 A force is a push or pull on an object.  Forces have both size and direction.  Forces can be balanced or unbalanced.

 Based on your prior knowledge, what is friction?

 Friction is a force that RESISTS motion. (works against it)  Occurs when two objects are in contact with one another.

 What two objects are experiencing friction in these examples? › A person walking › A car driving on the street › A boat sailing on a lake › An airplane flying in the air

 Which force represents friction in a car driving down the street? 525 N 1025 N

 Change the surface of the objects › Rougher surface=Greater Friction  Increase the force keeping the objects in contact › Ex. Rubbing your hands together

 Change the surface of the objects › Smoother surface=Less Friction  Decrease the force keeping the objects in contact › Ex. Rubbing your hands together  Use a lubricant (ex. Oil, grease, water)

 Why does a surface covered in ice have less friction than a surface not covered in ice?  Think of some activities that would change if there was no friction.

 Based on your prior knowledge and observations, what is gravity?

 Gravity is a force that attracts objects to each other.  Every object has its own force of gravity.  Gravity PULLS objects towards the center of the object.

 The strength of gravity depends on the objects MASS. › More MASS=More GRAVITY › Most objects are too small to exert a detectable amount of gravity  Gravity increases as you near the center of the object.  What objects gravity do we experience the most?

 The Less MASS an object has the Less GRAVITY it exerts.  The farther away from the objects center, the less gravity exerted by that object.

 Why do astronauts experience less gravity on the moon?  Denver is on average a mile above sea level while New Orleans is on average 100ft below sea level. At which city will you experience more gravity from the Earth?