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Transcript
st 1 Newton’s Law of Motion Net Force The net force is the sum of all forces acting on an object. Example: A weightlifter holds a weight above his head by exerting a force of 1.6 kN [up]. The force of gravity acting on the weight is 1.6 kN [down]. Draw a FBD of the weight. What is the net force on the weight? Net Force FA=1.6 kN Fg=1.6 kN The net force is zero: 1.6 kN + (-1.6 kN) = 0 Newton’s First Law of Motion If there is no net force acting on an object, the object will remain at rest Newton’s First Law of Motion If there is no net force acting on an object, the object will remain at rest or will keep moving at the same constant velocity. (Conversely, if an object is at rest or is moving at constant velocity, there is no net force acting upon it.) Inertia This is the principle of inertia, first articulated by Galileo: Inertia “Inertia is a property of matter.” Inertia is the property of matter that causes it to resist changes to its motion. The greater the mass of an object, the greater its inertia. Newton’s First Law of Motion Question: An object is being pushed along at constant velocity by a force of 5 N [left]. What is the force of friction acting on the object? If the velocity is constant, there is no net force, so the force of friction must be equal in magnitude and opposite in direction to the applied force: Ff = 5 N [right] More Practice Explaining Why with Newton’s 1st Law of Motion