Download Gravity and Friction

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GRAVITY AND FRICTION
IN THOUGHT
Write a paragraph describing the picture.
 Words to include in your paragraph:

Gravity
 Free fall
 Wind resistance
 Speed
 Acceleration
 Motion
 Velocity
 Terminal Velocity

OBJECTIVES
Describe friction and identify the factors that
determine the friction force between two
surfaces
 Explain how mass differs from weight
 State the universal law of gravitation
 Describe the effects of gravity and air
resistance on an object in free fall

KEY TERMS
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Force
Contact force
Noncontact force
Net force
Friction
Fluid friction
Rolling friction

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Static friction
Sliding friction
Gravity
Free fall
Projectile
Air resistance
Terminal velocity
Push
Pull
TYPES OF FORCE
Force – is a push or a pull
Some forces are only applied when objects are
in contact with one another; while others are
applied when objects do not touch
Contact force is a push or pull on one object by
another that is touching it – someone playing a
guitar
Noncontact force is a force that one object can
Using
the
apply to one
another
object without touching it
force
– electric force
Contact Force
Are the
following
pictures
contact or
noncontact
force
STRENGTH OF FORCES
SI unit for force is the newton (N)
 Forces can be added together if they
working in the same direction

10 N

15 N
5N
15 N
What is the
net force of
the squares
arebelow?
5N
10 N
Forces can be subtracted if they are working
against one another
10 N
10 N
5N
5N
0N
10 N
NET FORCE AND DIRECTION OF FORCES
The net force is the sum of all the forces added
together
 Vector analysis – is the analysis of the different
forces on an object resulting in the calculation
of net force
 Net force is given with a direction – in the
example below 27 N right

10 N
17 N
FRICTION
Surfaces may seem smooth but they have
many irregularities
 When two surfaces rub the irregularities of one
surface get caught on those of the other
surface
 Friction – the force one surface exerts on
another when the two rub against each other

NATURE OF FRICTION
The strength of the force of friction depends on
two factors: the types of surfaces involved and
how hard the surfaces push together.
 Rough surfaces produce greater friction than
smooth surfaces
 Objects with greater mass and weight produce
greater friction than lighter mass and weight
 Friction releases energy in the form of heat
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TYPES OF FRICTION
Fluid Friction – friction that occurs when an
object moves through a fluid
 Rolling Friction – friction that occurs when an
object rolls over a surface
 Sliding Friction – friction that occurs when an
object slides over a surface.
 Static Friction – prevents surface from sliding
past one another
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Sliding
Friction
Fluid
Friction
Static Friction – prevents you
from sliding. If the box doesn’t
move when force is applied it is
because there is not enough
force to overcome the static
friction
GRAVITY
Gravity – is the force that pulls objects towards
Earth
 Gravity – is an attractive force that exists
between all objects that have mass
 Gravity is calculated on Earth as 9.8 m/s/s or
9.8 m/s2

FREE FALL
Free fall – when the only force acting on a falling
object is gravity
 Objects in free fall accelerate at a constant rate of
speed – 9.8 m/s/s
1 sec – 9.8 m/s
F
2 sec – 19.6 m/s
l
a
3 sec – 29.4 m/s
s
 The mass of an object does
h
p
not affect the rate of speed at
h
o
Which the object falls

t
o
PROJECTILE MOTION
Flash photo of a
projectile
Projectile – an object that is thrown or shot
 An object that is dropped and one that is
thrown horizontally are both in free fall
 Horizontal motion does not interfere with free
fall and both objects will hit the ground at the
same time

Both cannon balls will hit
the ground at the end of 6
seconds. B will travel a
further distance.
AIR RESISTANCE
Air resistance – a type of fluid friction that all
objects encounter while falling through the air
 Friction is the direction opposite to motion, so air
resistance is an upward force
 This why some objects fall faster than others
 Eventually air resistance will equal out the force of
gravity – when this happens the object is no longer
increasing in velocity
 Terminal velocity – the greatest velocity that an
object reaches

WEIGHT
Weight – is the measure of force of gravity on
an object, and mass is the amount of matter in
that object
 Weight = mass (kg) x acceleration due to gravity
 Weight = mass x 9.8 m/s/s
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UNIVERSAL GRAVITATION
The law of universal gravitation states: that the
force of gravity acts between all objects in the
universe
 The moon has a gravitational effect on Earth,
the Earth has a gravitational effect on the
moon

Gravitational
pull on
objects
compared to
the gravity
rate on Earth
GRAVITATIONAL FORCE & MASS
When the mass of one or both objects increases,
the gravitational force between them also
increases
1
1
2
 F = G*m1*m2 / r
kg
kg
F = Force
G = Gravity
2 kg
2 kg
M1 = Mass of first object
M2 = Mass of second object
Gravitational force
increases if the mass of at
r = Distance between the objects
least one objects

increases.
GRAVITATIONAL FORCE & DISTANCE

The attraction between objects decreases as
the distance between objects increases
F
o
r
c
e
Distance
REVIEW
Weight and mass are not equal
 Weight = mass * gravity
 All objects have a gravitational force/attraction
to one another
 There are multiple types of friction
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REFERENCES
Anderson, M. et all (2012) Physical Science.
McGraw-Hill: Columbus
 Frank, D.V et al (2001). Physical Science.
Prentice Hall: New Jersey
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