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Newton’s Laws

Newton’s Laws. Inertia. Force  push or a pull Contact Force  physical contact Field Forces  No contact: gravity, magnetic force, etc. Inertia. Newton’s 1 st Law An object wants to keep on doing what it is already doing

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Newton’s Laws

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  1. Newton’s Laws

  2. Inertia

  3. Force push or a pull • Contact Force  physical contact • Field Forces  No contact: gravity, magnetic force, etc.

  4. Inertia • Newton’s 1st Law • An object wants to keep on doing what it is already doing • In order to change, it needs a net force not equal to zero • = Sum of all forces

  5. Mass is not Weight • Weightis a common force • Weight is the force exerted by gravityon an objects mass • Weight can change if gravity changes

  6. Mass: 70 kg Weight: 700 Newtons (157 pounds) Mass: 70 kg Weight: 112 Newtons (25 pounds) Mass is Same!!

  7. Free Body Diagrams Identify all forces acting upon an object Show direction and relative size of forces Vectors originate from the center of mass Scenario 1: 10 kg block falling at terminal velocity a 10 kg

  8. Free Body Diagrams Scenario 2: 10 kg block pulled across a frictionless floor by a string Y Does it fall through the floor? Just like projectiles, we treat x and yseparately 10 kg Normal Force • perpendicular () to the surface • equal to the force acting on the opposite side of surface

  9. Free Body Diagrams Scenario 2: 10 kg block pulled across a frictionless floor by a string X 3 Forces 10 kg net force only in x direction

  10. Free Body Diagrams A 1000kg car moving right using its brakes to slow down on a level freeway. Consider friction. Scenario 3: Y Does it fall through the floor? Just like projectiles, we treat x and yseparately 10 kg

  11. Free Body Diagrams A 1000kg car moving right, using its brakes to slow down on a level freeway. Consider friction. Scenario 3: X 1000 kg

  12. What’s up with f vector?! f does not actually act on the COM, but at the surface. X 1000 kg • Note: f is a resistiveforce. • it always acts against the motion

  13. Free Body Diagrams A 1000kg car moving right, using its brakes to slow down on a level freeway. Consider friction. Scenario 3: X 4 Forces 1000 kg net force only in x direction

  14. Free Body Diagrams Fnet= 0 because terminal velocity is maximum speed, thus there is no acceleration Scenario 1: 10 kg block falling at terminal velocity 10 kg

  15. Free Body Diagrams Scenario 2: 10 kg block pulled across a frictionless floor by a string Y Just like projectiles, we treat x and yseparately Does it fall through the floor? 10 kg

  16. Free Body Diagrams A 1000kg car moving right, using its brakes to slow down on a level freeway. Consider friction. Scenario 3: X 4 Forces 1000 kg net force only in x direction

  17. Free Body Diagrams Scenario 4: 10 kg block on a frictionless ramp at 30° 10 kg

  18. Free Body Diagrams Scenario 4: 10 kg block on a frictionless ramp at 30° 10 kg There are 2 forces

  19. Free Body Diagrams Scenario 4: 10 kg block on a frictionless ramp at 30° Choose x and y to line up with movement y x 10 kg There are 2 forces

  20. Free Body Diagrams y x Scenario 4: 10 kg block on a frictionless ramp at 30° Choose x and y to line up with movement 10 kg There are 2 forces Weight has both an x and y component

  21. Free Body Diagrams y x The angle between the weight and Fy is the same as the angle between the ramp and the ground

  22. Free Body Diagrams Scenario 4: 10 kg block on a frictionless ramp at 30° The y forces cancel out (block does not accelerate in the y direction) 10 kg Only force left is: thexcomponent of the weight

  23. Air Resistance • Twothings determine amount of air resistance • surface area (shape) • speed What happens when the force of air resistance is equal and opposite to the force of gravity?

  24. Fapplied • Direction of push • Fair • Opposite to motion • f • down • Weight / gravity • Normal • 90° to surface • Tension • Fapplied • Direction of push

  25. Review Test & HW Check & Correct your Free Body Diagrams HW Answer sheet in front • take a photo if you need to

  26. Practice! Go to pg. 310

  27. Newton’s 2nd Law: Acceleration • Recap of 1st law: • involves objects with no net force • When you do have a net force: • object will accelerate in same direction • This acceleration is proportional to the net force and inversely proportional to its mass

  28. Exploring 2nd Law • Put the two together: • mass units: kg • acceleration units: m/s2 • This makes force units: kg•m/s2 • Let’s call 1 kg•m/s2a Newton (N) 

  29. Lab Go to pg. 359

  30. 2nd Law Lab Prep To be able to complete the lab you need to be able to find acceleration in the situation below:

  31. 2nd Law Lab Prep 1. Draw a Free Body Diagram of the moving “body” (in this case: both masses connected by string) m1 m2

  32. 2nd Law Lab Prep 1. Draw a Free Body Diagram of the moving “body” (in this case: both masses connected by string) m1 m2 m2g Draw in all forces acting on the body Which forces affect the motion?

  33. 2nd Law Lab Prep m1 m2 m2g Use Newton’s 2ndLaw

  34. 2nd Law Hanging mass pulling a block on a frictionless tabletop • How does the acceleration of m2 change by connecting it to m1? • How does the acceleration of m1change by connecting it to m2? • Why? • Accelerationdecreases because m2 is pulling more mass • Moreinertia

  35. Start Lab Go to pg. 316-319

  36. Assembly Procedure • Screw in pulley • Screw in photogate and plug it in • Make sure the photogate “sees” the pulley – when the pulley spins the red light should blink

  37. Assembly Procedure • Put your cart on the rail • If the cart moves forward or backwards level the rail by turning the screws until the cart stays still • Record the mass of your cart and block • Record the mass of your mass hanger and masses • Plug sensors in & open the correct file

  38. Critical Points • Catch the cart before it hits the pulley or the ground • Accuracy is important! Make sure the % difference between your lowest and highest times is less than 1%

  39. Corrections to text #3 pg 317 • Tap mode and select Pulley (10 spoke) enter data collection4after 50 events #3 pg 322 • How much time would it take for the block to fall 30 cm if the 125 gram…

  40. Freefalling: 2nd Law • Weightis a very common force • In freefalling the only force is weight • In freefall, the acceleration of gravity is -9.8 m/s2 • But most objects DO NOT accelerate -9.8 m/s2 ! • Air resistancemakes objects accelerate slower

  41. Free Body Diagrams • When FAir resistance = Fw the object’s a =zero 10 kg block in freefall traveling at terminal velocity Scenario 1: 10 kg

  42. 2nd Law • This is because of net force • Pay close attention to the “net” • This is a reminder you need to add all the forces present upon that object • Let’s look at an example…

  43. 2nd Law: Example The Batman, with a mass of 70-kg, rappels down a rope from hisbat-copter with a downward acceleration of 3.5 m/s2. What vertical force does the rope exert on Batman?

  44. 2nd Law: Example The Batman, with a mass of 70-kg, rappels down a rope from his bat-copter with a downward acceleration of 3.5 m/s2. What vertical force does the rope exert on Batman? Start all force questions with a diagram showing the forces Pick the positive direction (make direction of motion positive) Identify all givens with symbols

  45. 2nd Law: Example The Batman, with a mass of 70-kg, rappels down a rope from his bat-copter with a downward acceleration of 3.5 m/s2. What vertical force does the rope exert on Batman? Given: Want:

  46. 2nd Law: Example Calculations:

  47. Time to practice Start your homework Turn to pg. 405

  48. Atwood Machine Video http://www.youtube.com/watch?v=i2bGTC27OJU

  49. Newton’s 3rd Law • Equal and Opposite • For every force that one object exerts on a second object, the second object exerts an equal (in size) and opposite (in direction) reaction force on the first object.

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