Name: ____________________________________
Date: _____________________________________
Flynt - ___ Period
___th Grade Science
There are several kinds of contact forces:
1. Friction
2. Tension
3. Compression
4. Buoyancy
5. Normal Force
FRICTION:

Definition/Key Points:
1. Friction is a contact force that occurs whenever two things rub against each
other.
2. Friction is a force that resists or opposes the motion of two objects that are
touching each other.
 The vector direction for friction will ALWAYS be in the direction opposite
to the motion of the object.
3. Friction ALWAYS transfers some energy to the surrounding environment through
the generation of heat, static electricity, and sound!
 This is why no mechanical movement is completely perpetual; as long as
there is friction, some energy will always be transferred (“lost”) to the
environment.

What causes friction:
1. An object might look smooth, but under a microscope, all surfaces would appear
very rough and jagged.
2. When rough surfaces grind against each other, it causes friction. The rougher the
surface, the greater the friction.

Three kinds of Friction:
1. Kinetic Friction
2. Static Friction
3. Rolling Friction

Benefits of friction: Without it, we wouldn't be able to walk, sit in a chair, climb stairs, or
use a mouse to surf the web. Everything would just keep slipping and falling all over the
place.
KINETIC/DYNAMIC FRICTION
 Kinetic (or dynamic) friction occurs when two objects are moving relative to each other
and rub together (like a sled on the ground).
 Types of Kinetic Friction:
1. Sliding Friction – when two objects are sliding past each other.
 Examples: sliding on a tile floor in your socks, ice hockey, sledding, skiing.
Summary
Name: ____________________________________
Date: _____________________________________
Flynt - ___ Period
___th Grade Science
2. Fluid Friction – the friction between a solid object as it moves through a liquid or
a gas.
 Fluid friction is also called drag, air resistance, or air friction.
 Examples of Fluid Friction: The drag of air on an airplane or the drag of
water on a swimmer.
 Since friction is exerted in a direction that opposes movement, kinetic friction usually
does negative work, typically slowing something down. There are exceptions, however, if
the surface itself is under acceleration. One can see this by placing a heavy box on a rug,
then pulling on the rug quickly. In this case, the box slides backwards relative to the rug,
but moves forward relative to the floor. Thus, the kinetic friction between the box and
rug accelerates the box in the same direction that the box moves, doing positive work.
STATIC FRICTION
 Static friction is the “sticking” force between two objects that are not moving relative to
each other.
 Example: sitting in your chair and not sliding off; an object resting on a slope that is NOT
sliding down the sloped surface; standing.
 The initial force needed to get an object moving is often dominated by static friction.
 Example: Sometimes an air rocket car will not roll unless you gave it a slight push. Once
you give it that extra push to overcome static friction, the car rolls along just fine without
any other intervention.
 Static or Kinetic??? Static friction is the force that prevents a car wheel from slipping as it
rolls on the ground. Even though the wheel is in motion, the patch of the tire in contact
with the ground is stationary relative to the ground. Thus, some scientists consider
rolling friction to actually be static rather than kinetic friction!!!
ROLLING FRICTION
 Rolling friction is the frictional force associated with the rotational movement of a disc or
other circular objects along a surface.
 Rolling friction is sometimes considered to be both kinetic friction and static friction.
o Why is it kinetic? Because two objects are moving relative to one another.
o Why is it static? Because the actual part of the tire in contact with the ground is
stationary relative to the ground (not sliding with respect to the ground)!
 Rolling friction produces less friction than most other kinds of friction.
 Rolling friction generates heat and sound as by-products.
Summary