Download Chapter 8 Section 3 Notes

Chapter 8 Section 3 Notes
Motion and Force
What is Force?
□ A force is the cause of acceleration or
change in an object’s velocity
Fundamental Forces
□ There are 4 fundamental forces in
nature:
□ Gravity
□ Electromagnetic force
□ Strong nuclear force
□ Weak nuclear force
Fundamental Forces
□ Gravity: very, very weak force; this force
acts over a long distance; much weaker
than the electromagnetic force; field
force
□ Electromagnetic force: moderate force;
acts over long distances; about 1/100 the
strength of the strong force; field force
Fundamental Forces
□ Strong nuclear force: strongest of all
forces, but acts over a very short
distance; only acts over distances the
size of the atomic nucleus
□ Holds together protons and neutrons in
the nuclei of atoms
□ Weak nuclear force: associated with
nuclear decay
Fundamental Forces
Types of Forces
□ Contact forces: caused when objects touch
□ Examples: when you push a shopping cart
or catch a baseball and it stops moving
□ Field forces: do not require that objects
touch each other
□ Examples: attraction of gravity or repulsion
between 2 north poles of a magnet
□ Both contact and field forces can cause an
object to move or stop moving!
Balanced and Unbalanced
Forces
□ Net force: the sum of all the forces
acting on an object
□ Whenever there is a net force acting on
an object, the object accelerates in the
direction of the net force.
Balanced and Unbalanced
Forces
□ An object will not accelerate if the net
force is equal to zero.
□ Balanced forces: net force of zero
□ Do not cause a change in motion of an
object.
Balanced and Unbalanced
Forces
□ Example of Balanced Forces:
□ When 2 teams are playing tug of war and
both teams are exerting equal amounts of
pressure, there is no acceleration, and
neither team moves.
Balanced and Unbalanced
Forces
□ Unbalanced forces: occurs when
forces acting on an object don’t
cancel out; there is a greater force
acting on 1 side of the object
□ Example: When 2 teams are playing tug
of war and one team exerts a greater
force and pulls the other team forward
Balanced and Unbalanced
Forces
□ When an object is at rest, are there still
forces acting on it?
□ Yes! It is just not moving because the forces are
balanced.
□ What happens if forces act in different
directions that are not opposite one
another?
□ The combination of forces acts like a single force
and cause acceleration in a combined direction
□ Example: You push your book east and your friend
pushes the book north; the book will move northeast
Friction
□ Friction: force between 2 objects that
opposes the motion of either object
□ Example: Imagine a car rolling on a flat, evenly
paved street. Eventually, the car will stop due to
the force of friction.
□ Friction depends on surfaces in contact:
□ Frictional forces are greater when both surfaces
are rough
□ Example: new tennis shoes walking along a carpeted
floor will keep you from falling down.
Friction
□ Two types of friction:
□ Static Friction: when 2 objects touch but
do not move
□ Example: A filing cabinet sitting on the floor
Friction
□ Kinetic Friction: friction between moving
surfaces. Two types of kinetic friction:
□ Sliding friction: when 1 object slides over
another
□ Example: Sliding a box across the counter
□ Rolling friction: when 1 object rolls across
another
□ Example: Rollerblading
Friction
□ The force necessary to make a
stationary object start moving is usually
more than the force necessary to keep
it moving; therefore, static friction is
usually greater than kinetic friction.
□ Rolling friction is usually less than sliding
friction.
Friction and Motion
□ Friction is not all bad; it is actually
required for many everyday tasks to
work correctly.
□ Examples:
□ Cars could not move without friction; as a
car’s wheels turn, they push against the road
and as a reaction, the road pushes forward on
the car.
□ You would not be able to write down these
notes either!
Friction and Acceleration
Air Resistance
□ Air Resistance: a type of friction
□ Air Resistance is caused by the
interaction between the surface of a
moving object and the air molecules
□ Air Resistance depends on 3 factors:
□ Size of the object
□ Shape of the object
□ Speed of the object
Gravity
□ Gravity: force of attraction between 2
particles of matter due to their mass
□ Gravity is different from other forces
because: gravity acts on an object even
when the objects don’t touch
□ All objects in the universe attract each
other through the force of gravity!
Gravity
□ Gravity depends on
two factors:
□ The mass of the
objects: the greater
the mass of the object,
the larger the
gravitational force
□ The distance between
the objects: as the
distance between 2
objects increases, the
gravitational force
decreases.