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Newton’s 2nd Law Chapter 5 Acceleration and velocity • Remember v = d/t and a = v/t • An object traveling at constant velocity has zero acceleration • An object at rest has zero acceleration • In terms of force, the forces acting on these objects are balanced Acceleration and velocity • Balanced forces means that the net force = zero. • Zero net force = zero acceleration • Net force is directly proportional to acceleration • F~a Acceleration and velocity • Mass resists acceleration • Acceleration is inversely proportional to mass. • This has to do with the inertia of the object. • Remember, inertia is the object’s ability to resist change (Newton 1). • a ~ 1/m Newton’s 2nd Law • Newton’s 2nd Law states: The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object. Newton’s 2nd Law • Restated, the law says F = ma • The units for force: F = 1N = 1kg•m/s² Forces • Forces occur in pairs. • Gravity is paired with normal force. • On an airplane, there are four forces involved. Forces - Plane • • • • Thrust – moves the plane forward Drag – pulls it backward Lift – moves it up Weight – holds it down Example • How much thrust must a 30,000 kg plane develop to achieve an acceleration of 1.5 m/s² ? • F = ma • F = (30000kg)(1.5 m/s²) • F = 45,000 N More forces • Friction is a force that affects motion. • It acts on materials that are in contact with each other. • It is dependent on surface irregularities. • Friction always acts in a direction to oppose motion. Friction • Friction occurs in solids, liquids, and gases. • In air, it is called air resistance. • In gas, it is called fluid friction. • Fluids include both gas and liquid. They both flow. Free-body diagrams • Free body diagrams show only the forces. • They are merely vector diagrams of all the forces acting on an object. Pressure • Pressure is the amount of force applied over a unit of area. • P = F/A • The unit is the Pascal • 1 Pa = 1N/m² Terminal velocity • When air resistance = weight, then net force = zero. • When net force = zero, acceleration = zero. • This is called terminal speed or velocity. Terminal velocity • For humans skydiving, terminal velocity is roughly 150 to 200 km/hr. • For a penny, terminal velocity is approximately 60 km/hr. Example • What is the minimal force required to lift a 5kg baby out of a crib? • F = ma • F = (5kg)(10 m/s²) • F = 50 N