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Do Now

Do Now. In a tug-of-war, 13 children pull westward on a rope with an average force of 150 N per child. Five parents pull eastward on the other end of the rope with an average force of 475 N per adult. What is the net force acting on this system?. AIM: What are Newton’s three laws of motion?.

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Do Now

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  1. Do Now In a tug-of-war, 13 children pull westward on a rope with an average force of 150 N per child. Five parents pull eastward on the other end of the rope with an average force of 475 N per adult. What is the net force acting on this system?

  2. AIM: What are Newton’s three laws of motion? SWBAT: Describe inertia and its relationship to mass (5.1.vii) Verify Newton’s second law for linear motion (5.1.ix) Use vector diagrams to analyze systems (5.1.viii)

  3. Newton’s First Law • “An object at rest will remain at rest, and an object in motion tends to stay in motion” • Motion in a straight line, at a constant velocity (Fnet = 0) • The law of inertia • Inertia: the tendency of an object to resist changes in its motion • Inertia is not a force • Inertia is directly related to mass • Heavier objects have greater inertia (and thus are harder to move) than lighter ones

  4. Which has the greater inertia? (a) m = 400 kg v = 5 m/s (b) m = 50 kg v = 3 × 108 m/s

  5. (a) m = 100 kg v = 0 m/s (b) m = 100 kg v = 20 m/s (c) m = 100 kg v = 50 m/s (d) I think this is a trick question!

  6. Newton’s Second Law Fnet = ma • Units: [N] = [kg][m/s2] • If Fnet is not zero, there will be an acceleration • If Fnet is zero, a will also be zero • Object is at rest or is moving at constant velocity • We say that an object is in equilibrium when Fnet = 0 • (Sounds an awful lot like Newton’s First Law, doesn’t it?)

  7. A force of 1 N is the only force exerted on a block (mA), and the acceleration (aA)is measured. • When the same force is the only force exerted on a second block (mB), the acceleration is three times as large (aB = 3aA). • What can you conclude about the masses of the two blocks? (a) mA = mB(b) mA > mB(c) mA < mB

  8. Practice with Newton’s 2nd Law James is learning to rollerblade in Mr. Babel’s class. He wants Mr. Babel to pull him along so that he has an acceleration of 0.9 m/s2. If James has a mass of 35 kg, with what force does Mr. Babel need to pull him?

  9. Two cosplayers at ComicCon are dressed as the Tenth and Eleventh Doctor. They both grab a lifesize, inflatable TARDIS (m = 16 kg) for sale at the same time, from opposite sides. If the Tenth Doctor pulls with a force of 47 N, and and the Eleventh Doctor pulls with a force of 53 N, what acceleration will the TARDIS experience?

  10. Newton’s 2nd Law + Motion A race car has a mass of 710 kg. It starts from rest and travels 40.0 meters in 3.0 seconds. The car is uniformly accelerated during the entire time. What net force is exerted on it?

  11. Mass vs. Weight • Remember that g is an acceleration? Fnet = ma Fg= mg • m is the object’s mass [kg] • Fg is the object’s weight • Do scales read your your mass or your weight? • 1 lb = 4.45 N

  12. You place a watermelon on a spring scale at the supermarket. If the mass of the watermelon is 6 kg, what is the reading on the scale (in N and in lbs)?

  13. Newton’s Third Law • When one object (A) exerts a force on a second object (B), the second object exerts an equal and opposite force on the first object • FAon B = - FB on A • We call these two forces interaction pairs • Action = - Reaction • Be cautious: These action/reaction forces act on different objects!

  14. Can you identify the interaction pair here?

  15. A bird sits on top of a statue of Newton. Draw free body diagrams for the bird and the statue. Specifically label any forces that are part of an interaction pair.

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