Download Gravity is a force exerted by masses

Gravity and Friction
Chapter 12
• Describe how friction affects motion.
• List the factors that affect friction
Gravity
• Masses attract each other
• Gravity is the force that objects exert on
each other because of their masses.
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Gravity
• Gravity is a universal
force because it acts
between any two
masses anywhere in
the universe.
• For example, there is
a gravitational pull
between the Sun and
the Moon.
Force of Gravity
• If there is a force between all masses, why
are you not pulled toward your desk by the
desk’s gravity when you walk away from
it?
• The force of gravity between your desk
and you is extremely small
• The strength of gravitational force is
important when determining what will be
attracted.
The Mass of Objects
• The more mass two objects have, the
greater the force of gravity the masses
exert on each other.
The Distance Between Objects
• As distance between
the objects increases,
the force of gravity
decreases.
• If the distance is
doubled, the force of
gravity is ¼ as strong
as before.
Gravity on Earth
• Page 382
Weight and Mass
• They are not the
same, but similar
• Mass is a measure of
how much matter an
object contains.
• Weight is the force of
gravity on an object.
• Balance=mass
• Scale= weight
Gravity keeps objects in orbit
• Sir Isaac Newton
hypothesized that the
force that pulls
objects to the groundgravity- also pulls the
Moon in its orbit
around Earth.
Orbit
• An Orbit is the
elliptical path one
body, such as the
Moon, follows around
another body, such as
the Earth, due to the
influence of gravity
Centripetal Force
• Centripetal Force is keeping one object in
orbit around another object is due to the
gravitational pull between the two objects.
• Page 385
Spacecraft in Orbit
• The minimum speed needed to send an
object into orbit is approximately 8000
meters per second.
• If a spacecraft is launched with a speed of
11,000 meters per second or more, it is
moving too fast to go into an orbit.
• This allows it to escape Earths gravity.
People in Space
• While astronauts are
in orbit, their weight
does not press
against the floor of
the spacecraft.
• This is known as a
microgravity
environment, in which
objects behave as if
there were no gravity.
Friction
• A force that resists the motion between
two surfaces in contact.
• Friction between your feet is what provides
the action and reaction forces that enable
you to walk.
• Like this penguin……
Friction and Surfaces
• Friction changes depending on surfaces
• Example: Hockey Puck sliding across an
ice floor and a wooden floor.
Friction and motion on surfaces
• You need a larger force to start something
moving than you do to keep something
moving.
Friction and pressing surfaces
together
• The harder two surfaces are pushed
together, the more difficult it is for the
surfaces to slide over each other.
• Page 391
Friction and Heat
• Friction between surfaces produces heat.
Friction in Fluid
• Objects falling through air have different
accelerations.
• This is because air is fluid
• Fluid is a substance that can flow easily.
• The friction due to air is called Air
resistance.
Skydiving
• Page 393
Chapter 12.3 & 12.4
• Explain how pressure is determined
• Describe how forces act on objects in
fluids.
• Describe pressure changes in fluids.
• Explain how fluids apply forces to objects.
• Describe how forces are transmitted
through fluids.
Pressure depends on force and
area
• Pressure is a measure of how much force
is acting on a certain area. In other words,
pressure describes how concentrated a
force is.
Pressure
• While increased pressure may make you
feel as if there is more force on you, the
force is actually the same!
Pressure
• One way to increase pressure is to increase
force.
• Example: If you press your finger on the desk
you increase pressure.
• Formula:
Pressure= Force
Area
P- Pressure, also known as Pascal’s
F-Force, A-area over with force is exerted
Pascal’s
• One pascal is the pressure exerted by one
newton of force on an area of one square
meter.
Knowing Pressure Important
• Sometimes, knowing pressure is more
useful than knowing force.
• For example, many surfaces will break or
crack if the pressure on them is too great.
A person with snowshoes can walk on top
of snow, while a person in hiking boots will
sink into the snow.
Pressure acts in all directions in
fluids
• Randomly moving
water molecules
collide with a diver.
The net force from the
many collisions
produces the
pressure on the diver.
• Page 397
Pressure in Air
• Although you do not notice the weight of
the air, air exerts pressure on you at all
times!
• Change in Elevation
• Change in Density
• Effects on Pressure
• Page 398
Pressure in Water
• Just as air pressure increases at lower
elevations, water pressure increases with
greater water depth.
• Water exerts more pressure on you than
air does because water has a greater
density than air.
How does water affect weight?
• Do you feel bigger or smaller in water?
• You should feel like you weight less
because water exerts an upward force on
objects.
• This upward force on objects in a fluid is
known as buoyant forces.
Buoyancy
• This is why it is easier
to lift people or a
heavy rock in water
than on land.
• Density and
Buoyancy are
similar—this is why
ice floats on water
Bernoulli’s Principle
• Daniel Bernoulli, a Swiss mathematician
who lived in the 1700’s described the
effects of fluid in motion on pressure.
• Bernoulli’s Principle- says that an increase
in speed of the motion of a fluid decreases
the pressure within the fluid. The faster a
fluid moves, the less pressure it exerts on
surfaces or openings it flows over.
• Page 405
Pascal’s Principle
• In the 1600’s Blaise Pascal, a French
scientist for whom the unit of measure
called the pascal was named,
experimented with fluids in containers.
• Pascal’s Principle- States that when an
outside pressure is applied at any point to
a fluid in a container, that pressure is
transmitted throughout the fluid with equal
strength.
• Page 406