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Section 3.5 Forces in the Real World
December 14, 2010
Page 103
•Gravity is the pull of pieces of matter on each other.
•On Earth, gravity goes down
•Bigger objects pull harder than smaller objects
•Closer objects pull harder than farther objects
•Planets’ orbits are based on gravity
•Friction is a force that resists the motion of objects or surfaces.
•Many kinds of friction exist.
•Friction depends on both of the surfaces in contact.
•More weight means more friction
•Friction works against the direction of the object’s motion
•If something doesn’t move at all when pushed, that’s static friction
•Sliding friction is a force that resists the motion of an object moving across a surface.
•Rolling Friction is the kind of friction you get when you move on wheels.
•There is much less friction in rolling friction because you are changing contact points.
•Fluid friction is friction when moving on or through gases or liquids.
•Because liquid and gas molecules move away (while solids stay in place) there’s much
less friction
•Fluid friction is why you put motor oil in car engines (reduces friction damage.)
•Gases cause less friction than liquids (We call that air resistance)
•Fluid friction increases as velocity increases
•The greater the force squeezing two surfaces together, the greater the friction force.
•Unless a force is constantly applied, friction will slow all motion to a stop eventually.
•It is impossible to completely get rid of friction, but it can be reduced.
•Friction is also important to anyone driving a car.
•Grooved tire treads allow space for water to be channeled away from the road-tire
contact point, allowing for more friction in wet conditions.
•Shoes are designed to increase the friction between their soles and the ground.
•Tension is a force that acts in a rope, string, or other object that is pulled.
•Tension always acts along the direction of the rope. (Pulling force)
•When you apply a compression force to a spring, it collapses to become shorter.
•The force created by stretching or compressing a spring always acts to return the spring
to its natural length. (Squeezing Force)