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Force

and Newton's Laws. Force. F = ma. When Making a sudden stop in your car, what pushes you into the seat belts?. Nothing, your own motion does it Your seat Nothing, your seat belts just tighten The Force. [Default] [MC Any] [MC All]. “The Force”.

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Force

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  1. and Newton's Laws Force F = ma

  2. When Making a sudden stop in your car, what pushes you into the seat belts? • Nothing, your own motion does it • Your seat • Nothing, your seat belts just tighten • The Force [Default] [MC Any] [MC All]

  3. “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

  4. 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)

  5. Which of the following is an example of a force? [Enter question here] • 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 [Default] [MC Any] [MC All]

  6. 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

  7. 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)

  8. 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

  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? [Enter question here] • Gravitational Force • Electric Force • Frictional Force • Magnetic Force • THE Force [Default] [MC Any] [MC All]

  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 net force. Or!!! An object in motion will stay in motion unless acted upon by an outside net 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? [Enter question here] • 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. [Default] [MC Any] [MC All]

  15. You pay a large amount of money to be sent to outer space. You finally reach a point where all the gravitational forces acting on you are close to zero. If you are traveling at the speed of light and then run out of fuel you will….. [Enter question here] • Slow to a stop • Travel in a parabola • Continue in a straight line. • Maintain constant speed. • Both A and B • Both C and D • All of the above [Default] [MC Any] [MC All]

  16. 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?

  17. What if…The net force acting on an object is zero? [Enter question here] • 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. [Default] [MC Any] [MC All]

  18. I have a mass attached to the ceiling by a thread. There is also a thread attached at the bottom. If you pull the bottom string which will break first? • Both at the same time • The top one • The bottom one • Depends on how hard you pull it [Default] [MC Any] [MC All]

  19. 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!!!

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

  21. Weight • Find your weight on different places! • 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

  22. What is the weight of a 60 kg person on earth? [Enter question here] • 5.8 N • 58 N • 588 N • 5880 N • 60 kg • None of the above [Default] [MC Any] [MC All]

  23. What is the weight of a 60 kg person on the moon? (gm= 1.67 m/s2) [Enter question here] • 1 N • 10 N • 100 N • 1000 N • 1.67 N [Default] [MC Any] [MC All]

  24. Constant acceleration from gravity • Galileo found that balls of different masses fell at the same rate. • The force of gravity is stronger for more massive objects. • But it requires more force to accelerate a more massive object (inertia). • These two effects cancel each other out.

  25. 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

  26. 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).

  27. 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

  28. Force of Friction Static Friction is the force which keeps an object stationary Kinetic Friction slows an object in motion Force is dependent on surface type and pressure pushing the two objects together Ff = m FN m is the coefficient of friction, and describes the “roughness” of a surface. Force is directed opposite motion or expected motion

  29. Find the Force of Friction… • For a 5 kg box that is being pushed across a table which has a µ of 0.655. :)

  30. Find the Force of Friction… • For the shoes of a 60 kg student as they walk across a floor with a µ of 0.654.

  31. 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

  32. What is the mass of a 294 N box • 30 kg • 2880 kg • 294 N • 66 lbs [Default] [MC Any] [MC All]

  33. 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.

  34. 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!

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

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

  37. 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.

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

  39. 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?

  40. FBD Practice • A 5 kg box slides across a table that has a coefficient of friction of 0.236. If the force applied to the box is 25 N, what is the net force acting on the box in the direction of motion?

  41. 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?

  42. 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.

  43. The mass of a refrigerator that slides across a kitchen floor that has a µ of 0.245 and an applied frictional force of 300 N. • 125 kg • 1220 kg • 73.5 kg • 7.50 kg [Default] [MC Any] [MC All]

  44. Basic Info: balanced Forces • Objects are balanced only if their net force is zero in both the vertical and horizontal directions • Meaning all the forces in the x direction add up to zero AND all the forces in the y direction add up to zero • All forces that act in a angle needs to be broken into components using trig. • Meaning using a right triangle with x and y components.

  45. Vector Components • In order to find the components of a vector (like force) you will need to use those timeless Trigonometric Functions.

  46. Vector Components • So we have a person pulling a sled 30o with respect to the horizontal at a force of 50 N. • We need to think of it like the sled being pulled vertically and horizontally at the same time, giving it both components. Fy F=50N Fx Θ=30o

  47. Vector Components • In order to calculate the components, we need to shift Fy to make a right triangle. • Then we can use trig functions to solve for Fy and Fx like they are sides of a right triangle. • To solve for Fx, we will use cosine because it is adjacent and we have the hypotenuse. Fy F=50N Fx Θ=30o

  48. Vector Components • To solve for vx, we will use cosine because it is the adjacent side and we have the hypotenuse. • To solve for vy, use the same process but with sine. F=50m/s Fy Θ=30o Fx

  49. Fyequals? • 43.3 N • 25 N • 28.9 N • 50 N F=50m/s Fy Θ=30o [Default] [MC Any] [MC All]

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