F ORCES Friction Applied Normal Tension Spring Force Gravity Electricity Magnetism Contact ForcesNon-Contact Forces

G RAVITY Force that pulls objects toward each other Law of Universal Gravitation: The force of gravity acts between all objects in the universe that have mass. We are attracted toward Earth and Earth is attracted towards us. our mass is very small

F ACTORS AFFECTING G RAVITY Mass More mass – stronger the gravitational pull Weight is greater on Jupiter than on Earth Distance Greater distance – less force 10 20

F RICTION Friction is a force that opposes motion between two surfaces that are in contact. When two surfaces are in contact, the microscopic hills and valleys of one surface stick to the tiny hills and valleys of the other surface. This contact causes friction.

FRICTION Two factors that affect the force of friction are the types of surfaces involved and how hard they are pushed together. The rougher the surface – the greater the force Sandpaper vs. Ice The heavier the object – the greater the force of friction. The harder the surfaces are pushed together, the stronger the force of friction.

F RICTION – F RIEND OR F OE ? With your table partner, discuss these four questions and come up with a list of at least three answers per question. Be ready to share with the class. In what ways is friction helpful in the world? In what ways is friction harmful? What are some ways to reduce friction? What are some ways to increase friction?

F RIEND AND FOE Friction can be both harmful and helpful, so it may be necessary to decrease or increase friction.

F RICTION Rougher surface Sand, gravel on ice Increase the applied force Use force when sanding Lubricants Oil, wax, grease Rolling Ball bearings Smoother surface IncreasesDecreases