1. Projectile Motion A projectile is an object on which the only force acting is ______ and the direction of this force is always _____. (In real life, a projectile will also have ______ or ___________ acting on it and the direction of this force is always _____________________.)

2. Projectile Motion A projectile is an object on which the only force acting is gravity and the direction of this force is always down. (In real life, a projectile will also have friction or air resistance acting on it and the direction of this force is always opposite the direction of motion.)

3. An Introduction to Forces: Student Learning Goal The student will analyse, in quantitative terms, the forces acting on an object, and use free-body diagrams to determine net force on the object in one dimension. (B2.9) The student will analyse, in quantitative terms, the forces acting on an object, and use free-body diagrams to determine net force on the object in one dimension. (B2.9)

4. An Introduction to Forces SPH4C

5. A Definition A force is a push or a pull.

6. A Definition A force is a push or a pull. It is a vector quantity and is symbolized by:

7. A Definition A force is a push or a pull. It is a vector quantity and is symbolized by:

8. A Definition A force is a push or a pull. It is a vector quantity and is symbolized by: In the SI system, force is measured in Newtons (N). 1 N = 1 kg m/s 2

9. Applied Force Applied Force is a general term for any contact force, e.g.

10. Applied Force Applied Force is a general term for any contact force, e.g. Tension Tension

11. Applied Force Applied Force is a general term for any contact force, e.g. Tension Tension Friction Friction

12. Applied Force Applied Force is a general term for any contact force, e.g. Tension Tension Friction Friction Normal Force Normal Force

13. Tension Tension is the force exerted by strings, ropes, cables, etc. attached to an object.

14. Tension The tension along the string is constant.

15. Friction Friction acts to oppose any (attempted) motion.

16. Friction Static friction: the force that prevents a stationary object from starting to move Static friction: the force that prevents a stationary object from starting to move

17. Friction Friction acts to oppose any (attempted) motion. Static friction: the force that prevents a stationary object from starting to move Static friction: the force that prevents a stationary object from starting to move Kinetic friction: the force that acts against an object’s motion Kinetic friction: the force that acts against an object’s motion

18. Friction Friction acts to oppose any (attempted) motion. Static friction: the force that prevents a stationary object from starting to move Static friction: the force that prevents a stationary object from starting to move Kinetic friction: the force that acts against an object’s motion Kinetic friction: the force that acts against an object’s motion Air resistance (drag): friction on an object moving through air Air resistance (drag): friction on an object moving through air

19. Friction Friction acts to oppose any (attempted) motion. Static friction: the force that prevents a stationary object from starting to move Static friction: the force that prevents a stationary object from starting to move Kinetic friction: the force that acts against an object’s motion Kinetic friction: the force that acts against an object’s motion Air resistance (drag): friction on an object moving through air (many physics problems with neglect this) Air resistance (drag): friction on an object moving through air (many physics problems with neglect this)

20. Normal Force The normal force acts to keep objects apart.

21. Normal Force The normal force acts to keep objects apart. i.e. if you push on a wall, the wall will exert a normal force on your hand that prevents your hand from pushing through the wall.

22. Action-at-a-Distance Forces There exist forces for which contact between objects is not necessary. These forces are called action-at-a- distance forces.

23. Action-at-a-Distance Forces There exist forces for which contact between objects is not necessary. These forces are called action-at-a- distance forces.

24. Action-at-a-Distance Forces There exist forces for which contact between objects is not necessary. These forces are called action-at-a- distance forces. One example is gravitational force, the force of attraction between all objects with mass.

25. Action-at-a-Distance Forces There exist forces for which contact between objects is not necessary. These forces are called action-at-a- distance forces. One example is gravitational force, the force of attraction between all objects with mass. (The gravitational force the Earth exerts on an object is called its weight.)

26. Free-Body Diagrams Usually an object will have more than one force acting upon it.

27. Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object

28. Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object – and only the forces acting on the object.

29. Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object – and only the forces acting on the object. A representation of the object is drawn in the centre of the diagram

30. Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object – and only the forces acting on the object. A representation of the object is drawn in the centre of the diagram and the forces acting on it are drawn as arrows pointing outwards.

31. Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object – and only the forces acting on the object. A representation of the object is drawn in the centre of the diagram and the forces acting on it are drawn as arrows pointing outwards. The arrows must be labelled!

32. FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball.

33. FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball.

34. FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball. FgFg

35. FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball. FgFg F air

36. FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book.

37. FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book.

38. FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FAFA

39. FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FAFA FfFf

40. FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FgFg FAFA FfFf

41. FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FgFg FNFN FAFA FfFf

42. Net Force FBDs are drawn to help determine the net force (the sum of all forces) acting on an object. Resolve all vertical and horizontal vectors and determine the resultant.

43. FBD: Example 2 with Numbers A book is being pushed across a table with a force of 5 N [right]. The force of friction is 2 N [left], the gravitational force is 10 N [down], and the normal force is 10 N [up]. Find the net force on the book.

44. FBD: Example 2 with Numbers A book is being pushed across a table with a force of 5 N [right]. The force of friction is 2 N [left], the gravitational force is 10 N [down], and the normal force is 10 N [up]. Find the net force on the book. F g = 10 N F A = 5 NF f = 2 N F N = 10 N

45. FBD: Example 2 with Numbers A book is being pushed across a table with a force of 5 N [right]. The force of friction is 2 N [left], the gravitational force is 10 N [down], and the normal force is 10 N [up]. Find the net force on the book. Horizontal forces: F A + F f = 5 N + (- 2 N) = 3 N Vertical forces: F g + F N = (- 10 N) + (10 N) = 0 The net force F net = 3 N [right]

46. More Practice Estimating and Measuring Forces