Chapter 6 Forces and Motion

1. Chapter 6 Forces and Motion Section 1 Gravity and Motion

2. Essential Questions for 6-1 • What is a force? • What are the effects of gravity and air resistance on falling objects? • Why are objects in orbit in free fall? • Why do they appear to be weightless? • How is projectile motion affected by gravity?

3. Forces and Fields • A Forceis a push or a pull • A Force Field is an area in which a force is experienced.

4. Vectors and Scalars • Vectors are quantities that have both magnitude and direction. • For example: Velocity: 45mph West • Scalars are quantities that just have magnitude • For example: Speed 45mph

5. Why use vectors • They show the forces acting on an object. We can add them or subtract them to determine the Net force acting on an object.

7. This is Galileo Galilei (1564-1642)

8. Galileo went to the Leaning Tower of Pisa…

9. … dropped two cannon balls of different masses to the ground…

10. …and this is what happened.

11. Force of Gravity • Gravity = A force that pulls objects together. • On Earth, gravity makes any object of any mass accelerate toward Earth at 9.8 m/s2.

12. Relationships in FG = m x 9.8 m/s2 • Heavier objects do not fall faster than lighter objects. • Heavier objects hit the ground with a Stronger force, but they accelerate toward Earth at the same rate.

13. Gravity and Falling Objects • Objects fall to ground at the same rate because acceleration due to gravity is the same for ALL objects • Why? Acceleration depends on both force and mass. • A heavier object experiences a greater gravitational force, BUT it is also harder to accelerate. • Galileo was a genius.

14. Acceleration and Velocity • Acceleration is the rate velocity changes over time. The acceleration of an object affected by gravity is 9.8 m/s. • Change in velocity of falling objects can be measured by the following equation: ∆v = g x t OR 9.8 m/s times the number of seconds an object falls…

15. Formula for Force of G where: • F is the force between the masses, • G is the gravitational constant, • m1 is the first mass, • m2 is the second mass, and • r is the distance between the centers of the masses. The force of gravity we experience on Earth is primarily due to the pull of the sun on the Earth. Although, we are far from it, its’ mass is so great it creates the strongest gravitational pull in our solar system

16. Which of these objects will fall at the fastest rate when dropped? • The ball with a mass of 75 kg • The ball with a mass of 25 kg • The ball with a mass of 10 kg • They all fall at the same rate.

18. Mr. I Earth Moon a = 1.6 m/s2 m = 70 kg a = 9.8 m/s2 m = 70 kg

19. Calculate Your Weight Earth Moon a = 1.6 m/s2 m = your mass in kg a = 9.8 m/s2 m = your mass in kg

20. Air Resistance • Air resistance – the force that opposes the motion of objects through air • Amount of air resistance depends on the size, shape, and speed of the object • A place where all the particles have been sucked out is called a vacuum. There is no Air resistance in a vacuum, because there is nothing to cause friction with.

21. Discussion • Why would a feather dropped from the same height as an acorn fall to Earth more slowly than the acorn?

22. Discussion • How can a sky diver benefit from air resistance?

23. Terminal Velocity • Terminal Velocity – when air resistance = gravity • 0 N or a balanced force! • It’s the fastest an object will fall • It’s also why rain drops don’t kill you… think about it…

24. Free Fall • Free fall - when gravity is the only force acting upon an object • Can only happen where there is NO air resistance: SPACE (or a vacuum) • - Orbiting objects are in free fall. Orbit is caused by two motions: - Orbiting objects move forward, but are also in free fall – so are the astronauts

25. Discussion • How does free fall in an orbiting spacecraft cause the appearance of weightlessness?

26. Orbiting and Centripetal Force • Orbit is caused by two things that create an unbalanced force • Gravity provides centripetal force to the orbiting object • Centripetal force – the unbalanced force that causes an object to move in a circular path • Ex: planets around the sun or the moon around the Earth

27. Discussion • What does free fall in an orbit look like? • Which is • The path of the orbiting spacecraft? • The path of free fall for the spacecraft? • The path the spacecraft would follow if there was no gravity? • The curve of the Earth’s surface?

28. Projectile Motion • The curved path an object follows when it is thrown near the surface of the Earth • Composed of horizontal motion and vertical motion • Horizontal motion (like throwing a ball) causes the object to go forward • Vertical motion (gravity) causes the object to fall • Together they create a curved path: BUT still hits the ground the same time as a dropped object • This is why you always have to aim above a target when trying to hit it…

29. An example of projectile motion

30. Quiz 6-1: 4 Questions 1. If a baseball and a cannonball are dropped from the same height at the same time, and there is no air resistance, which ball will hit the ground first? a. The cannonball lands first. b. The baseball lands first. c. The balls land at the same time. d. The ball with the larger volume lands first.

31. 2. What feature of an object does not affect air resistance? a. its size b. its chemical properties c. its shape d. its speed

32. 3. Why do astronauts in space appear weightless? • There is no gravity in space. b. Air resistance opposes gravity. c. They have no mass in space. d. They are in free fall.

33. 4. In what directions is a projectile accelerated? a. both vertically and horizontally b. vertically downward c. vertically upward d. horizontally forward

34. Chapter 6 Forces and Motion Section 2 Newton’s Laws of Motion

35. Essential Questions for 6-2 • What is Newton's first law of motion? • How does it relate to objects at rest and objects in motion? • What is Newton's second law of motion? • What is the relationship between force, mass, and acceleration? • What is Newton's third law of motion? • What are some examples of force pairs?

36. 1st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

37. 1st Law • Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force.

38. 1st Law • Once moving, unless acted on by an unbalanced force (gravity and air – fluid friction), an object would never stop!

39. 1st Law • Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever. • Ex. So far…

40. Discussion 1: • You are a passenger in a car that is moving rapidly down a straight road. As the driver makes a sharp left turn, you are pressed against the right side of the car. Explain why this happens.

41. Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – friction.

42. What is this unbalanced force that acts on an object in motion? Friction! • There are four main types of friction: • Sliding friction: ice skating • Rolling friction: skateboarding • Fluid friction (air or liquid): air or water resistance • Static friction: initial friction when moving an object

43. Newton’s 2nd Law

44. 2nd Law The net force of an object is equal to the product of its mass and acceleration, or F = m x a

46. Newton’s 2nd Lawproves that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate. (Gravity = 9.8 m/s) • However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s

48. Discussion 2: • How does Newton’s second law explain why it is easier to push a bicycle than to push a car with the same acceleration?

49. 3rd Law • For every action, there is an equal and opposite reaction.

50. 3rd Law There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.