An Introduction to Forces: Student Learning Goal

1. 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)

2. An Introduction to Forces SPH4C

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

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

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

6. 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/s2

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

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

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

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

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

12. Tension Tension is the force exerted by strings, ropes, cables, etc. attached to an object. The tension along the string is constant.

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

14. Friction Friction acts to oppose any (attempted) motion. • Static friction: the force that prevents a stationary object from starting to move

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

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

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

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

19. Normal Force The normal force is a force exerted by a surface (or another object) on any other object at 90° (perpendicular to the surface) e.g. if you push on a wall, the wall will exert a normal force on your hand e.g. a table exerts a normal force on a textbook

20. 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.

21. 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.

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. One example is gravitational force, the force of attraction between all objects with mass.

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. Example 1: Gravitational force - the force of attraction between all objects with mass. (The gravitational force the Earth exerts on an object is called its weight.)

24. Action-at-a-Distance Forces • Example 2: Electrostatic force – the attractive or repulsive force between charged objects (+ vs. +; + vs. -)

25. Action-at-a-Distance Forces • Example 3: Magnetic forces – the force between objects with magnetic poles (i.e. two magnets repel with N-poles facing each other; attract when N-pole facing the S-pole)

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. Fg

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

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. FA

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

40. FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. Ff FA Fg

41. FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FN Ff FA Fg

42. An Introduction to Forces: More Practice Draw a free-body diagram for each of the following objects: (a) A car engine being lifted from a car by a rope attached to a pulley

43. An Introduction to Forces: More Practice Draw a free-body diagram for each of the following objects: (b) an car moving with constant velocity on a level road

44. An Introduction to Forces: More Practice Draw a free-body diagram for each of the following objects: (c) an apple hanging from a tree branch

45. An Introduction to Forces: More Practice Draw a free-body diagram for each of the following objects: (d) a skydiver being slowed by a parachute

46. An Introduction to Forces: More Practice Mr. Wong is pulling across level snow a sled on which is sitting his daughter. Tied to the back of his daughter’s sled is another tiny sled on which his daughter’s baby doll is sitting. Draw the free-body diagrams for (a) Mr. Wong

47. An Introduction to Forces: More Practice Draw the free-body diagrams for (b) his daughter

48. An Introduction to Forces: More Practice Draw the free-body diagrams for (d) his daughter’s sled

49. An Introduction to Forces: Answers to More Practice Draw the free-body diagrams for (d) the doll’s sled

50. 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.