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Transcript
Newton’s Laws of Motion Forces cause changes in motion. Newton’s Laws of Motion • All forces balance so the airliner moves at constant velocity • Unbalanced force from the club accelerates the golf ball Newton’s Second Law: Acceleration An object accelerates when a net unbalanced force acts on it. Newton’s Second Law: Acceleration • Definition of acceleration: • acceleration is measured in m/s2 • Net force causes acceleration: Unbalanced forces acting on an object cause the object to accelerate Example from our lab: • When you pushed, the ball accelerated Newton’s Second Law: Acceleration • Stronger kick causes greater acceleration • NFL Physics Newton’s Second Law: Acceleration • What forces act on a race car? – – – – Friction F traction Traction Gravity Support from the road (Normal force) • Do these forces balance? • Represent each force with an arrow: – How strong? Which way? F normal F friction F gravity • Unbalanced forces produce acceleration Newton’s Second Law: Acceleration • The combination of all forces acting on an object is called the net force. • Acceleration depends on the net force. • To increase the acceleration of an object, you must increase the net force acting on it. • An object’s acceleration is directly proportional to the net force acting on it: • acceleration ~ net force • (The symbol ~ stands for “is directly proportional to.”) Newton’s Second Law: Acceleration • Acceleration depends on the mass being pushed • The same force applied to twice as much mass results in only half the acceleration. • The acceleration is inversely proportional to the mass. • Inversely means that the two values change in opposite directions. As the denominator increases, the whole quantity decreases by the same factor. Newton’s Second Law: • Less mass – greater acceleration Newton’s Second Law: Newton’s second law states that 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 If a is acceleration, F is net force, and m is mass or F=ma Newton’s Second Law: • The acceleration is equal to the net force divided by the mass. • If the net force acting on an object doubles, its acceleration is doubled. • If the mass is doubled, then acceleration will be halved. • If both the net force and the mass are doubled, the acceleration will be unchanged Newton’s Laws of Motion: Jumping Lab • Acceleration and Net Force are proportional to each other • F = ma, where m is the slope of the graph • m – is the mass of the object • Mass is the measure of inertia • • • • F – force is measured in Newtons 1 Newton approximately equal to the weight of a small apple m – is the mass of the object in kg a – is the acceleration in m/s2 Newton’s Second Law: 1. A car has a mass of 1000 kg. What is the acceleration produced by a force of 2000 N? 2. If the car has an identical car in tow, what will be the acceleration of the car? Answer: The same force on twice the mass produces half the acceleration, or 1 m/s2. 3. If the force is 4000 N, what is the acceleration? Answer: Doubling the force on the same mass simply doubles the acceleration. Newton’s Second Law: How much force, or thrust, must a 30,000-kg jet plane develop to achieve an acceleration of 1.5 m/s2? Arrange Newton’s second law to read: force = mass × acceleration F = ma = (30,000 kg)(1.5 m/s2) = 45,000 kg•m/s2 = 45,000 N Newton’s Second Law Writing Exercise: • What is the relationship among an object’s mass, an object’s acceleration, and the net force on an object? • Use the example of a rocket launch to illustrate the relationships expressed in Newton’s Second Law