1. An Introduction to Forces SPH4C

2. 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

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

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

5. 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 will neglect this) Air resistance (drag): friction on an object moving through air (many physics problems will neglect this)

6. 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.

7. 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.)

8. 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!

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

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

11. 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.

12. 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

13. 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]

14. More Practice Estimating and Measuring Forces