1. Forces and Friction Week of October 15th

2. What is a force?  A push or a pull exerted on an object in order to change the motion of the object; force has size and direction.

3. Changes  Forces cause changes in motion.  A force can change acceleration; can be a change in the speed or direction of an object.

4. Newton  The SI unit for force  Symbol, N

5. Forces Acting on Objects  All forces act on objects.  Something must receive the push or pull.  Just because a force acts on an object doesn’t mean that motion will occur.

6. Net Force  The combination of all of the forces acting on an object.

7. Same direction  Forces in the same direction are added together to determine the net force.

8. Different direction  Forces in opposite directions are subtracted to determine the net force.

9. Balanced forces  Forces are balanced when the forces on an object produce a net force of 0 N.  Balanced forces do not cause a change in motion.

10. Examples of Balanced Forces  House of cards  Chair you’re sitting on  Table you lean on

11. Unbalanced forces  Forces are unbalanced when the net force on an object is not 0 N.  Unbalanced forces produce a change in motion.

12. Examples of unbalanced forces  Action in sports  Pushing your chair in  Sliding your books across a table.

13. Friction

14.  A force that opposes motion between two surfaces that are in contact.

15. Source of Friction  All surfaces have hills and valleys.  When two surfaces are in contact, the hills and valleys of one surface sticks to the hills and valleys of another.

16. Amount of Friction  Amount of friction depends on many factors.  Two factors are the force pushing the surfaces together and the roughness of the surfaces.

17. Amount of friction  More force exerted causes more friction.  The rougher the surface is, the greater the friction is.

18. Types of Friction  Kinetic Friction  Static Friction

19. Kinetic Friction  The friction between moving surfaces.  The amount of kinetic friction depends on how the surfaces move.

20. Examples of Kinetic Friction  Brakes on a bicycle  Writing  Wheels

21. Static Friction  Static friction occurs when a force is applied to on object but does not cause the object to move.

22. Static vs. Kinetic Friction  Static friction disappears as soon as an object starts moving and then kinetic friction immediately occurs.

23. Friction  Harmful and Helpful  Increase and decrease friction