Transcript Energy and Forces in Motion MS

Energy and
Forces in Motion
Physical Science
Chapter 11 and Section 1 of
Chapter 13
What is Energy?
Energy is the ability to do work.
The Law of Conservation of Energy says:
Energy cannot be created or destroyed, it can only
change form.
An example of energy conservation is when potential
energy becomes kinetic energy (and sometimes
back into potential energy!)
Potential Energy
Potential Energy is the energy an
object has because of its position or
shape. The object is ready to move,
but not yet in motion.
Examples: A stretched rubber band,
a wind-up toy, a kid at the top of a
slide.
Potential Energy Due to
Gravity
Gravitational Potential Energy occurs
when gravity is the force used to
create the potential energy. The
more force you build UP against
gravity, the greater the GPE.
Examples: the low dive vs. the high dive
at a swimming pool. Throwing a penny
off a chair vs. the Empire State Building!
Chemical Potential
Energy
Remember that a chemical change occurs
when 2 or more different elements are
combined to create a new substance.
Chemical potential energy is greatest just
before the actual chemical change.
Example: The side of the marshmallow is
completely brown just before it begins
burning!
Kinetic Energy
Kinetic energy is the energy of motion. All
moving objects have kinetic energy.
Kinetic energy depends on speed and
mass. The faster the object is moving,
the more kinetic energy. Kinetic energy
increases as the mass of the object
increases.
From Potential to
Kinetic
An object’s greatest potential energy is
just before it begins to move
(winding up a toy).
Once the object begins moving, it has
kinetic energy. The object has it’s
greatest kinetic energy just before it
begins to slow down or stop.
Everyday Use of Potential and Kinetic
Energy
http://www.youtube.com/watch?v=Jnj8
mc04r9E&safety_mode=true&persist_s
afety_mode=1&safe=active
Forces
A force is a push or pull
A net force is the combination of all
forces involved.
For example, if you and a friend were
each pushing a heavy box, then the
force would be the combination of the
force used by both you and your friend.
Balanced vs Unbalanced
Forces
A balanced force means that the opposing
forces are the same, so they cancel each
other out. When you have a balanced
force, you have a net force of 0.
An unbalanced force is when the forces
are not equal (one is stronger). Net force
= greater force – lesser force.
3 Natural Forces
On Earth the 3 natural forces are
- Gravity
- Friction
- Air Resistance (friction of the air)
Centripetal Force
Centripetal force is the force needed
to move an object in a circle (i.e., an
ice skater)
Because an object (moving with
centripetal force) is always changing
direction, it is always accelerating.
Centripetal Force Demo
http://www.youtube.com/watch?v=56QC
I4Ig4EY&safety_mode=true&persist_saf
ety_mode=1&safe=active
Gravity!
Gravity is the attractive force between 2 objects.
Gravity is dependent upon 2 things
- The mass of the objects, and
- The distance between them.
In other words, the bigger the objects, the closer
they are, the more gravitational pull!
Gravity and Motion
Aristotle believed that the rate an object falls
to Earth depends on the object’s mass (the
larger the mass, the faster it would fall).
Galileo believed that objects fall at the same
rate because the rate of gravity is the same
(9.8m/s/s). He was correct. It is hard to
prove on Earth because of air resistance.
Proving Galileo Correct!
http://www.youtube.com/watch?v=h9xw
KXcZqbA
Acceleration at a
Constant Rate
the rate of acceleration on Earth is 9.8 m/s2
When an object is dropped on Earth, it is
falling at a rate of 9.8 m/s faster than the
second before (no matter the size)
1 sec
9.8 m/s downward
2 sec
19. 6 m/s downward
3 sec
29.4 m/s downward
4 sec
39.2 m/s downward
And so on…..
Slowing Down
Acceleration
Air Resistance is fluid friction, which
slows down the acceleration of
gravity a force that acts against a
falling object. The longer an object
falls, the more force of air resistance
is built up.
Slowing Down
Acceleration
Terminal Velocity - When the force of an
object falling and the force of the air
resistance pushing up on that object are
the same (net force of 0), then the object’s
velocity towards the ground will stop
accelerating (falls at a constant speed.)
Terminal Velocity
Terminal velocity is a good thing. If
hailstones didn’t have terminal velocity, they
would cause a great deal of harm and
damage by the time they hit the ground.
Because (most) hailstones are small, their
terminal velocity is between 5 m and 40 m/s.
If there was no terminal velocity, the
hailstones would be hitting us at a velocity of
up to 350 m/s!
Terminal Velocity
http://www.youtube.com/watch?v=kNED
5RzqxOo&safety_mode=true&persist_s
afety_mode=1&safe=active
Creating Terminal Velocity
and
Free Fall
We use items to help us achieve terminal
velocity faster by increasing the force of air
resistance. An example would be a
parachute.
Skydivers say they are in free fall before the
parachute opens, but that isn’t correct.
Free fall means that there is no other
force acting upon the falling object except
gravity, and that means you can’t have
free fall if there is any air resistance.
Free Fall in Space
There is no such thing as weightlessness,
even in space. That’s because gravity always
exists, and weight is dependent on gravity.
When you see astronauts “floating” in space,
they still have weight, because there are still
objects around you (planets, stars, the space
craft). The amount of gravity is so slight, this
is why you appear to float.
Free Fall from the
Fringes of Space
http://www.youtube.com/watch?v=Tamt
hU7wKJc
Orbiting Objects in Free
Fall
When the shuttle is orbiting the Earth, it has two
motions:
- It is traveling forward at a constant
speed,
- It is being pulled by gravity downward towards
the Earth. This is called Satellite Motion.
The reason why astronauts don’t hit their heads on
the ceiling of the shuttle during free fall is because
the astronauts are also in free fall towards the
Earth.
The Role of Gravity and
Orbiting
All orbiting objects move in a circular path
(the moon around Earth, the Earth around the
Sun, etc.).
Any object moving in a circle is constantly
changing direction. Any object in motion
must be acted upon by an unbalanced force.
The unbalanced force that causes objects in
orbit to move in a circular motion is called
centripetal force.
Projectile Motion and
Gravity
Projectile motion is the curved path an
object follows when thrown or propelled
near the surface of the Earth.
Projectile motion can be vertical or
horizontal. When they are combined, they
form a curved path. The horizontal
velocity remains constant, but the vertical
velocity slows down because of gravity.
Projectile Motion and
Gravity
Example: throw a baseball. When the
ball goes forward, it starts to fall to
Earth. This motion goes in a curved
path along the surface of the Earth.
So, if you were to try and hit a bull's-eye
with an arrow, where should you aim
the arrow before letting go?
Projectile Motion Demo
http://www.youtube.com/watch?v=sFcs
MDzW1Yg&safety_mode=true&persist_
safety_mode=1&safe=active
Friction
All moving objects
encounter friction,
an opposing force
to motion
Without it most
motion would be
impossible
4 types, static,
sliding, rolling,
fluid
Static Friction
1. Normal friction is the outward force from
the surface. This creates static force keeps
an object from moving (outward force from a
surface and bottom of object.)
It is the largest frictional force
Always opposite direction of the
applied force (pushing a cart,
walking)
2. Sliding Friction
Once the object is in motion it
experiences sliding friction
Opposite direction from applied force
Less than static friction so less force is
needed to keep it in motion
3. Rolling Friction
As something rolls, the
object and floor bend
slightly. This bend
causes rolling friction
It is a much smaller
force than static friction
 As much as 1000
times smaller
Allows you to move
heavy objects
Ball bearings reduce
friction
4. Fluid Friction
It opposes the
motion in the
LIQUID or GAS
Like swimming, it is
hard to move
If you are in the air,
fluid friction is
called air
resistance
At higher speeds it is
very noticeable.
Isaac Newton
Remember Newton and the apple?
What is a unit of force called?
Isaac Newton wrote a book about his
observations on motion (Principia). He
didn’t actually come up with the official
laws, but his findings led to the laws we
call Newton’s Laws of Motion.
Newton’s First Law of
Motion
“An object at rest remains at rest and an
object in motion remains in motion at a
constant speed and in a straight line
unless acted upon by an unbalanced
force.”
An unbalanced force doesn’t just mean
someone stopping the object. Gravity, air
resistance and friction are all unbalanced
forces.
Inertia and Mass
Inertia is a resistance to change in motion.
If there were no gravity, air resistance or
friction, then the object would continue to
move at the same speed and in the same
direction (Earth around the Sun, etc.).
The more mass of an object, the greater
its inertia (try stopping a car in the same way
you stop a bicycle!)
Newton’s 2nd Law of
Motion
“The acceleration of an object depends on the
mass of the object and the amount of force
applied.”
Force = mass x acceleration (F = ma)
The amount of force depends on the amount of
mass and the acceleration rate. If you increase
either mass or acceleration, you increase the
force. If you decrease one, you will decrease he
force. You can make up the difference if you
decrease one, but increase the other.
Momentum vs Force
Momentum is how difficult it is to
stop a moving object (so the object
is in motion):
P(momentum) = mass x velocity
Force is how much force the object
would have at the moment it collides
with another object:
F = mass x acceleration
Law of Conservation of
Momentum
As with all conservation laws, momentum
is not created or destroyed, but is
transferred.
When you bowl, the momentum of (the
rolling bowling ball transfers to the pins.)
This law illustrates Newton’s 3rd law.
When a moving object strikes another
object, the momentum of the moving
object (action) transfers and causes the
2nd object to move (reaction).
Newton’s 3rd Law of
Motion
“ Whenever one object exerts a for ce on
second object, the second object exerts
an equal and opposite force on the first.”
Forces work in pairs (action/reaction).
Reaction is not always evident on falling
objects (such as a bouncing ball).
Action/Reaction occurs at the same time!
Normal Force
When you stand on the floor, the floor
pushes back on your feet. The normal
force is the outward force from the
surface. The stronger the surface,
the more normal force. (Ex: Which
would have more normal force: a
concrete wall or a wall of
marshmallows?)