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Forces Using Newton’s Laws Objectives Describe how weight and mass are related. Differentiate between gravitational force weight and apparent weight. Define friction force, static and kinetic friction. Describe Simple Harmonic Motion and explain how the acceleration due to gravity influences such motion. Mass and Weight Mass and Weight are NOT the same thing! Weight is a force. It incorporates gravity. On a falling object, both Force and acceleration are downward. Mass and Weight On Earth, the magnitude of an object’s weight is its mass x acceleration (9.8 m/s2) On other planets the acceleration due to gravity varies, so the object’s weight will vary, but its mass will not. Scales The scale exerts an upward force on the bottom of your feet. It reads Fn, the number that is necessary in order to put your weight in equilibrium. Fnet = 0 Fspg – Fg = 0 Fspg = Fg Apparent Weight F = ma Fg = mg Fg changes when g changes. Either from planet to planet or… Apparent Weight If there are other contact forces, then the weight will change. Contact forces may include a push or a pull (e.g. someone pushing on your shoulders while you’re standing on a scale in an elevator). Or, no contact forces pushing up on you (e.g. the elevator cable breaks while you’re standing on a scale inside the elevator). Your zero! apparent weight would be Friction Force Static friction force Exerted on one surface by the other when there is no relative motion between the two surfaces. µs = Static coefficient of friction Kinetic friction force: Exerted on one surface by the other when the surfaces are in relative motion. µk = Kinetic coefficient of friction Terminal Velocity As an object falls, the downward force of gravity causes the object to accelerate. As the speed of a falling object increases, the upward force of air resistance also increases. This causes the net force on a falling object to decrease. Finally the upward air resistance force because large enough to balance the downward force of gravity Terminal Velocity The net force on the object is zero and the velocity of the object doesn’t change The object falls with a constant velocity called the terminal velocity. The terminal velocity is the highest velocity the falling object will each. The terminal velocity depends on the size, shape, and mass of the falling object. Simple Harmonic Motion (SHM) When the force that restores the object to its equilibrium position is directly proportional to the displacement of the object Period (T)and Amplitude Period The time needed to repeat one complete cycle of motion. The period depends on the length of the pendulum and acceleration due to gravity. Amplitude The maximum distance the object moves from equilibrium. Period (T)and Amplitude T = 2π√l/g T = period l = length of pendulum g = gravitational acceleration