College Physics: A Strategic Approach with Mastering Physics

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10/7/2008 1 1 Newton’s Newton’s Laws Laws Ch Ch-4 2 Newton’s Laws of motion Issac Newton (1642-1727). One of the greatest scientists of all time. Made fundamental contributions in mathematics, astronomy, optics, gravitation and formulated the laws of motion. Newton s Laws constrain and regulate our very existence. What is a force? 3 FORCE A Force is a push or a pull 4 Examples of forces " Push " Pull " Friction " Gravity " Electromagnetic force " Nuclear force Newton’s laws of motion First we need to define some terms. Force. A vector. Causes a push or pull on a body. Unit is the Newton (N). Mass. A measure of of the quantity of matter in an object. Unit is the kilogram (kg). 6 How do we write forces or draw them in physics? " Forces are vectors " Therefore rules of vectors apply. " The way we draw them is through the use of free body diagrams.
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10/7/2008 2 7 F REE B ODY D IAGRAM A critical first step for solving force problems 1. Draw a simple neat diagram that isolates the object whose motion is being analyzed. 2. Draw ALL the forces that act ON the object; do NOT draw any other forces, including those exerted BY the object on its surroundings. FORCE 8 FBD: Free Body Diagram F g n S F = F g + n = 0 n F g FORCE Draw ONLY forces acting ON the stone 9 Normal Force? " The force that a surface exerts on an object is call the Normal force. " Normal means Perpendicular. " The normal force that a surface exerts on an object is always perpendicular to the surface. Demo: Platform scale, mirror on table and Laser 10 FBD: Free Body Diagram F g T S F = F g + T = 0 T F g FORCE Draw ONLY forces acting ON the stone 11 F g T 1 S F = F g + T 1 + T 2 = 0 F g T 1 T 2 T 2 FORCE 12 F g F g T 1 T 1 S F = F g + T 1 + T 2 = 0 T 2 T 2 FORCE
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10/7/2008 3 13 F g F g T T S F = F net = F g + T = m a F net FORCE 14 F g S F = F net = F g + n + f = f = m a f n f n F g F net FORCE 15 F g F g FORCE T T F s F s S F = F net = F g + T + F s " ` 0 F net Draw ONLY forces acting ON the stone 16 F g n 1 S F = F g + n 1 + n 2 = 0 n 2 F g n 2 n 1 FORCE 17 F g n S F = F g + n = m a F g n F net FORCE 18 PRS How many forces act on the 3 kg mass? 1) 1 2) 2 3) 3 4) 4 5) 5 FORCE
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10/7/2008 4 19 Newton s laws. A force is a push or pull, it occurs at a point of contact between the object and some identifiable agent that exerts the force. The only non contact force we care about is Gravity. Free body diagrams. Label only those forces that act on the object. 20 FORCE Newton’s Three Laws of Motion (1672) 1. In the absence of a net external force, an object at rest remains at rest, or an object in motion remains in motion with constant velocity. Net force = Total force = S F Examples of F net = 0 : ¤ You sitting in your seat. ¤ Car moving at constant speed on straight
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Lecture Notes Ch4 - 10/7/2008 Newton s Laws ChCh-4 Examples...

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