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Newton’s First and Second Laws of Motion

Newton’s First and Second Laws of Motion. Key Concepts What is Newton’s first law of motion? What is Newton’s second law of motion? Key Terms - Inertia. The First Law of Motion.

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Newton’s First and Second Laws of Motion

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  1. Newton’s First and Second Laws of Motion Key Concepts What is Newton’s first law of motion? What is Newton’s second law of motion? Key Terms - Inertia

  2. The First Law of Motion • An object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless acted upon by an unbalanced force • Clothes on the floor will stay there unless someone uses a force to pick them up • A tennis ball that was hit will continue until a force stops it • Gravity and friction are the 2 forces on Earth that often change an object’s motion

  3. Inertia • Tendency of an object to resist a change in motion • Newton’s first law is also called the Law of Inertia • Example – a person continues to move forward when a car stops

  4. Inertia Depends on Mass • The greater the mass of an object, the greater the inertia • The greater the inertia, the greater the force needed to move the object, and the greater the force needed to change its motion

  5. The Second Law of Motion • Acceleration depends on the object’s mass and the net force acting on the object • Acceleration = Net Force Mass • Force = mass * acceleration Units = Newton (N) = kg *m/s2

  6. Sample Problem • Calculating Force: A speedboat pulls a 55kg water skier. The force causes the skier to accelerate at 2.0m/s2. Calculate the net force that causes this acceleration. • Information given: • Mass of water skier =55kg • Acceleration of water skier= 2.0 m/s2 • What quantity are you trying to calculate? • Net force Fnet • What formula should you use? • A = fnet or Fnet = m*a m • Perform calculation – 55kg *2.0m/s2 = 110kg*m/s2 = 110N

  7. Math Practice 1 • What is the net force on a 1,000 kg object accelerating at 3 m/s2?

  8. Math Practice 2 • What net force is needed to accelerate a 25 kg cart at 14m/s2?

  9. Changes in Force and Mass • If you keep the mass constant, a change in force will have the same change in the acceleration • If you keep the force constant, an increase in mass will cause a decrease in acceleration • Opposite is also true – If the force is constant, a decrease in mass will cause an increase in acceleration

  10. Sources • http://www.physicsclassroom.com/class/newtlaws/u2l1b.cfm • http://www.rcs.k12.va.us/csjh/sieber/PhysicalSci_1stSem.htm

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