F f = F app again, the velocity will be constant. JMc

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Label the four forces. Is there a net force? If so, what is it causing the book to do? Redraw the diagram at a steeper angle. JMc

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