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Final Exam Review Newton’s Laws Friction & Weight Newton’s 1st Law • Objects in motion stay in motion and objects at rest stay at rest unless a force acts on it. • (Law of Inertia) Example • • The meteor in Armageddon would have continued toward the Earth unless Ben and Bruce blew it off course. • (Applied a net force to change • it’s motion) Newton’s 2nd Law • F = ma • Key Point: A net force will change the velocity or direction of motion of an object. Example • Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2kg book toward Roberto. If the net force acting on the book is 1.6N, what is the book’s acceleration? Example • Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2kg book toward Roberto. If the net force acting on the book is 1.6N, what is the book’s acceleration? • Givens • • m = 2.2 kg F = 1.6 N Example • Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2kg book toward Roberto. If the net force acting on the book is 1.6N, what is the book’s acceleration? • Givens • m = 2.2 kg • F = 1.6 N • F = ma → 1.6N = (2.2kg)a • a = 0.73 m/s2 Practice • Pg 132- Practice C Newton’s 3rd Law • “Every action has an equal and opposite reaction” • Forces come in pairs. Weight • Def: Force of gravity on an object’s mass. • If “g” changes, so does weight. You “weigh” less on Mars, because g= 3.7 m/s2 there instead of 9.8 m/s2 like Earth. • BIG SMALL Earth Mars Example • Determine the weight of a 150kg man on the surface of the earth. • F = ma • F = (150kg)(9.8m/s2) = 1470N Practice • Determine the weight of the 150kg person on the following planets: • Planet Gravity • Mercury 3.6 m/s2 • Venus 8.6 m/s2 • Jupiter 25.9 m/s2 • Saturn 11.3 m/s2 Friction • Def: The force that resists motion between two objects in contact together. • Formula: Ff = μFn • Examples: Sliding friction (box on floor) • Air resistance (falling objects) • Water resistance (swimming) Example • A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. Example • A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. • Givens • Ff = 75N • m = 24kg Fn = ? Example • A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. • Givens • Ff = 75N • m = 24kg Fn = ? • Fn = mg = (24kg)(9.8m/s2) = 235N • Ff = μFn → 75N = μ(235N) • μ = 0.32 Practice • Pg 139- Practice D • Problem 2