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Force

and Newton's Laws. Force. F = ma. “The Force”. “An energy field created by all living things. It surrounds us, penetrates us, and binds the galaxy together.” The Force has two components: Light side Dark side. The Real Force.

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Force

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  1. and Newton's Laws Force F = ma

  2. “The Force” • “An energy field created by all living things. It surrounds us, penetrates us, and binds the galaxy together.” • The Force has two components: • Light side • Dark side

  3. The Real Force • Something that causes an object’s motion to change (causes acceleration). • A “push” or a “pull.” • Common Examples of forces: Gravity fields, pushing on something, compressing a spring, a magnetic field, tension, friction, and the “normal” force. • Units are Newtons (N)

  4. Which of the following is an example of a force? • Your weight • A magnet pulling on another magnet • A bully pushing you down the stairs • A rope pulling a cartful of turnips • All of the above • None of the above

  5. Types of Forces • Contact • Me slapping you upside the head • Baseball bat on a baseball • Field • Balloon bending stream of water • Projectile Motion • Magnetic fields

  6. Easily forgotten Forces!! Important contact force: FRICTION • Acts in the opposite direction of motion • Produces HEAT • AIR RESISTANCE is a form of FRICTION!! • SLOWS MOTION • Can be desired (tires on road) or undesired (skates on ice)

  7. Easily forgotten forces!! Important field forces: GRAVITY and Magnetic Fields • Opperate at a distance (don’t need to “touch”) • Decreases with the distance between objects. • WEIGHT is a FORCE caused by gravity • Fweight = mass x gravity • As gravity changes, WEIGHT changes, NOT MASS

  8. As a meteor passes the earth our gravity causes its path to bend. Another meteor passes us and its path is not bent nearly as much. Which meteor is closer to the earth? • The first meteor • The second meteor • What meteor? • Yes

  9. So I’m cruising around Belton in my awesome car when I see a dog in the middle of the road! What is the main force that lets me stop my car so the doggy can live? • Gravitational Force • Electric Force • Frictional Force • Magnetic Force • THE Force

  10. Force is a vector • Has magnitude and direction • Amount of force makes a difference • Little push vs. big shove • Earth gravity vs. moon gravity • Direction makes a difference • Push vs. Pull • Thrust vs. Drag • Forces “add” to give a net force. • Net force determines what an object will do.

  11. Newton's 1st Law!! The law of inertia: An object at rest will stay at rest unless acted upon by an outside force. Or!!! An object in motion will stay in motion unless acted upon by an outside force.

  12. Galileo’s Unique Idea • Objects don’t need a force to keep moving! • Every object naturally wants to maintain its state of motion or rest • INERTIA! (resistance to change in motion) • Refined by Newton in 1800’s:

  13. Basic Info: Inertia • Inertia depends on: • Mass • Shape/Mass Distribution of object- rotational inertia • Solid Cylinder (like a wheel of cheese,) • Hoop (like a bicycle tire) • Inertia does NOT depend on: • Velocity/Speed of object • It takes the same amount of force to speed a bus up as to slow it down!

  14. Which of these objects has the greatest amount of inertia? • A 0.5 mg cockroach. • A 2.0 g lump of cheese. • A 35 g ball of goo. • A 15 kg baby goat • A 300 kg circus freak on roller skates.

  15. Net Force • Net Income: How much money did you make after you counted up all profit and expense? • Net Force: How much force is there after all individual forces are added up?

  16. What if…The net force acting on an object is zero? • The object will slow to a stop. • The object will begin to fall. • The object’s motion will not change. • The universe will open up and swallow the object. • Time will stop for us but the object will grow old and smelly.

  17. Fun Stuff • Wear your seatbelt! • Fun stuff • Other one

  18. Newton's 2nd Law!! In other words… You already know this! the important part is the Σ. This means “Sum” or “Net”

  19. Net Force • Net Income: How much money did you make after you counted up all profit and expense? • Net Force: How much force is there after all individual forces are added up?

  20. The 2 components of force F = ma X Mass Acceleration

  21. Typical Situations • Which is easier to accelerate? • A tiny sports car? Or a ridiculously large and completely unnecessary SUV? • Your car has stalled in the middle of the road! • Easier to push it to the side by yourself? Or with friends? More mass requires more Force!

  22. Don’t forget the Force • Causes accelerations • Gravity leads to: Projectile Motion, and more projectile motion • “Pushes and Pulls” can also cause accelerations (like this). • Measured in Newtons (N) • The amount of force that causes a 1 kg mass to accelerate at 1 m/s2

  23. The 2nd Law: F=ma Force (N) = mass (kg) x acceleration (m/s2) Always make sure your mass is in kilograms (kg)!

  24. Calculate the following • What force is required to push your 1,100 kg car with an acceleration of 0.5 m/s2?

  25. Calculate the following • If a 10 N force is applied to an object that causes it to accelerate at 22 m/s2, what is the mass of the object?

  26. Calculate the following • The net force on the propeller of a 3.2 kg model airplane is 7.0 N forward. What is the acceleration of the airplane?

  27. Calculate the following • A force is applied to a 1.5 kg ball that causes it to accelerate from rest to a velocity of 4.5 m/s in 0.5 seconds. What is the magnitude of the force?Remember: Fnet = maand a = (Vf-Vi)/t

  28. Calculate the following • A 12 kg box starts from rest and slides down a frictionless ramp. If the gravitational force pulling it down the ramp is 35 N, what is the final velocity of the box after 3.4 seconds?

  29. Gravity“Gravitational Force” Force which is exerted by any object with mass. Earth’s gravitational force is measured as Fg = mg, g = 9.8m/s2 Direction is always down Designated Fg sometimes called weight

  30. Gravitational Force Newton’s Law of Universal Gravitation The amount of force is directly proportional to the mass of each of the objects. The gravitational force between two objects decreases as the distance between them (r) increases! This is an inversely proportional relationship! Don’t worry, we wont be using this equation yet!

  31. weight • The amount of gravitational force exerted on an object • 1 lb = 4.448 N 1 N = 0.225 lb FW = mg FW (also W or Fg): weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) Remember.... MASS always the same (kg) WEIGHT depends on gravity (N) Weight and mass are two completely different things!!!

  32. weight • Would you weigh more on Earth or Jupiter? • Jupiter because... • greater mass • greater gravity • greater weight

  33. Weight • Find your weight on different planets! • First, find your mass in kilograms: take your weight and divide it by 2.2 (because 1 kg weighs 2.2 lbs) • Now, to find how many Newtons you weigh, take your mass (m) and multiply it by the gravitational acceleration (g). Remember: F=ma, or FW = mg • gearth: 9.8 m/s2 • gmoon: 1.67 m/s2 • gmars: 3.7 m/s2 • gsun: 274.13 m/s2 • gpluto: 0.42m/s2 • To convert Newtons to Pounds, remember: 1 lb = 4.448 N

  34. What is the weight of a 60 kg person on earth? • 5.8 N • 58 N • 588 N • 5880 N • 60 kg • None of the above

  35. What is the weight of a 60 kg person on the moon? (gm= 1.67 m/s2) • 1 N • 10 N • 100 N • 1000 N • 1.67 N

  36. Force of Normal The force that a surface exerts on an object. Force is always in a direction perpendicular to the surface the object or system is in contact with Usually a balancing force when an object is in contact with another object Designated FN or N

  37. Force of Normal - Example Let’s say the box in this diagram has a mass of 25 kg. That means it has a weight of 245 N. Since the ground is level, the Normal Force is going to be equal to the box’s weight, but the force is exerted in a different direction (up).

  38. Force of Normal - Example 2 That last one was easy! Let’s say this box is seated on an incline and still has a mass of 25 kg. The weight of the box (mg) is still directed straight down, but the Normal Force is at an angle, which makes it a little harder to find… FN = wcosθ

  39. Tension Force applied by a string or cable when the object or system is hanging Direction can be variable A force of tension is described as FT or T

  40. Basic Info: Force Diagrams • Definition: A Diagram that shows all the forces acting on a body • Does NOT include forces exerted by the body! • Forces are drawn as vectors.

  41. Free Body Diagram Simple drawing of all forces working on an object or system Use a box to represent the object or system All forces move away from the box. Remember: gravity will always affect an object and so Fg will always be in a F.B.D!

  42. Draw a Force body diagram • Diagram the forces acting on a car stuck in a traffic jam.

  43. Draw a Force body diagram • Diagram the forces acting on a car driving on IH-35.

  44. Basic Info: Unbalanced Forces • Objects that are NOT in equilibrium • Will accelerate! • Objects in equilibrium will not accelerate (inertia) • Net force required to speed up, slow down, turn, etc. • Demos: • Frictionless Track at an angle- Remember Forces are Vectors and can have 2 components!!

  45. Which of these situations shows balanced forces? A B Which will accelerate? D: all of them E: none of them C

  46. FBD Practice • As a 1150 kg car is driving on the highway the engine is supplying a force of 900 N. The total force of friction (including air resistance and the internal friction of the engine) is 460 N. What is the net force on the car in the direction of motion?

  47. FBD Practice • A force of 150 N is applied to a 15 kg wood block to make it slide across a piece of sandpaper. • What is the force of normal acting on the wood block? • What is the force of friction acting on the wood block if the μ of the sandpaper is 0.995? • What is the net force acting on the wood block? • What is the acceleration of the block?

  48. FBD Practice • A 8 kg block of cheese is sitting on a ramp with a 30o incline and a µ of 0.745. Draw a force diagram showing all the forces acting on the cheese.

  49. Formulas! • Fnet = ma • W = mg • Ff = μFN • Fnet = FA – Ff • Object on a flat surface: • FN = Weight • Object on an incline: • FN = Wcosθ • FA = Wsinθ

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