Friction

-the force that one surface exerts on another when the two rub against each other.

-acts in a direction opposite to the direction of motion-opposes motion

-without friction the object would continue to move at a constant speed forever

- strength depends on

          a.  types of surfaces involved

          b.  how hard the surfaces push together

-as the surface roughness increases, so does the friction

Uses- necessary to walk, drive a car, to prevent injuries

3 kinds of friction

a.  Sliding- two solid surfaces move against each other

b.  Rolling- object rolls over another-like skateboard

c.  Fluid- when objects move through a liquid or a gas

-air resistance-type of fluid friction- whirly birds fall slowly because air pushes up against their surface area. Greater surface area= greater air resistance. Greater velocity =greater air resistance.  Air resistance will eventually equal gravity, thus preventing acceleration but not falling. The final velocity the object gets to is called the terminal velocity.  

the most force is required to overcome sliding friction

Gravity

Freefall- when the only force acting on an object is gravity

-objects accelerate in freefall because gravity is an unbalanced force- like in the skydiver who jumped from the balloon high in the atmosphere where the air is very thin=less friction to balance out gravity

-acceleration due to gravity near the earth’s surface= 9.8 m/sec

for every sec it falls its accelerated 9.8 m

 Projectile motion-how an object that is thrown falls.

- all objects whether thrown or falling will land at the same time

The Law of Universal Gravitation states that the force of gravity acts between all objects in the universe.

-gravity exists between all objects in the universe no matter how big or small

depends on size of masses involved and distance between them

can be expressed as