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Chapter 4 Forces and the Laws of Motion

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Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force. “Equilibrium”

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Is the object in equilibrium? Describe the motion of the object.

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Newton’s Second Law

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Example The net force on a golf cart is 390 N north. If the cart has a total mass of 270 kg, what are the magnitude and direction of the cart’s acceleration?

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Practice Two students reach for a jar of mustard at the same time. One student pulls to the left with a force of 13.2 N, while the other students pulls to the right with a force of 12.9 N. If the jar has a net acceleration of 0.44 m/s 2 to the left, what is the mass of the jar? 13.2 N 12.9 N

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Practice A 2.0-kg fish pulled upward by a fisherman rises 1.9 m in 2.4 s, starting from rest. Assuming the acceleration is constant, find the magnitude and direction of the net force acting on the fish during this interval.

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Practice F T,1 = 23.2 N F T,2 = 23.2 N

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Practice * Note: force of gravity = 25,000 N (a) F net = 1830 N at 41.0° below the horizontal (b) a net = 0.718 m/s 2 (c) ∆x = 51.5 m

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Example A Goliath beetle with a mass of 0.080 kg is placed on a slope that makes an angle of 37.0° with the horizontal. If the force of gravity is 0.784 N, find the acceleration of the beetle along the slope (a) if there is no friction, (b) if there is a force of friction applied upward along the slope equal to 1.4 N. 37.0°

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y x 0.784 N

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Practice * Note: force of gravity = 4008 N (a) F net = 76 N at 30.0° above the horizontal (b) a net = 0.186 m/s 2 at 30.0° above the horizontal (c) ∆t = 8.03 seconds

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Newton’s Third Law If two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction. “For every action, there is an equal and opposite reaction.” These two forces are called an “action-reaction pair”

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Action-reaction pair

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Action-reaction pair: Force of Gravity Which force is bigger?

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Weight

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Mass vs. Weight Which is greater: your mass on Earth or your mass on the moon? Which is greater: your weight on Earth or your weight on the moon?

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Normal Force The normal force acts on a surface in the direction perpendicular to the surface

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Example A student attaches a rope to a 20.0 kg box of books. He pulls with a force of 90.0 N at an angle of 30.0° from the horizontal. Find the magnitude of the normal force and the net acceleration of the box. 90.0 N 30.0°

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Force of Friction

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Friction opposes the applied force Static friction: resists the initiation of sliding motion between two surfaces that are in contact and at rest Kinetic friction: opposes the movement of two surfaces that are in contact and sliding over each other

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Force of Friction The force of friction depends on – the normal force – the materials in contact Coefficient of friction (µ): the ratio of the force of friction to the normal force

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Force of Friction

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