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Newton’s Laws of Motion

Newton’s Laws of Motion. get an acceleration a. apply a force F. Newton’s First Law. Resistance to changes in motion. “The Law of Inertia”. An object at rest remains at rest if no force acts on it. An object in motion remains in straight-line motion if no force acts on it.

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Newton’s Laws of Motion

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  1. Newton’s Laws of Motion

  2. get an acceleration a apply a force F Newton’s First Law Resistance to changes in motion “The Law of Inertia” An object at rest remains at rest if no force acts on it. An object in motion remains in straight-line motion if no force acts on it. No force  no acceleration constant velocity What makes these cars have different velocities? A change in speed (magnitude) A change in direction A change in both magnitude (speed) and direction mass m
  3. Objects at Rest Normal Force ↑ Are there any forces acting on the block? An object remains at rest because the forces acting on it are balanced; not because there are NO forces acting upon it Gravitational Force ↓ The downward force (mg) of gravity is balanced by an upward force of the table (-mg). Web Links: Truck and ladder, Car and person
  4. To give 1 kg an acceleration of 1 m/s2 requires a force of: 1 Newton (N) = SI unit for force

    Newton’s Second Law

    "Force equals mass times acceleration (F = ma): the net force on an object is equal to the mass of the object multiplied by its acceleration." F = ma Variables & Units m = mass = kilogram (kg) a = acceleration = m/s2 F = force = ma = kg∙m = Newton (N) s2
  5. Fx = max Fy = may F = ma the sum of forces on an object the object’s mass the object’s acceleration = x Since force is a vector F has components : Fx = the sum of the x-direction forces Fy = the sum of the y-direction forces And since motion in the x and y directions is independent: What relationship does force have with mass? Acceleration? What relationship does mass have with acceleration?
  6. Relationships (direct or inverse) F = ma F= ma F= ma ↑F= ma___ What happens to acceleration? Why? F= ↑ma___ What happens to acceleration? Why?
  7. Calculate Three people are pulling on a wagon applying forces of 100 N,150 N, and 200 N. The wagon has a mass of 25 kilograms. Determine the acceleration and the direction the wagon moves. G U E S S Σ F1= F2= F3= m= a= F = ma Fnet= 100N +150N - 200N= 50 N a= Fnet/m
  8. Unbalanced Forces Resultant= net force If there is a net force there is an acceleration b/c forces are unbalanced Fnet= 10N -5N= 5N 10N 5N
  9. Problem What is the net force? What force would be needed to stop the object from moving? Fnet= 12N -10N= 2 N 12N 10N Equilibriant- vector with equal magnitude but opposite direction 2N 2N
  10. Balanced Forces What is the net force? Balanced forces are at equilibrium Object is either at rest or has a constant velocity (Newton’s 1st law) Acceleration =0 Fnet= 10N -10N= 0 N 10N 10N
  11. The 3rd Law and the Slam Dunk The player exerted a downward force against the earth (court) The earth exerted a reciprocal, upward force upon the player The force exerted by the earth elevated the player to the rim!

  12. Newton’s Third Law

    Forces always occur in pairs. If object A exerts a force F on object B, then object B exerts an equal and opposite force –F on object A. "Every action has an equal and opposite reaction” Free body diagram (Force diagram)- visual representation of the forces acting upon an object Web Link: Skydiver
  13. Free-Body Diagram pg. 123 shows the size and direction of each force vector (agent) and the overall direction of the net force, Fnet Always draw weight force first, then normal force (⏊ to surface) Push and pull (applied force)- force acting upon object causing it to move Friction- force parallel to the surface and opposes motion (always points away from the direction of movement) Static (not moving) or kinetic (moving) What if object is on an incline? Fnet=
  14. Draw Free-Body Diagrams w/labeled forces Normal ↑ Normal ↑ ⟵applied friction⟶ ⟵applied friction⟶ Weight ↓ Weight ↓ A. Forces: normal, friction, applied, weight C. Forces: normal, friction, applied, weight E. Forces: weight, thrust, lift (normal), drag Normal ↑ tension⟶ D. Forces: weight Weight ↓ Weight ↓ B. Forces: Buoyant (normal), weight F. Forces: normal, tension, weight
  15. Ex: 5 N T T Tension (T) – force applied by a string or rope T = 5 N We say “the tension in the string is 5 N” The tension is the same throughout a string, even when it passes over pulleys:
  16. F3  F2 F1 Ex: The force table If it is in equilibrium(acceleration = 0), how are F1, F2, and F3 related?
  17. Ex: A person pulls a crate across the ice with a rope Find the horizontal acceleration of the crate. 15 N m=35 kg 30
  18. Newton’s Laws Visual FN Title10 pts, subtitles10 pts, visual10 pts, free-body diagram10 pts, explanation of visual using vocab10 pts Summarize (attach sheet to back of visual) Scales section on pg. 127 25 pts Common Misconceptions about Forces on pg. 125 25 pts Fg 1st Law 3rd Law 2nd Law Fn Ff Fp Fg 50 pts= NEWTON’S LAWS 50 pts=
  19. acceleration a mass m scale Ex: Weight in an elevator Mass and weight are not the same thing! Mass is an intrinsic property of an object Weight depends on the value of gravity Going up Draw a free body diagram of the person Apply Newton’s 2nd Law Solve for FN (the normal force) FN = the scale reading = apparent weight Repeat the whole process for Going down Web Link: Forces and an elevator
  20. Acceleration Due to Gravity The acceleration due to gravity, g, is considered a constant for objects near the earth’s surface : g = 9.8 m/s2 In the SI system: m = mass = kilograms mg = weight = Newtons

  21. Friction

  22. Friction (f) – a force that opposes motion between two surfaces in contact friction applied force Two types of friction: Static friction (fs) – keeps objects from moving Kinetic friction (fk) – opposes motion while moving μkinetic Both depend on: Surfaces in contact Amount of normal force (FN) Friction Force = coefficient of friction x Normal Force Ffriction= Fnormal
  23. Complete-Balanced Friction Forces pg. 131 2) Kinetic friction (v ): FN fk = kFN k = coefficient of kinetic friction (depends on surfaces) fk Fapp mg FN fs,max = sFN s = coefficient of static friction (depends on surfaces) Fapp fs mg Web Link: Friction Fs ›Fk Why?
  24. Ex: If the crate has a mass of 230 kg, and the static coefficient of friction between the crate and sidewalk is .62, what horizontal force must they exert to be able to start the box moving? F G U E S S m= μs= fs= fs= sFN Fg= g=
  25. Reaction Force From Ramp Normal Force Friction Force Decompose Vector Weight of block Weight of block Inclined Plane:“Normal” Forces and Frictional Forces “Normal” means perpendicular Friction Force = coefficient of friction x Normal Force Ffriction = Fnormal
  26. Review and Quiz! Brain Pop
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