Transcript Notes Forces- Gravitational, Mag., Elec. File

Forces: Gravitational, Magnetic, &
Electrical
Forces between objects act when the objects are in
direct contact
or when they are not touching.
Magnetic, electrical and gravitational forces can act at a
distance.
Gravity
Sir Isaac Newton is credited with the discovery of gravity.
Now, of course we know that he didn’t really discover the
thing – let’s face it, people knew about gravity for as long as
there have been people. Gravity didn’t have to be
discovered for crying out loud! Your basic tiny little toddler
figures it out in the first few months of life. So what is the
big deal with Newton and gravity? Well, what Newton did
was to describe gravity, extend its effects from the surface of
the earth (which everyone understood) out into space to
explain the behavior of the planets (which nobody
suspected), and to formulate a mathematical law that
accurately described the force of gravity between objects
with mass.
Gravity
• Newton’s theory is very simple. Gravity is a force of
attraction between any two objects that have mass.
Two objects sitting on a desktop attract each other
with a force that we call gravity. They don’t go flying
together because gravity is a very weak force and is
only significant when one or the other of the masses
is enormous – planet size. This is why we aren’t
attracted to objects that we pass on our daily
wanderings.
Gravity
• The size of the force of gravity is given by this
equation:
mm
F  G 12 2
r
Newton’s law of gravity
• G is the universal gravitational constant.
N m
G  6.67 x 1011
2
• m1 is the mass of one of the bodies and m2 is the
mass of the other body.
• r is the distance between the two bodies.
• The value of G is the same everywhere throughout
the universe.
kg
2
Gravity
G m1m2
FG  
2
r
• Newton’s law of gravity is an inverse-square Law. This
means that the force of gravity gets smaller or larger by the
square of the distance. The force is directly proportional to
the masses, so if the mass of one of the objects doubles, the
force of gravity would double. But if the distance doubled,
the force of gravity would decrease by a factor of four. That’s
because it decreases by the square of the distance. Inversesquare laws are very common in physics. We’ll see more of
them in our explorations.
Gravity
1.
2.
3.
4.
5.
6.
Universal Law of Gravitation
Gravitational force is a field force between two particles -- in
all mediums.
Force varies as the inverse square of the distance
Force is proportional to mass of objects.
The gravity force acts from the center of the two objects.
The gravitational force is always attractive.
The gravitational force cannot be shielded or canceled.
Gravity
• Solving gravity problems is quite simple. Let’s do one.
• A girl, Brandy (42.5 kg), sits 1.50 m from a boy (63.0 kg),
George. What is the force of gravity between them? (This will
tell us how attracted they are to each other.)
F G
m1m2
r
2
F  7940 x 10
2  42.5 kg 63.0 kg




N

m
11
F   6.67 x 10
2 
2


kg
1.50
m




11
N

8
7.94 x 10 N
You can see that this is a tiny force, one so small that Brandy will never notice its
presence. George must generate some other attractive force if there is to be a
relationship between the two of them.
A field model can be used to explain how two objects can exert
forces on each other without touching. An object is thought to
have a region of influence, called a field, surrounding it. When a
second object with an appropriate property is placed in this
region, the field exerts a force on and can cause changes in the
motion of the object.
In physics, a gravitational field is a model used to explain the influence
that a massive body extends into the space around itself, producing a
force on another massive body. Thus, a gravitational field is used to
explain gravitational phenomena, and is measured in newtons per
kilogram (N/kg).
Uniform Gravitational Fields
We live in what is essentially a uniform gravitational field. This means
that the force of gravity near the surface of the Earth is pretty much
constant in magnitude and direction. The green lines are gravitational
field lines. They show the direction of the gravitational force on any
object in the region (straight down). In a uniform field, the lines are
parallel and evenly spaced. Near Earth’s surface the magnitude of the
gravitational field is 9.8 N/ kg. That is, every kilogram of mass an
object has experiences 9.8 N of force. Since a Newton is a kilogram ·
meter per second squared, 1 N/ kg = 1 m/s2. So, the gravitational field
strength is just the acceleration due to gravity, g.
continued on next slide
Earth’s surface
Uniform Gravitational Fields
(cont.)
A 10 kg mass is near the surface of the Earth. Since the field
strength is 9.8 N/ kg, each of the ten kilograms feels a 9.8 N
force, for a total of 98 N. So, we can calculate the force of
gravity by multiply mass and field strength. This is the same
as calculating its weight (W = mg).
10 kg
98 N
Earth’s surface
Nonuniform Gravitational Fields
Near Earth’s surface the gravitational field is approximately uniform.
Far from the surface it looks more like a sea urchin.
The field lines
• are radial, rather than
parallel, and point toward
center of Earth.
Earth
• get farther apart farther from
the surface, meaning the
field is weaker there.
• get closer together closer to
the surface, meaning the
field is stronger there.
How are Weight and Mass related?
 Mass is the measure of the
amount of matter in an
object
 Weight is a measure of the
gravitational force exerted
on an object
 Weight varies with the
strength of the
gravitational force, but
mass does not
How does gravity affect motion?
• Free fall –occurs when gravity is the only
force acting on an object
• An object in free fall is accelerating because
the force of gravity is an unbalanced force
Free Fall
• Near the surface of the Earth the acceleration
due to gravity is 9.8m/s2
• For every second an object is falling its speed
increases by 9.8 m/s
• All objects in free fall accelerate at the same
rate
How does Air Resistance Affect
Gravity?
• Objects falling through air experience a type
of fluid friction called air resistance
• Friction is a force in the opposite direction of
motion so air resistance is an upward force
• Falling objects with greater surface area
experience more air resistance
• In a vacuum there is no air, all objects fall at
the same rate of acceleration
How does Air Resistance Affect
Gravity?
• Air resistance increases with velocity
• Eventually the falling object will fall fast
enough that the upward force of air resistance
will equal the downward force of gravity
• At this point, the forces are balanced and the
objects stops accelerating
• The object continues to fall at constant speed
• This is called terminal velocity – when the
force of air resistance = weight of the object
Terminal Velocity
Every object exerts a gravitational force on every other object
with mass. These forces are hard to detect unless at least one of
the objects is very massive (e.g., sun, planets). The gravitational
force increases with the mass of the objects, decreases rapidly
with increasing distance and points toward the center of
objects. Weight is gravitational force and is often confused with
mass. Weight is proportional to mass, but depends upon the
gravitational field at a particular location. An object will have the
same mass when it is on the moon as it does on Earth. However,
the weight (force of gravity) will be different at these two
locations.
Show video : (Techbook)
Field Forces: Gravity: No Contact
Between Objects
Electric fields exist around objects with charge. If a second object
with charge is placed in the field, the two objects experience
electric forces that can attract or repel them, depending on the
charges involved. Electric force weakens rapidly with increasing
distance.
Show video(Techbook)
Field Forces: Electromagnets
Electric Field Hockey
https://phet.colorado.edu/en/simu
lation/electric-hockey
Magnetic fields exist around magnetic objects. If a second
magnetic object is placed in the field, the two objects experience
magnetic forces that can attract or repel them, depending on the
objects involved. Magnetic force weakens rapidly with increasing
distance. Magnetic field lines can be seen when iron filings are
sprinkled around a magnet.
Electricity is related to magnetism. In some
circumstances, magnetic fields can produce electrical
currents in conductors. Electric currents produce
magnetic fields. Electromagnets are temporary
magnets that lose their magnetism when the electric
current is turned off.
Generators convert mechanical energy into electrical
energy and are used to produce electrical energy in
power plants. Electric motors convert electrical energy
into mechanical energy. Motors are in blenders and
washing machines. Both motors and generators have
magnets (or electromagnets) and a coil of wire that
creates its own magnetic field when an electric current
flows through it.