Forces, Free Body Diagrams, and Newton’s Laws of Motion

1. Forces, Free Body Diagrams,and Newton’s Laws of Motion (Oh my!)

2. Forces • Force is: • a push or pull upon an object resulting from the object's interaction with another object. • any action that has the ability to change an object’s motion. • a vector quantity. • measured in Newtons. ***Remember, you only have to write the underlined portion in your notes.

3. Forces All forces (interactions) between objects can be placed into two broad categories: • Contact forces • Forces resulting from “action-at-a-distance”

4. Forces • Contact forces are those types of forces that result when the two interacting objects are perceived to be physically contacting each other. • Action-at-a-distance forcesare those types of forces that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation.

5. Contact Forces Frictional Force Tension Force Normal Force Air Resistance Force Applied Force Spring Force Action-at-a-Distance Forces Gravitational Force Electrical Force Magnetic Force Examples of Forces

6. Important Forces to Remember… • Normal force is the supporting force and is always perpendicular to the supporting surface. • Friction is the force exerted by a surface as an object moves across it or makes an effort to move across it.

7. We draw them!!!! #physics

8. Free Body Diagrams This book is being pushed across the top of a table. Here is a free body diagram of the scenario shown to the left.

9. Free Body Diagrams • Free Body Diagrams (FBDs) are tools used to analyze physical situations • Free Body Diagrams show all the forces acting on a single object • The net force is the sum of all forces acting on an object

10. Free Body Diagrams • The object itself may be drawn as a circle or box • Draw and label all the external forces on the object • The only “rule” for drawing free-body diagrams is to depict all the forces that exist for that object in the given situation.

11. Free Body Diagrams • A circus elephant falls off a tight rope. Neglecting air resistance, draw a free body diagram for the falling elephant.

12. Free Body Diagrams • Remember that we ignored air resistance. Fgrav

13. Free Body Diagrams • A circus elephant falls off a tight rope. Including air resistance, draw a free body diagram for the falling elephant.

14. Free Body Diagrams Fair • This time we included air resistance. Fgrav

15. Free Body Diagrams • Draw an FBD of the scenario below. Assume the dog is moving right. Include air resistance.

16. Free Body Diagrams Fair

17. Free Body Diagrams • Free Body Diagrams are useful to quickly see if there are unbalanced forces on an object. • If an object is in equilibrium, the net force is zero. *An object can still be in motion while in equilibrium • If the forces are unbalanced, then acceleration occurs.

18. Free Body Diagrams Math Time!!!

19. Free Body Diagrams Let’s calculate the magnitude and directions of this FBD! Assign positive and negative directions in both the horizontal and the vertical. Vertically Up is + Down is – Horizontally Right is + Left is –

20. Free Body Diagrams Let’s calculate the magnitude and directions of this FBD! Yay!

21. Free Body Diagrams Let’s calculate the magnitude and directions of this FBD! Woohoo!

23. Free Body Diagrams Suppose three people are trying to keep an injured polar bear in one place. Each person has a long rope attached to the bear. Two people pull on the bear with forces of 100 N each . What force must the third person apply to balance the other two? The bear will not move if the net force is zero.

25. Newton’s Laws of Motion Isaac Newton- • developed the laws of motion • discovered that gravity is universal

26. And now for the first one… • Suppose you want to move a box from one side of the room to the other. What would you do?

27. Newton’s 1st LOM

28. Newton’s 1st LOM • You would pushor pull it across theroom. • In physics terms, you would apply a force to the box.

29. Newton’s 1st LOM

30. Newton’s 1st LOM Forces have the ability to : • Increase the speed of an object • Decrease the speed of an object • Change the direction in which an object is moving

31. Newton’s 1st LOM • Anytime there is a change in motion, a force mustexist, even if you cannot immediately recognize the force.

32. Newton’s 1st LOM • Newton’s first law states that an object will continue indefinitely in its current state of motion, speed, and direction, unless acted upon by a net force.

33. Newton’s 1st LOM • Generally speaking, objects tend to keep doing what they are doing; if they are moving they keep moving, if they are at rest, they stay at rest. Demos - https://www.youtube.com/watch?v=T1ux9D7-O38

34. Newton’s 1st LOM • Inertia is a term used to measure the ability of an object to resist a change in its state of motion.

35. Newton’s 1st LOM • Because inertia is a key idea in Newton’s first law, the first law is sometimes referred to as the law of inertia.

36. Newton’s 1st LOM • The amount of inertia an object has depends on its mass. • More mass=more inertia Less mass=less inertia

37. Newton’s 1st LOM • An object with a lot of inertia takes a lot of force to start or stop; an object with a small amount of inertia requires a small amount of force to start or stop.

38. Newton’s 1st LOM • Which one would you rather have to push to the gas station? Why?

39. Newton’s 2nd LOM Newton’s 2nd law states: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

40. Newton’s 2nd LOM • If you apply more force to an object, it accelerates at a higher rate. • If the same force is applied to an object with greater mass, the object accelerates at a lower rate because mass adds inertia.

41. Newton’s 2nd LOM Math Time!!!

42. Newton’s 2nd LOM F = ma (Force = mass x acceleration) Force is measured in Newtons (N) Mass is measured in Kg Acceleration is measured in m/s2 ***Remember that acceleration is speeding up, slowing down, or turning

43. Newton’s 2nd LOM • One Newton of force is needed to give a mass of one kilogram an acceleration of one meter per second squared 1 N = 1 kg * m/s2 • So, if you see units of kg*m/s2, you know that they are the equivalent of Newtons (N)

44. Force = mass x accelerationFind the missing variable

45. Newton’s Second Law of Motion Force is equal to ________ x ___________. Acceleration is directly proportional to ________. Acceleration is inversely proportional to_______.

46. And now, the 3rd one… • Newton’s 3rd law: For every action, there is an equal and opposite reaction

47. Newton’s 3rd LOM • Forces always occur in action-reaction pairs • One force is called the action force. The other force is called the reaction force

48. Newton’s 3rd LOM • The action and reaction forces are equal in strength and opposite in direction • Forces always come in pairs

49. Newton’s Third Law of Motion When you ride a skateboard, strictly speaking, what is the force that moves the skateboard?

50. Newton’s Third Law of Motion All forces come in pairs. When you push on the ground (action), the ground pushes back (reaction) on your foot.