Chapter 12

Section 1

Forces

It was named after Sir Isaac Newton (the guy under the apple tree) – who explained how force, mass, and acceleration are related.

If the object is not moving, it will continue to not move.If the object IS moving, it will continue to move at the same speed and in

the same direction.

This acceleration can be a change in speed, in direction, or both.

Without friction, everything would be slippery. (even more slippery than a sheet of ice)

Note that “touching” each other includes solid objects that are directly touching one another as well as objects moving through a liquid or gas.

There are 4 main types of friction:

static friction, sliding friction, rolling friction, and fluid friction

* Always opposes your push

* Because sliding friction is less than static friction, less force is needed to keep an object moving than to start it moving

NOTE: rolling friction does not actually exist – this is one of those things textbook authors

make up in a misguided attempt to help you.

* Rolling friction is much less than static and sliding friction – which is why wheels are so useful for moving heavy objects.

* Air resistance is fluid friction acting on an object moving through the air. (So the faster you drive, the more air resistance there is.)

* Fluid friction increases as the speed of the object moving through it increases. (So the

faster you stir, the greater the friction is.)

We can decrease our terminal velocity by increasing the (upward force of) air resistance by increasing our surface area.• This is what a skydiver’s parachute does.

• Cats and flying squirrels spread all four legs out to increase their surface area – which increases their air resistance – to decrease their terminal velocity.

(air resistance and gravity)