Download Chapter 8.1 and 8.2

Chapter 8
Vocabulary review:
Speed: the distance an object travels per unit of time
Velocity: speed in a given direction
Acceleration: the rate at which velocity changes with time
8.1 The Nature of Force
How Are Forces Described?
A force is a push or a pull. When one object pushes or pulls
another object, the first object exerts a force on the second
object.
Like velocity and acceleration, a force is described by its strength
and by the direction in which it acts.
The direction and strength of a force can be represented by an
arrow. The arrow points in the direction of the force. The length
of the arrow tells you the strength of the force – the longer the
arrow, the greater the force.
The strength of force is measured in the SI unit called the newton
(N), after scientist Sir Isaac Newton.
How Do Forces Affect Motion?
Often, more than one force acts on an object at the same time.
The combination of all the forces on an object is called the net
force. The net force determines if and how an object will
accelerate.
You can find the net force on an object by adding together the
strengths of all the individual forces acting on the object.
A nonzero net force causes a change in the object’s motion.
8.2 Friction and Gravity
If you slide a book across a table, the surface of the book rubs
against the surface of the table. The force that two surfaces exert
on each other when they rub against each other is called friction.
Two factors that affect the force of friction are the types of
surfaces involved and how hard the surfaces are pushed together.
In general, smooth surfaces produce less friction than rough
surfaces. Friction also increases when surfaces push harder
against each other.
Friction acts in a direction opposite to the direction of the object’s
motion. Without friction, a moving object will not stop until it
strikes another object.
Sliding Friction: occurs when two solid surfaces slide over each
other. Sliding friction is what makes moving objects slow down
and stop.
Static Friction: acts between objects that aren’t moving. Think
about trying to push a couch across a room. If you don’t push
hard enough, it won’t move. The force that is keeping you from
moving it is static friction. Once you push hard enough to
overcome static friction, the couch starts moving and there is no
more static friction (it turns into sliding friction).
Fluid Friction: Fluids, such as water and air, are materials that
flow easily. Fluid friction occurs when a solid object moves
through a fluid. Fluid friction is easier to overcome than sliding
friction.
Rolling Friction: When an object rolls across a surface, rolling
friction occurs. Rolling friction is much easier to overcome than
sliding friction for similar materials. That’s why it is easier to push
a bike along the sidewalk when the wheels can turn, but harder to
push if you apply the brakes.
What Factors Affect Gravity?
Sir Isaac Newton concluded that a force acts to pull objects
straight down toward the center of Earth. Gravity is a force that
pulls objects toward each other.
Newton realized that gravity acts everywhere in the universe, not
just on Earth. It is the force that keeps the moon orbiting around
Earth. It is the force that keeps all the planets in our solar system
orbiting around the sun.
What Newton realized is now called the universal law of
gravitation. The Law of Universal Gravitation states that the force
of gravity acts between all objects in the universe that have mass.
However, you don’t see your pencil fly toward the wall the way
you see it fall to Earth. That’s because the gravitational force
between some objects is stronger than the force between others.
You observe only the effects of the strongest gravitational forces.
Two factors affect the gravitational attraction between objects:
mass and distance
The more mass an object has, the greater the gravitational force
between it and other objects. Example: the more massive
planets have a greater gravitational force than the less massive
planets.
Gravitational force also depends on the distance between the
objects centers. As distance increases, gravitational force
decreases.
Weight and Mass – Mass is sometimes confused with weight.
Mass is a measure of the amount of matter in an object. Weight
is a measure of the force of gravity on an object.