BellRinger

1. BellRinger EQ: What caused the matchbox car to accelerate? EQ: What caused the Felix Baumgartner to accelerate?

2. Free Fall a = 9.8 m/s2 6.5 m/s2 a = vf – vo = t Did Felix defy gravity? Felix jumped from 24 miles above the Earth. That is 128,000 feet, or 39014.4meter. In 50 second he reached a maximum of velocity of 325m/s (729mph) breaking the sound barrier. Calculate the acceleration Felix as he fell toward the Earth.

3. Newton's Laws

4. Newton’s Contributions • Calculus • Light is composed of rainbow colors • Reflecting Telescope • Your objectives are to learn. • Laws of Motion • Theory of Gravitation

5. What is a force? Here are some ways of describing forces: • A push • A pull • A stretch • A squeeze • A catch • A twist We can’t see forces but we can see the effects of a force.

6. Forces can make things: • Speed up • Slow down • Change direction • Change shape

7. Types of Force There are two types of force field force and contact force. An examples of a field force are electricity, magnetism, nuclear force, and gravity. An example of a contact force is kicking a ball.

8. Measuring forces A measure of the change in velocity over time. Anything that has mass and takes up space. mass * acceleration or F = ma Force is a measure of how much acceleration an object has Acceleration- Object- Force- therefore is

9. Measuring forces • Forces are can be large so we typically do not use the gram unit for mass, we instead use the kilogram. • Newtons. • We can write this as N. F = ma m = kg a = m t2 F= kg * m t2

10. Force Diagrams • The effect of a force depends on both magnitude and direction. Thus, force is a vector quantity. • Diagrams that show force vectors as arrows are called force diagrams, or Free Body Diagrams. • Force diagrams that show only the forces acting on a single object are called free-body diagrams.

11. Force Diagrams Free-Body Diagram In a force diagram, vector arrows represent all the forces acting in a situation. Force Diagram • A free-body diagram shows only the forces acting on the object of interest—in this case, the car. The sum of all the forces is called the Net Force.

12. Free-Body diagram N = Natural Force Fw = Force of gravity Fh = Force of Homer Ff = Friction Force N Ff Fh Fw N = Fw Ff >Fh Ff <Fh

13. Essential Question EQ: A fan blows on two balls, a boiling ball and a balloon. Describe what you think will happen. boiling ball balloon

14. Newton’s First Law(law of inertia) An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

15. Inertia: video • Inertia is the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction. • Newton’s first law is often referred to as the law of inertia because it states that in the absence of a net force, a body will preserve its state of motion. • Mass is a measure of inertia.

16. Newton’s First Law (law of inertia) much it will resist change to mass inertia INERTIA is a property of an object that describes how ______________________ the motion of the object more _____ means more ____

17. Net Force • Newton's first law refers to the net force on an object. The net force is the vector sum of all forces acting on an object. • The net force on an object can be found by using the methods for finding resultant. • Resultant is the result of the combined effect of all the forces acting on an object. Although several forces are acting on this car, the vector sum of the forces is zero. Thus, the net force is zero, and the car moves at a constant velocity.

18. Force 1 (F1) and force 2 (F2) are applied to the ball at the same time with the same magnitude. Draw a new arrow describing the combined direction of F1 & F2 on the ball. This is called the vector sum. F1 = F2 Graphing assignment

19. Graphing Assignment What is the direction and magnitude of the force the pole is acting against the two ropes? F1 = (5, 5) F2 = (-3, 6) (X, Y) (X, Y) F1 = (5Nx, 5Ny) F2 = (-3Nx, 6Ny) Two horizontal ropes are attached to a post that is stuck in the ground. The ropes pull the post producing the following vector forces

20. Graphing Assignment • If F1 + F2 = -F3diagram the direction and magnitude that the pole pulls back (F3) on the ropes? Hint: -F3XY = (F1X +F2X)(F1y +F2y)

21. Graphing Assignment • What is the angle between F2 & F3? • What is the angle between F1 & F3? • What is the angle between F1 & F2? • Hint: tan(θ) = Opposite / Adjacent  • θ = tan -1 (Opposite/Adjacent) or • (Y/X) • (-1) means (1/tan) or inverse.

22. BellRinger EQ1: Why did the matchbox car not move?

23. Normal Force • The normal force acts on a surface in a direction perpendicular to the surface. • The normal force is not always opposite in direction to the force due to gravity. • In the absence of other forces, the normal force is equal and opposite to the component of gravitational force that is perpendicular to the contact surface. • In this example, Fn = mg cos q.

24. BellRinger EQ2: Why did the matchbox car move? EQ3: Why did the matchbox car not continue to move forever?

25. If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? Things don’t keep moving forever because there’s almost always an unbalanced force acting upon them. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

26. Balanced Force Equal forces in opposite directions produce no motion

27. Unbalanced Forces Unequal opposing forces produce an unbalanced force causing motion

28. Terminal Velocity

29. Friction • A force that opposes motion • Acts parallel to the surfaces in • contact. Cause of Friction: the microscopic roughness between surfaces…like two gears locking together.

30. Friction is Everywhere!!! sand on icy streets walking tires brakes Friction allows us to Move!..........

31. 2 Types of Friction • Static friction is a force that resists the initiation of sliding motion between two surfaces that are in contact and at rest. • Kinetic friction is the force that opposes the movement of two surfaces that are in contact and are sliding over each other.

32. Hypothesis- • What will happen if we try to push our desk? Observe the force. • Conclusion: • ____________________ . • is greater than • _____________________. Kinetic friction is always less than the maximum static friction. Static Kinetic

34. Force vs. Time Maximum Static Friction Constant Kinetic Friction F t

35. Friction Forces in Free-Body Diagrams • In free-body diagrams, the force of friction is always parallel to the surface of contact. • The force of kinetic friction is always opposite the direction of motion. • To determine the direction of the force of static friction, use the principle of equilibrium. For an object in equilibrium, the frictional force must point in the direction that results in a net force of zero.

36. Free-Body diagram N = Natural Force Fw = Force of gravity Fh = Force of Homer Ff = Friction Force N Ff Fh Fw N = Fw Ff >Fh Ff <Fh

37. Normal Force • The normal force acts on a surface in a direction perpendicular to the surface. • The normal force is not always opposite in direction to the force due to gravity. • In the absence of other forces, the normal force is equal and opposite to the component of gravitational force that is perpendicular to the contact surface. • In this example, Fn = mg cos q.

38. The Coefficient of Friction • The quantity that expresses the dependence of frictional forces on the particular surfaces in contact is called the coefficient of friction, m. • Coefficient of kinetic friction: • Coefficient of static friction:

39. The Coefficient of Friction Found in the lab

40. Factors that affect Sliding Friction: • The normal Force acting on an object. FN(Fg only equals FN in a special circumstance, ie. lab) • The coefficient of Friction ( µk < µs, unitless) The texture of the surfaces. The stickiness between surfaces.

41. General Formula for Friction: friction force = coefficient of friction x normal force Ff = µFN Ff = µma or FN = mg Static - µs Kinetic - µk

42. Which object would have the greatest normal force? So which would experience the greatest friction force? (assume all blocks are moving)

43. What about surface area? Which would have a greater friction force? Friction force does not depend on the area of contact. Same FN and µ!

44. 2. Rolling Friction – (< sliding) wheels and ball bearings

45. 3. Fluid Friction – (< sliding) occurs when a solid object moves through a gas or a liquid substance Liquids lubricants

46. Gases Hammer Feather

47. Now for the problems… just like before… draw your FBD but now we need: Then: Ff = µFN ΣFx = ma ΣFy = ma

48. Pause for a Cause A sled sits on packed snow. The coefficient of kinetic friction is 0.12. If a penguin and the sled together weigh 650N, what force is needed to pull the sled across the snow at constant speed? Ff = µFN

49. Pause for a Cause Overcoming Friction A student attaches a rope to a 20.0 kg box of books. He pulls with a force of 90.0 N at an angle of 30.0° with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is 0.500. Find the acceleration of the box.

50. Pause for a Cause 1. Define Given: m = 20.0 kg mk = 0.500 Fapplied = 90.0 N at q = 30.0° Unknown: a= ? Diagram: