2-2 Notes Types of Forces
Chapter 2, Lesson 2
Gravity
• Many types of forces act on objects.
• Gravity is an attractive force between
all objects that have mass.
Gravity
The ball does
not travel in a
straight line
because of the
unbalanced
force of gravity
acting on it.
The Law of Universal Gravitation
• The Law of Universal Gravitation
states that all objects with mass are
attracted to each other.
• The magnitude of attraction depends
on the mass of each object and the
distance between them.
The Law of Universal Gravitation
• The gravitational force becomes
stronger as either or both objects
increase in mass.
• The gravitational force becomes
weaker as the distance between the
objects increases.
The Law of Universal Gravitation
Weight and Mass
• Mass is the amount of matter in an
object and does not change with
location.
• Weight is the gravitational force on a
object and changes with location.
Weight and Mass
• Weight is a force and a vector.
• Weight changes with height above
Earth.
The gravitational pull of the
earth has been mapped in this
new geoid image from the
GOCE satellite - with the yellows
and red indicating a strong pull,
while the blues show less
gravity.
2011
Weight and Mass
Friction
• Friction is a force that opposes the
movement between two surfaces in
contact.
Friction
• A book pushed across a table slows
down because of friction.
• Friction is an unbalanced force acting
on the book to slow it down.
MOTION
FRICTION
Friction
• Without friction, you could not walk,
drive, skate, etc.
• Sometimes we want to INCREASE it.
(cleats, surfboard wax)
Static Friction
• Static friction is the force between two
surfaces in contact that keeps them
from sliding when a force is applied.
Static Friction
• A force is applied to a heavy box, but
the box doesn’t move.
Static Friction
• The forces are balanced, the force
pushing the box equals the force of
static friction pushing in the opposite
direction.
Sliding Friction
• When the force pushing on the box is
larger than the maximum static
friction force, the box begins to slide.
• The frictional force that acts on the
sliding box is called sliding friction.
Sliding Friction
• The book pushed across the table
slows down because of sliding
friction.
• Without sliding friction, the book
would continue moving without a
force being applied.
Sliding Friction
• Usually friction is present and
an unbalancing force must be
applied to keep an object
moving.
• When friction is greatly
reduced, objects move with
nearly constant velocity
without an applied force.
Rolling & Fluid Friction
• Rolling Friction – when an
object rolls over a surface.
• Fluid Friction – when an
object moves through a gas
OR a liquid (a fluid).
Elastic Forces
• An elastic force occurs when a
material is stretched or compressed.
• A diving board exerts an upward
elastic force on the diver when it is
bent downward.
• 2 types: tension and compression.
Tension Forces
• A tension force is a pulling force
exerted by an object when it is
stretched, such as a rubber band.
Compression Forces
• A compression force is a pushing
force exerted by a material when it is
squeezed or compressed.
• The size of the compression force
exerted by a material is equal to the
size of the force that compresses the
material.
Normal Forces
• A normal force is the force exerted by
an object that is perpendicular to the
surface of the object.
Normal Forces
• The cup is exerting a downward force
on the table, caused by gravity.
• The table is exerting an upward
normal force on the cup, caused by
compression.
Phonebook Friction (2:41)
2.2 Types of Forces
A(n) ____ force is the force exerted
by an object that is perpendicular to
the surface of the object.
A compression
B elastic
C normal
D tension
2.2 Types of Forces
Which force causes a rolling ball
to slow down?
A sliding friction
B static friction
C normal force
D gravity
2.2 Types of Forces
The gravitational force between two
objects ____ as ____ increases.
A increases; distance
B decreases; mass
C increases; velocity
D decreases; distance
1.
2.
3.
4.
A
B
C
D
If a student pushes a book across a
table with a force of 10 N and the
force of friction is 2 N, what is the
net force on the book?
A 0N
B 2N
C 8N
D 12 N
SCI 2.d
1.
2.
3.
4.
A
B
C
D
What balanced forces are acting on a
book sliding across a table?
A gravity
B gravity and friction
C sliding friction, normal
force, and gravity
D gravity and normal force
2-2 Vocab – Types of Forces
text p.96-104
velocity
10.
11.
gravity
law of universal gravitation 12.
13.
weight
1.
elastic force
tension force
2.
compression force
3.
normal force
4.
5. friction
6. static
7. sliding friction – a type of friction that occurs when
2 surfaces slide against each other
8. rolling friction – a type of friction that occurs when
1 object rolls over a surface
9. fluid friction – a type of friction that occurs when 1
object moves through a fluid (liquid or gas)