1. FRICTION Friction = a force that opposes the motion of objects that touch as they move past each other. ALL moving objects are affected by friction. Without friction…the world would be a very different place.

3. 4 Main Types of Friction

4. Static Friction Static friction = the friction force that acts on objects that are not moving. Always act in the direction OPPOSITE to the applied force. APPLIED FORCE STATIC FRICTION

5. Sliding Friction Sliding Friction = a force that opposes the direction of motion of an object as it slides over a surface. Sliding friction is less than static friction…less force is needed to keep an object moving that to start it moving.

6. Rolling Friction Rolling Friction = the friction force that acts on rolling objects. (100 to 1000 times less than sliding friction)

7. Ball bearings in these wheels greatly reduce friction by replacing sliding friction with rolling friction.

8. Fluid Friction Fluid Friction = the force that opposes motion of an object through a fluid. (water, air) Fluid friction INCREASES as speed of object INCREASES. Fluid friction on an object moving through the air is known as AIR RESISTANCE

9. Gravity Gravity is a force that acts between any two masses. Gravity is an attractive force…it pulls objects together. Gravity can act over large distances.

10. Earth’s Gravity Earth’s gravity acts downward toward the center of the Earth. Fortunately, an upward force usually balances the downward force of gravity.

11. Falling Objects Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.

12. A flying squirrel takes advantage of air resistance to slow its fall and increase the distance covered in its jump.

13. Terminal Velocity Terminal velocity = the constant velocity of a falling object when the force of air resistance equals the force of gravity. http://www.youtube.com/watch?v=p0IZsfzDS4s

14. Projectile motion Projectile motion = the motion of a falling object after it is given an initial forward velocity. The combination of an initial forward velocity and the downward force of gravity causes the ball to follow a curved path.

16. Review 1.) You push on a box and are unable to move it. What force opposes your push? a. static friction b. rolling friction c. sliding friction d. air resistance

17. Review 2.) Air resistance depends on a. the velocity of a moving object b. the weight of a moving object c. the mass of a moving object d. the inertia of a moving object

18. Review 3.) What force besides gravity acts on a projectile? a. weak nuclear b. electrical c. magnetic d. air resistance