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Chapter 2 Forces in Motion

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  1. Chapter 2 Forces in Motion Gravity and Motion Newton’s Laws of Motion

  2. Section 1: Gravity and Motion • All Objects Fall with the Same Acceleration • **Galileo’s experiment: 2 cannon balls of different weight but the same size. Dropped them off the Leaning Tower of Pisa. Hit Ground at same time. • Video clip • changed peoples idea about gravity and falling objects.

  3. Section 1: Gravity and Motion • Acceleration Due to gravity • objects fall to the ground at the same rate because of acceleration due to gravity. • **See Figure 1 pg.36 • Ping Pong vs. golf ball…who will win???

  4. Section 1: Gravity and Motion • Accelerating at a constant rate • All objects accelerate toward the earth at a rate of 9.8 m/s/s downward • **See figure 2 pg. 37

  5. Section 1: Gravity and Motion

  6. Section 1: Gravity and Motion • Galileo’s experiment on falling objects clip • Math Break pg 37 • 1. Δ v=9.8 m/s/s x 2s=_________ • 2. Δ v=9.8 m/s/s x ___=_______

  7. Section 1: Gravity and Motion • Air Resistance Slows Down Acceleration • Crumpled paper and flat sheet example. What happens???? • Air resistance:fluid friction • occurs between the surface of the object and the surrounding air. • Air resistance Affects Some Objects More Than Others • Depends on Size and Shape of the object!!!! • Ex. Crumpled piece of paper dropped at the same time as a full sheet of paper. --On Earth ALL OBJECTS ARE AFFECTED BY AIR RESISTANCE • **See figure 3 pg. 38 video clip on falling

  8. Section 1: Gravity and Motion • Acceleration Stops at the Terminal Velocity • Terminal velocity:object falling at a constant velocity (not changing) • upward force of air resistance=downward force of gravity (no more acceleration) • Video Clip • Terminal velocity a good thing????? • **Hailstones…no air resistance then they would hit at 350 m/s.

  9. Section 1: Gravity and Motion • Free Fall Occurs When There is No Air Resistance • Free Fall:falling object that only gravity is acting on it (no air resistance). • Video clip • Orbiting Objects Are in Free Fall • Floating Astronauts are in “free fall”

  10. Section 1: Gravity and Motion • Two Motions Combine to Cause free fall • **See Figure 7 pg. 40 How an Orbit is formed (spaceship) • Orbit:an object traveling in a circular or nearly circular path around another object. • Why doesn’t the moon fall down? Video clip • The Role of Gravity in Orbiting • Centripetal Force:unbalanced force that causes objects to move in a circular path. • **See Figure 8 pg. 40 (Example the moon)

  11. Section 1: Gravity and Motion • Projectile Motion and Gravity • Projectile Motion:the curved path an object follows when thrown or propelled near the surface of the Earth. • **See Figure 9 pg. 41

  12. Section 1: Gravity and Motion

  13. Section 1: Gravity and Motion • Examples of Objects in Projectile Motion: • A football being passed Water sprayed by a sprinkler • Balls being juggled A swimmer diving into water • An athlete doing a high jump A leaping frog • Horizontal Motion • motion that is parallel to the ground • example: throwing a ball • Vertical Motion • motion that is perpendicular to the ground • Example: throwing a dart to a bulls eye • Gravity Pulls objects in projectile motion down with an acceleration of 9.8 m/s/s (if no air resistance) • **See Figure 10 pg.42

  14. Section 2: Newton’s Laws of Motion BrainPop—Issac Newton BrainPop—Newton’s Laws of Motion Newton’s First Law of Motion Definition: An object at rest and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force

  15. Section 2: Newton’s Laws of Motion • Part 1: Objects at Rest • anything not moving will stay still until an unbalanced force moves it. • Ex. A chair in the middle of the room will not move until you push it. See Figure 11 pg.43

  16. Section 2: Newton’s Laws of Motion • Part 2: Objects in motion • anything moving will stay moving until an unbalanced force stops or changes the direction it was moving. • Ex. Bumper Cars—car stops you fly forward. See Figure 12 pg.44

  17. Section 2: Newton’s Laws of Motion • Friction and Newton’s First Law • once you push your chair it should sail across the room forever right? No! The unbalanced force stopping it is FRICTION! • *****Turn to pg. 44 and answer the APPLY Stopping Motion question

  18. Section 2: Newton’s Laws of Motion • Inertia is Related to Mass • Inertia: the tendency of all objects to resist any change in motion. • Mass Is a Measure of Inertia • Small mass=less inertia than a large mass object. • Ex. Playing softball with a bowling ball See Figure 13 pg. 45**DO SELF CHECK pg. 45

  19. Section 2: Newton’s Laws of Motion Newton’s Second Law of Motion Definition:The acceleration of an object depends on the mass of the object and the amount of force applied.

  20. Section 2: Newton’s Laws of Motion • Part 1: Acceleration Depends on Mass • Using the same amount of force to push a near empty grocery cart and full grocery cart. • Which one will accelerate faster? • See figure 14 pg. 46

  21. Section 2: Newton’s Laws of Motion • Part 2: Acceleration Depends on Force • Giving a cart a soft push verses a large push. • Which will accelerate faster? • See figure 15 pg. 46

  22. Section 2: Newton’s Laws of Motion Expressing Newton’s Second Law Mathematically a=acceleration m=mass F=force  a= FF= m x a m  m=F a See figure 16 pg. 47

  23. Section 2: Newton’s Laws of Motion

  24. Section 2: Newton’s Laws of Motion • Newton’s Third Law of Motion • Definition: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. • See Figure 17 pg. 48 • action force=swimmers hands pushing on the water • reaction force=water pushing back on hands • Ex. Your weight pushing on a chair, chair pushing Up

  25. Section 2: Newton’s Laws of Motion • Force Pairs Do Not Act on the Same Object • if they acted the same the net force would be ZERO=no motion!!!!!

  26. Section 2: Newton’s Laws of Motion • The Effect of a Reaction Can Be Difficult to See • See figure 18 pg. 49 • Action force—ball falling to Earth • Reaction force—Earth being pulled toward the ball • Why can you not see it? • the force on the Earth=the force on the ball • Earth’s mass is greater than ball • Acceleration of the Earth is much smaller than the acceleration of the ball cannot see or feel it

  27. Section 2: Newton’s Laws of Motion • More Examples of Action and Reaction Force Pairs • See pictures bottom of pg. 49 • Momentum Is a Property of Moving Objects • Compact car and large truck traveling at the same velocity. • Which will take longer to stop? • 2 compact cars: car 1 velocity is 10 m/s east; car 2 20 m/s east. Which will take longer to stop?

  28. Section 2: Newton’s Laws of Motion • Momentum: a property of a moving object that depends on the object’s mass and velocity. • **the more momentum an object has the harder it is to stop. • Momentum formula: p= m x v • units for momentum are kg x m/s • Example: Find the momentum of an 80 kg basketball player driving to the basket with a constant velocity of 8 m/s.

  29. Section 2: Newton’s Laws of Motion • Momentum is Conserved • moving object hits another object, some or all of the momentum of the 1st object is transferred to the other object. • If only some of the momentum is transferred then the rest of the momentum stays with the 1st object. • See figure 19 pg. 50 • Ex. Pool and bowling

  30. Section 2: Newton’s Laws of Motion Conservation of Momentum and Newton’s 3rd law • billiard ball: the cue(white) ball hit the billiard ball with a certain amount of force (action force). The reaction force was = but opposite force exerted by the billiard ball on the cue ball. • Action force made the ball start moving; reaction force made it stop moving. • See figure 20 pg. 51