What is a force?
How does friction affect motion?
Motion
A change in position, also
known as an object’s
displacement
Speed (rate)
Rate of change in position
How fast or how slow
Measured in m/s (meters
per second)
Velocity
Speed with a direction
Vector
Constant speed
A speed that does not
vary
Graph – a straight line, no
turns
Acceleration
Rate of change in velocity
Change in speed over time
Change in direction
Acceleration
Units for acceleration – m/sec2
Acceleration refers to increasing
speed, decreasing speed
(deceleration, negative
acceleration), or changing
direction
Calculating Acceleration
 Acceleration = Final speed – Initial speed
Time
A plane travels from 0 m/s to 24 m/s in
3 sec. What is its acceleration?
Acceleration = 8 m/s2
Calculating Acceleration, alt.
 Force = mass x acceleration
 Acceleration =
 A force of 6 N is applied to an object that has a
mass of 3 kg. What is the acceleration?
 Acceleration = 2 m/s2
What is a Force?
A force is a push or a pull.
All forces have two properties:
Direction and Size
A Newton (N) is the unit that
describes the size (magnitude) of a
force.
 The student is pushing
down on the chair, but
the chair does not
move.
 The floor is balancing
the force by pushing on
the chair, which in turn
pushes on the student.
What is friction?
Friction is a force that opposes motion
between two surfaces that touch each other.
A force between two surfaces that touch and
are sliding, or trying to slide across one
another, for example when you try to push a
toy car along the floor.
Friction always works in the direction
opposite from the direction the object is
moving, or trying to move. It always slows
a moving object down.
Moving
Friction
 The surface of any object is rough.
 Even an object that feels smooth is covered with
tiny hills and valleys.
 The contact between the hills and valleys of two
surfaces causes them to stick, resulting in friction.
Friction converts kinetic
(motion) energy into
thermal (heat) energy.
Principles of Friction
 Friction acts parallel to the surfaces that are
in contact.
 The direction that friction acts is OPPOSITE
the direction of the motion (or intended
motion).
Fnormal
Ffriction(k)
Fapplied
Fweight
What Affects Friction?
 The amount of friction depends on:
 Roughness of the surfaces
 Force pushing the surfaces together
What Creates More Friction?
 Rough Surfaces
 Rough surfaces have more microscopic hills & valleys.
 The rougher the surface, the greater the friction will be.
 Greater Force (caused by increased mass)
 The amount of force exerted by the object increases the
amount of friction
 Increase the amount of force exerted by the object by
increasing the mass of the object
What Does Not Affect Friction?
 Surface Area
 Changing the amounts of the surfaces that touch does
NOT change the amount of friction
4N
4N
What Does Not Affect Friction?
5 m/s
20 m/s
2 N
Velocity
2 N
Friction forces are independent of speed or
velocity
The force of kinetic friction is the same at 5
m/s as it is for 20 m/s.
Types of Friction
There are two types of friction:
• Static Friction – the frictional force present just
before motion begins.
• Kinetic Friction – the frictional force present
with motion
Note: Static Friction is usually higher than Kinetic
Friction
Static Friction
 Static friction occurs when force applied to an object
does not cause the object to move.
 In order to move something, you must first overcome
the force of static friction between the object and
the surface on which it is resting.
 Kinetic friction occurs when force is applied to an
object and the object moves.
 Examples:
Sliding Friction: pushing an object across a surface
Rolling Friction: between wheels and a surface
Fluid Friction: opposes the motion of objects
traveling through a fluid (air or water), also called
drag force or air resistance
Reducing Friction
 Reduce the contact area by using rollers/ball-
bearings/wheels
 Change the surfaces of the materials that are
touching by using lubrication like motor oil,
wax, or grease
 Roll objects instead of sliding them
 Create a cushion of air
Eg. Like a hovercraft or air hockey table
Friction Can Be Harmful or Helpful
 Harmful friction
 Holes in socks or knees of jeans
 Wind & water cause erosion
 Helpful friction
 Friction between pencil & paper
 Without friction, you would just slip & fall when
you tried to walk
Useful Friction
• Friction can be a useful force because it
prevents our shoes slipping on the pavement
when we walk and stops car tires skidding on
the road.
• Ice causes very little friction, which is why it
is easy to slip over on an icy day. But this is a
good thing for ice skating and sledding.
Recall the Sliding Penny
 Why do things not continue to move at constant
velocity?
 Friction
 If the sliding penny slows down, what’s the
force responsible?
 Friction
 How could I keep it moving at a constant
velocity?
 Reduce the friction
 Do I need to apply a force to keep it moving?
Why?

No, inertia will keep it moving once the
initial force was applied.
How is friction measured?
Measures of friction are based on the type of materials
that are in contact.
Each material has what is known as a coefficient of
friction. The coefficient is a measure of how easily one
object moves in relationship to another.
When you have a high coefficient of friction, you have a
lot of friction between the materials.
Concrete on concrete has a very high coefficient of
friction.
A material such as Teflon (on most things) has a very
low coefficient. Teflon is used on surfaces where we
don't want things to stick; such as pots and pans.
Standard Friction Equation
 Friction = coefficient of friction x normal force.
 Normal force comes into play any time two objects are
in direct contact with one another, and always acts
perpendicular to the object that applies the force.
 The simplest example of the Normal force can be seen
in the situation of an object on a horizontal surface.
 If the object is on a horizontal
surface, then
Normal force = Weight
Friction and the Normal Force
n
12 N
8N
n
4N
n 2N
4N
The force required to overcome static or kinetic
friction is proportional to the normal force,
fs = ms
n
n
fk = mk
n.
6N
μ
μk
μs
FN
= Symbol for the Greek letter mu, used in
the formula to represent the coefficient.
= coefficient of kinetic friction
= coefficient of static friction
= the normal force
Sample Coefficients of Friction
Material on Material
ms = static friction
mk = kinetic friction
steel / steel
0.6
0.4
add grease to steel
0.1
0.05
metal / ice
0.022
0.02
brake lining / iron
0.4
0.3
tire / dry pavement
0.9
0.8
tire / wet pavement
0.8
0.7
Spring 2008
36
The coefficient of kinetic friction between a block and the level
surface it slides on is 0.45.
If the mass of the block is 10.0 kg, what is the minimum force
needed to keep the block moving with uniform motion?
Fk = µ kFN
Fk = µ kFN
Fk = (0.45)(10.0)(9.8 m/s2)
Fk = 44 N
The applied force need only balance the kinetic frictional
force in order to maintain uniform motion.
A student pulls a 5.00 kg object and discovers that she
needs to exert 30.0 N of force before the object moves.
What is the coefficient of static friction between the
object and the surface on which it rests?
Since the applied force is equal to 30 N just before moving, the
friction force must be equal in size (30 N).
Therefore: m = 30 N ÷(5 kg)(9.8 m/s2)
m = 0.61