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Chapter 12

Chapter 12. 12.2 Newton’s First and Second Laws of Motion. Aristotle. 384 B.C. – 322 B.C. Greek scientist and philosopher Incorrectly proposed that force is required to keep an object moving at a constant speed. This delayed progress in the study of motion for almost 2000 years. Galileo.

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Chapter 12

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  1. Chapter 12 12.2 Newton’s First and Second Laws of Motion

  2. Aristotle • 384 B.C. – 322 B.C. • Greek scientist and philosopher • Incorrectly proposed that force is required to keep an object moving at a constant speed. • This delayed progress in the study of motion for almost 2000 years.

  3. Galileo • 1564 - 1642 • Italian scientist • Studied how gravity produces constant acceleration. • Concluded that moving objects not subjected to friction or any other force would continue to move indefinitely.

  4. Newton • Late 1600s • Left college in England because of the plague • Defined mass and force • Introduced the laws of motion

  5. Newton’s First Law of Motion • According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.

  6. Newton’s First Law of Motion • This is sometimes called the law of inertia. • Inertia is the tendency of an object to resist a change in its motion. • In other words: an object at rests tends to remain at rest and an object in motion tends to remain in motion with the same direction and speed

  7. Newton’s Second Law of Motion • According to Newton’s second law of motion, the acceleration of an object is equal to the net force acting on it divided by the object’s mass. • Acceleration = net force ÷ mass • Or • a = F/m

  8. Mass • Mass is a measure of the inertia of an object and depends on the amount of matter the object contains.

  9. Units • N/kg = m/s2 • These are equal because a newton is kgxm/s2 . • In force the kg’s cancel and leave you with m/s2. • See page 367 for more details.

  10. a = F/m • A boy pushes forward a cart of groceries with a total mass of 40.0 kg. • What is the acceleration of the cart if the net force on the cart is 60.0 N? • a = 60.0 N / 40.0 kg • a = 1.50 m/s2

  11. a = F/m • What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction? • a = 10,000 N/ 5000 kg • a = 2 m/s2

  12. a = F/m • An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. • What is the net force acting on the automobile? • 3.0 m/s2 = F/1200 kg • 3.0 m/s2 X 1200 kg = F • 3600 N = F

  13. a = F/m • A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2. • What is the mass of the boy and the wheelchair? • 0.5 m/s2 = 25 N/m • 0.5 m/s2 X m = 25 N • m = 25 N/ 0.5 m/s2 • m = 50 kg

  14. Weight and Mass • Weight is the force of gravity acting on an object. • An object’s weight is the product of the object’s mass and acceleration due to gravity acting on it. • Weight = mass X acceleration due to gravity • W = mg

  15. Weight • Earth’s gravity is 9.8 m/s2. • What is the weight of a 112 kg person on Earth? • W=mg • W=112 kg x 9.8 m/s2 • W = 1100 kgxm/s2 or 1100 N

  16. Mass • What is the mass of a person who weighs 1500 N on Earth? • W=mg • 1500 N = m x 9.8 m/s2 • 1500 N / 9.8 m/s2 = m • 153 kg = m

  17. Mass vs. Weight • Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object.

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