1. Newton’s 2nd Law of Motion

2. Forces A push or pull The cause of an acceleration Cause of a change in an object’s state of motion Cause objects to speed up or slow down Cause a change of direction Unit of force: Newton (N)

3. Free Body Diagrams One object only Forces represented as arrows placed where they act Length of the arrows should represent the magnitude of the force F g down, F N perpendicular to surface, F f parallel to surface opposing motion

4. Finding F net We define forces pointing up or right as positive We define forces pointing left or down as negative To get F net : Add all parallel forces and subtract off antiparallel forces

5. Example A mover pushes a box with a 20 Newton force to the right. His friend sees him struggling so he helps push the box by applying a 25 Newton force in the same direction. What is the net force on the box?

6. Example Now a 3 rd mover, not being too bright, decides to help by pushing with a 14 Newton force in the opposite direction. What is the net force now?

7. Newton’s 2nd Law The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the mass of the object or: F net = ma

8. Types of Forces Contact forces: Tension - (F t or T) - The force that a “string” pulls on an object Normal Force - (F N or N) - Force that a surface applies to an object (the direction is  to the surface) Friction - (F f ) - To be defined later

9. F, applied force  A physical push or a pull

10. F N, Normal Force (N) A force that a surface applies to an object “Normal” means perpendicular The direction of the normal force is perpendicular to the surface surface

11. Tension - (F t or T)  Tension (force) in a string or rope  Strings only pull

12. Types of Forces Field Forces (Action at Distance) Force of Gravity (F g ) or Weight F g =Weight, - WEIGHT IS THE FORCE OF GRAVITY F g = mg Where g = 9.8 m/s 2 Electricity/Magnetism

13. Free Body Diagrams Used to analyze the forces affecting the motion of a single object Shows only the forces acting on an object

14. Free Body Diagrams One object only Forces represented as arrows starting at the center of the object Length of the arrows should represent the magnitude of the force F g down, F N perpendicular to surface, F f parallel to surface opposing motion

15. Examples

16. Force of Friction F f Always parallel to the surface Acts opposite to the direction of motion F f = μ F N Where μ is the coefficient of friction It is determined between the two surfaces in contact (it will vary depending upon the surface)

17. Common μ’s Materials μ Oak on oak, dry0.30 Steel on steel, dry0.41 greasy0.12 Steel on ice0.01 Rubber on asphalt, Dry1.07 wet0.95 Rubber on ice0.005