youtube/watch?v=45yabrnryXk&feature=fvw

3. http://www.youtube.com/watch?v=45yabrnryXk&feature=fvw

9. Frictionis a force that resists motion. It involves objects that are in contact with each other.

10. Cause of friction: Some scientists believe that friction is caused mainly by the uneven surfaces of the touching objects. As the surfaces are rubbed together, they tend to interlock and thus offer resistance to being moved over each other. It has been shown that tiny particles are actually torn from one surface and become imbedded in the other. Cause of friction

11. The only factors affecting friction is the amount of mass and the nature of the surface. The surface area and the speed the block is pulled should not matter. Factors affecting Friction • Friction acts parallel to the surfaces that are in contact and in the direction opposite to the motion of the object or to the net force tending to produce such motion. • 2. Friction depends on the nature of the materials in contact and the smoothness of their surfaces. The friction between two pieces of wood is different from the friction between wood and metal. • 3. Sliding friction is less than or equal to starting friction. Starting friction prevents motion until the surfaces begin to slide. When the object begins to slide, less force is required to keep it sliding than was to start it sliding. • 4. Friction is practically independent of the area of contact. The force needed to slide a block along a table is almost the same whether the block lies on its side or on its end. The surfaces are in contact in more places when the area of contact is large, but the pressure is greater when the area is small.

12. http://www.fearofphysics.com/Friction/friction.html

13. "Drag" (sometimes called air resistance or fluid resistance) refers to forces that oppose the motion of a solid object through a fluid