CHAPTER 3 SECTION 1
Forces
IPC
Types of Friction Foldable
• Make a foldable to help you keep track of the
types of friction
• 3 Headings on cover flaps:
• Static Friction
• Sliding Friction
• Rolling Friction
What is force and what does it do?
• Force: a push or a pull
• A force can cause the motion of an object to change
Net Force
• Net Force: the sum of all forces acting on an object
• As we move forward keep in mind the rules for adding forces
Balanced and Unbalanced Forces
Which is balanced? Which is unbalanced? Why?
• Balanced forces do not cause a change in motion.
• The net force will be the difference between the
two forces if they are in opposite directions.
Friction
• Friction: the force that opposes the sliding motion of
two surfaces that are touching each other
• What causes friction?
• Surfaces that appear smooth actually have many
bumps and dips
Types of Friction
• Static friction: frictional force that prevents two
surfaces from sliding past each other, caused by
microwelds that form between two surfaces
• These microwelds are stronger when pushed
together with larger force
• This means that when you are pushing an object and
it is not moving the force opposing you is static
friction
Types of Friction
cont.
• Sliding friction: frictional force that opposes the
motion of two surfaces sliding past each other
• Think about the resistance you feel when trying to
slide your shoes along carpet
• The force of sliding friction is usually smaller than the
force of static friction!
• This is why once you get an object moving it is easier
to keep it moving.
Types of Friction cont.
• Rolling friction: friction between an object that is
rolling in one direction and the surface of another
object
• Rolling friction, aka traction, is very useful and is the
reason your tires move on your vehicles
• In icy weather sand is used to increase the friction
with vehicle tires or people’s shoes
Gravity
• Gravity: an attractive force between any two objects that depends
on the masses of the objects and the distance between them
• Gravity is one of four fundamental forces
• Newton’s Law of Universal Gravitation: the relationship between
the gravitational force between two objects, the object’s masses,
and the distance between them
•F = G×m1m2
d2
G = universal gravitational constant (6.67×10-11 m3/kg×s2)
m1 and m2= the masses of the two objects
d= the distance between the centers of the two masses
In the 1660’s, British scientist Sir Isaac Newton used data on the motions of planets to
write the law of universal gravitation.
More About Gravity
• Why do I not feel the gravity of the textbook on my desk?
• While the distance from the book to me is very close, the mass of the
book is not enough for me to feel it’s gravitational pull
• The range of gravity
• According to Newton’s law of universal gravitation, the gravitational force
between two objects decreases rapidly as the distance between the masses
increases
• However, no matter how far apart the gravitational force between two objects
is never zero
• Gravitational fields
• A field is a region of space that has a physical quantity (such as force) at every
point.
• All objects are surrounded by a gravitational field that becomes weaker as
distance from the object increases
Weight
• Weight: the gravitational force exerted on an object
results in the object’s weight
• Weight Equation:
• Weight (N) = mass (kg) × gravitational strength
(N/kg)
Fg = mg
Do you think the
weight of a plane is
the same on earth’s
surface as it is in
earth’s
atmosphere? Why?
Weight and Mass
• Weight and mass are NOT the same
• Weight is a force
• Mass is a measure of the amount of matter an object
contains
• Weight on earth and weight on the
moon are not the same… Why?
• Is the mass on earth the same as the
mass on the moon? Why?
Force Activity
• In groups of 3-4, model one of the following forces
• Balanced/unbalanced forces
• Static/sliding friction
• Gravity
• Weight (on and off earth)
• Be able to justify your reasoning and explain the finer
details behind your chosen force
Homework
• Complete the handout and turn it in next class time
Wrap Up
• Are mass and weight the same thing?
• What happens to gravitational force the further you get from
an object?
• What are three types of friction?
• What is net force? Why is it useful?
Have a Great Day!