Describe how forces act upon objects and
create motion
 Describe how friction influences the
motion
 Explain the different types of friction
 Explain why friction is important

Is an _____ exerted on an object in order
to change the object’s ________ or
______
 A push or pull in other words
 It has magnitude & direction
 What does that mean?


Force can be balanced or unbalanced
Unbalanced Forces
Balanced Forces






Balanced forces occur when the forces
applied to an object produce a net force of
zero
They do not cause a change in the motion
of an object or move an object out of rest
Example: a hat on your head
Unbalanced forces occur when forces do
not cancel out completely
They cause a change in the motion of an
object
Because the net force is greater than zero
the object will accelerate in the direction
of the greater force
Friction is the force that opposes motion
between 2 surfaces that are in contact
 Friction happens because the surface of
any object is rough (microscopically
speaking)
 Friction opposes the applied force on an
object
 Friction also affects objects that aren’t
moving


When a car is accelerating, the forces are
__________________
› The force moving the car forward is greater
than the opposing force of friction
When a car is cruising at a constant
speed, the force moving the car forward
is ___________ by the force of friction
 Balanced forces could also mean that the
car is not in motion

Kinetic Friction
 Is the force of
friction against an
object already in
______
 There are different
kinds of kinetic
friction
› Sliding friction
› Rolling friction
› Fluid friction
Static Friction
 Is the force of
friction opposing the
initial movement of
an object from
________________
 It is _______ than
kinetic force
___________ is the friction that occurs
between objects that are sliding past each other
 ____________ is the friction of a round object
rolling over a flat surface
› It is usually less than sliding friction
 ___________ is the friction of a moving object
traveling through a fluid (a liquid or a gas)
› It can be minimized by very smooth surfaces
› When air is the fluid, the force of friction
is called ___________


Why is friction important?

How can friction cause problems?
1. Using lubricants such as motor oil, wax and
grease
› Lubricants are usually liquids but can be
solids or gases
2. Replacing sliding friction with rolling friction
› Such as using ball bearings between wheels
and axles
3. Make surfaces smoother
1. Making surfaces rougher
2. Increase the force pushing the
surfaces together