OBJECTIVES Sec II Describe forces & explain how forces act on objects Determine the net force when more than one force is acting on an object Compare balanced and unbalanced forces Describe ways that unbalanced forces cause changes in motion

II. What is a force? A. Force= a push or a pull Has both size and direction Can change the acceleration (speed or direction)of an object newton (N) = units force is measured in Remember: 1 N = 100g

B. Forces Acting on Objects All forces act on an object You cannot push or pull on nothing Applying a force does NOT mean that there has to be motion Pushing against the wall Sitting in a chair Forces can be unseen Static electricity Gravity Magnetism

C. Determining Net Force Net force= combination of all forces acting on an object Forces in the same direction will be added together to figure out the net force Forces in the opposite direction will be subtracted from each other to get the net force

Balanced forces = zero When the forces are equal in opposite directions There is no movement Sitting on a chair

Unbalanced forces produce a change in motion Change in motion could be speed or direction Fnet does NOT equal 0 N Cause an unmoving object to move

Surface of any object is rough (even if it feels smooth) III. Friction: A force that opposes motion between 2 surfaces that are in contact https://youtu.be/miB7HzUvmM0 A. The source of friction Surface of any object is rough (even if it feels smooth) When 2 surfaces come into contact with each other they will “stick together” Microscopic hills and crevices “stick & slip” 2 factors that effect the amount of friction: The force pushing the surfaces together the more weight the more force pushing down the more friction How rough the surface is rougher equals more friction because of more “hills & valleys” to cause the “stick & slip”

B. Types of Friction: https://youtu.be/VUfqjSeeZng Kinetic friction: friction between moving surfaces Sliding friction is usually greater than rolling friction (depending on the surface) Breaks on a car or bike Writing with a pencil Rolling friction is used in transportation

Static means= not moving Static friction: force applied to an object but does not cause the object to move Static means= not moving Force of the static friction balances the applied force Leaning an object against the wall and it doesn’t fall Static friction can be overcome with a large enough force As soon as movement occurs static friction disappears & kinetic friction immediately occurs

https://youtu. be/AX_lCOjLCTo (phone book friction) https://youtu https://youtu.be/AX_lCOjLCTo (phone book friction) https://youtu.be/QMW_uYWwHWQ C Friction: Helpful or Harmful??? Without friction it would be hard to walk, drive a car, or write on paper Friction wears down engine parts, can cause “rug” burns, & leads to erosion of land

To reduce friction lubricants are used Substances that are applied to surfaces to reduce friction Motor oil in an engine Air on an air hockey table Sanding wood

Making the surface rough also increases friction To increase friction you could increase the force used to push against something Making the surface rough also increases friction Rosin Batting gloves Sand/salt on icy roads