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Newton’s Laws

Newton’s Laws. Force  push or a pull Contact Force  physical contact Field Forces  No contact: gravity, magnetic force, etc. Inertia. 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. Force push or a pull • Contact Force  physical contact • Field Forces  No contact: gravity, magnetic force, etc.

  3. Inertia

  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. Newton’s 2nd Law: Acceleration • When you do have a net force… • object will accelerate in direction of the net force • This acceleration is proportional to the net force and inversely proportional to its mass

  6. Exploring 2nd Law • Put the two together: • mass units: kg • acceleration units: m/s2 • This makes force units: • 1 kg•m/s2 is called a Newton (N)

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

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

  9. Mass vs. Weight You can convert mass into weight using Newton’s Second Law On Earth, a = 9.8m/s2

  10. Example 1: An object has a mass of 10.0 kg, find its weight

  11. Example 2: A 2500 kg car is pushed with a 250 N force, what is the acceleration acting on the car due to the force? If the force is doubled, what will happen to the acceleration?

  12. Example 3: An artillery shell has a mass of 55 kg. The projectile is fired from the piece and has a velocity of 770 m/s when it leaves the barrel. The gun barrel is 1.5 m long. Assuming the force and therefore the acceleration is constant while the projectile is in the barrel, what is the force that acted on the projectile?

  13. Equilibrium vs. Static Equilibrium • If an object is in equilibrium it isnot acceleration (no change in motion) • If an object is in Static equilibrium it isat rest (not moving) • An object is in equilibrium if the sum of the forces acting on the object is zero

  14. Free Body Diagrams Identify all forces acting upon an object Show direction and relative size of forces Vectors originate from the center of mass

  15. Which boxes are in equilibrium? Which boxes are definitely in static equilibrium? Only # 3 NONE! b/c it’s possible that they are all moving 1 2 3

  16. Free Body Diagrams Scenario 1: 10 kg block going upwards 10 kg

  17. 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

  18. 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

  19. 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

  20. 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

  21. What’s up with friction?! 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

  22. 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

  23. Free Body Diagrams 10 kg block sliding up a ramp at 30° with friction Scenario 4: 10 kg

  24. Free Body Diagrams 10 kg block sliding up a ramp at 30° with friction Scenario 4: There are 3 forces 10 kg

  25. Free Body Diagrams 10 kg block sliding up a ramp at 30° with friction y x Scenario 4: y There are 3 forces x 10 kg Choose x and y to line up with the direction of motion Weight has both an x and y component

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

  27. Free Body Diagrams Scenario 4: 10 kg block on a frictionless ramp at 30° The y forces cancel (block does not accelerate in the y direction) 10 kg the only Net Force is:

  28. Practice! Drawing a FBD for # 5 from HW 3

  29. HW 3: #5 • A 34.5 kg block rests on the ramp as shown in the drawing. What is the tension in the line that is connected to the top of ramp?

  30. Language Objective Students will be able to explain to each other what force causes a ball to accelerate down a hill

  31. F1,… F2 • Direction of push • Fair • Opposite to motion • fk • fs • Opposite to motion • Weight / gravity • Straight down • Normal • 90° to surface • Tension

  32. Practice! Complete the Free Body Exercises in your HW packet

  33. 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?

  34. 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

  35. 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:

  36. 2nd Law: Example Calculations:

  37. Practice! • Try # 7 from HW 1

  38. Air Resistance • Most falling objects don’t accelerate at 9.8 m/s2! • Because air resistancemakes objects accelerate less 9.8 m/s2! • Twothings determine the magnitudeof air resistance • surface area (shape) • speed

  39. Terminal Velocity • Terminal velocity is the fastest an object can fall due to air resistance! • When FAir resistance = W an object is falling at terminal velocity and not accelerating

  40. Terminal velocity • When FAir resistance = W the object’s a =zero 10 kg block in freefall traveling at terminal velocity Scenario 1: 10 kg

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

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

  43. 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.

  44. Newton’s 3rd Law Subtleties • Forces only come in pairs (you cannot create a single force) • Does action cancel out reaction? • NO! Each force acts on a different object. • Each force has different effect on their object!

  45. Action Fore: A bat applies a force to a baseball. Action force effect: Reaction force: Reaction force effect: • The baseball accelerations a LOT • The baseball applies a force to a bat • The baseball bat (de)accelerates a little

  46. When I am zooming down the highway I crush small bugs that hit my car Which exerts the larger force? a. the bug on my car b. My car on the bug • Neither! The forces are equal (but opposite)

  47. How does a rocket work in space if there is no air to push it? • Lets watch WALL-E • What happens when the pod reaches “cruising speed”? • How does WALL-E move in space when there is no air or ground for him to push against?

  48. Time to practice Check Yourself #11-15

  49. Answers 11. A 12. B 13. B 14. B 15. A

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