Transcript Forces and Motion

Energy
Energy is the ability/potential to do work (move matter).
When matter interacts energy is transferred back and forth.
Something with a lot of energy could do a lot of work
Or
To do a lot of work (moving something massive), it requires a lot of energy
Energy associated with the motion and position of objects is called mechanical
energy. It is split into two types, kinetic energy and potential energy.
Kinetic energy is the energy of motion, describing how an object moves.
Potential energy is the energy of position, describing the potential an object has
to move as a result of its position in the universe.
Mechanical energy = Kinetic energy + Potential Energy
Kinetic energy depends on the mass of an object and how fast it is
moving (its velocity).
Potential Energy comes in two forms, elastic potential and gravitational potential.
Elastic potential energy is an objects potential to spring back after
being stressed somehow.
e.g- stretching rubber band, drawing back a bow,
compressing a spring
Gravitational potential energy is an objects potential to move as a result
of gravity (to fall). It depends on the height of an object and its weight
(mass and force of gravity)
When an object falls or an elastic is released, its potential energy is converted into
kinetic energy, but the total amount of energy stays the same.
When the ball hits the ground and stops
moving, its mechanical energy is gone.
Where did it go?
Energy that appears to be lost is really just transferred into a new form. There
are seven forms of energy (e.g. thermal, sound, radiant/light)
Energy is converted from one form to another constantly and is involved in
nearly every event or occurrence in the universe
This year we’ll be focusing on motion.
Forces and Motion
How can we describe motion?
How can we measure it?
We can measure how fast an object is moving, how much its speed is changing,
or how hard it will be to stop.
How fast something goes is actually how far it goes in a period of time.
This is speed.
Speed = distance / time
The base unit used for speed is the unit for distance (meters) and time
(seconds) together, so meters/second or m/s.
-prefixes can be added to the meters (e.g. kilo-, hecto, centi. milli)
-and seconds can be converted to minutes or hours
Since few things move at a constant speed, calculations are done of an objects
average speed not its instantaneous speed.
Speed is often shown graphically on distance vs. time graphs.
These are done on a traditional coordinate plane, but you usually only
need to use Quadrant I.
Time is always the x-axis
Distance is always the y-axis
Velocity is more useful than speed, it always has the same numerical value, but it
also tells you in which direction the object is moving.
Example: Speed = 2 m/s
Velocity = 2 m/s south
Directions are given using the same directions as appear on a
compass.
What is acceleration?
Acceleration describes how an object’s velocity is changing (speeding up,
slowing down, or changing direction).
If acceleration is greater than 0, then speed is increasing.
If acceleration is 0, then speed is staying the same.
If acceleration is less than 0, then speed is decreasing.
A car in cruise control can be going 100km/h, but it will have an
acceleration of 0, since its speed isn’t changing.
Since it describes how much m/s has changed in a certain amount of time (s),
the unit is m/s².
The formula is: average acceleration = Final velocity –initial velocity
Time
or
a = Δv / t
Since acceleration can also indicate a change in direction, an object going in a
circle is constantly accelerating even if its speed doesn’t change.
A graph of acceleration would be similar to one for velocity, but instead of
distance vs. time it would be velocity vs. time.
Try a few calculations:
A car pulls out of the driveway, accelerating from parked to 12 m/s in 4
seconds.
A person after a sprint slows down from 8 m/s to 0 m/s in 4 seconds.
A person running uphill slows down from 8 m/s before the hill to 4 m/s on
the hill in 1 second.
A baseball thrown at 15 m/s slows down to 12 meters per second in 2
seconds.
A football punted straight up slows down from 19.6 m/s to 0 m/s as it
climbs in 2 seconds.
The same football then falls, speeding up from 0 m/s to 19.6 m/s before it
hits the ground in 2 seconds.
Challenges:
A car drives 10 meters in 2 seconds. 5 seconds later it is going 20 m/s.
What was its acceleration?
If a car accelerated at 10 m/s², starting at 15 m/s, how long would it take to
get to 65 m/s?
If the breaks on a car allow it to accelerate at -15m/s². How long would it take
it to come to a complete stop if going 60 m/s?
How hard an object will be to stop, how much power it has as a result of its motion,
is related to its amount of kinetic energy.
Kinetic energy = ½ mv²
Momentum = (m)(v)
Just like energy is conserved, momentum is too.
The law of conservation of momentum states that when objects collide the total
amount of momentum remains the same.
-the momentum of each object can change, but the sum will remain the
same.
What happens when two objects of equal mass and velocity collide?
What happens when a large object and small object at equal velocity collide?
What happens when a moving object collides with a stationary object?
What changes when objects collide, mass or velocity?
What is a force?
A force is a push or pull.
A force tries to move an object in a particular direction, so they not only have a
size (magnitude or strength) but a direction.
The unit for force is the Newton (N).
Any change in the state of motion of an object is the result of the sum of all the
forces acting on it.
The result, the combination of all these forces is called the net force.
Forces that act in the same direction add together.
Forces that act in opposite directions take away from each other.
Forces acting at angles can partially add or subtract from each other, depending
on the angle.
If the net force works in one direction
more than others the object will move.
If they add up to zero, then it will not.
Two of the most common and important forces are friction and gravity.
-gravity works whenever two objects with mass are near each other.
-friction works whenever two objects touch each other.
If two objects are touching and a force is applied, friction works in the opposite
direction to resist movement.
-if friction is strong enough to keep the object from moving it is called
static friction.
-After the object starts moving it is called kinetic friction.
The strength of friction depends on how much the objects are being pressed
together (usually weight pressing it to the ground) and the structure of the
surfaces in contact.
Rough surfaces create more friction than smooth surfaces.
Energy used fighting friction is wasted energy, so most machines are
designed to minimize friction (car oil).
Gravity is the attractive force that works between bodies of mass.
The law of universal gravitation states that gravitational force increases as
mass increases and the distance between objects decreases.
Force of gravity = (Gravitational constant) __(Mass1)(Mass2)__
(Distance between objects)²
Your weight is the force of gravity between you and the earth.
How fast an object falls is a result of the force of gravity.
As an object falls it gets closer to earth, so the force of gravity increases.
Increased force means that the object will accelerate.
Free falling objects all accelerate at a constant rate (ignoring wind resistance).
-9.8 m/s² = g
The velocity of a free falling object is this value multiplied by time.
Δv = (g)(t)
Orbital motion is the net of gravity and the forward motion an object already has
(its inertia).
Projectile motionHow far a ball thrown forwards goes before
hitting the ground is decided by the net forces
of gravity and horizontal velocity (how hard
you threw it).
Newton’s First Law of Motion
An object at rest remains at rest, and an object in motion remains in motion
at constant speed and in a straight line unless acted on by an unbalanced
force.
Basically, an object won’t accelerate in anyway unless a force
makes it.
This is sometimes called the Law of Inertia, because it describes an objects
resistance to a change in its state of motion.
Newton’s Second Law of Motion
The acceleration of an object depends on the mass of the object and the
amount of force applied.
The law is summarized by the following equation.
F= ma
So force equals mass times acceleration
What happens to force if mass increases? Decreases?
What happens to force if acceleration increases? Decreases?
What happens to acceleration if force increases?
Newton’s Third Law of Motion
Whenever one object exerts a force on a second object, the second object exerts
an equal and opposite force on the first.
If you jump off of something high, it hurts because the ground hits you as hard
as you hit it.
A rocket works because as it pushes the gases downwards, they push it upwards.
This is why hitting things hurts…
Fluid substances also exert force on any object in them.
-This is why some things float while others sink and why temperature changes with
elevation.
What forces are acting on an object floating in water?
The two main ones are gravity and buoyant force.
Gravity, or the objects weight, pushes it down.
Buoyant force pushes it up.
Weight depends on the mass of the object.
Buoyant force depends on its volume.
So, whether an object floats or not depends on its
density.
Archimedes Principle- The buoyant force acting on an object is equal to the weight
of the fluid displaced by the object but acts in an upward direction.
If an object is less dense than water,
displaced water will weigh more, so
buoyant force will be stronger than
gravity and the object will float.
Since fluids are composed of active molecules, at least compared to solids, they
are constantly moving, pushing outwards, and applying force to molecules (objects)
around them.
The amount of force per unit area is called pressure.
Since fluid molecules have a lot of energy, the smaller a space you
squeeze them into, the more pressure they will exert.
-e.g. putting too much air into a balloon or tire.
Our atmosphere is a giant mixture of gases (a fluid), so it exerts pressure on
everything in and around it.
An air molecule at a low elevation has a lot of air molecules above it, and
their weight is pressing down on it. This means that it is being squeezed into
a smaller space and pushed closer to the molecules around it, which means
that it will exert more pressure on the objects around it.
As you increase in elevation, this atmospheric pressure decreases.
As atmospheric pressure decreases, temperature also decreases, and
volume increases.
Water (a fluid) also exerts pressure on any objects in it. Just like air, the deeper
you go the more pressure you feel.
Our bodies are adapted to the amount of pressure present at sea level.
If we go up into the air or below water our bodies have to acclimatize.
However, the range that our bodies can acclimatize to is pretty small.
Our bodies are filled with fluids, especially the air in our lungs, so when we
change elevations the pressure in those parts of the body also change, and
they do not work properly.
We can change the amount of pressure a fluid exerts by changing its speed.
Bernoulli’s Principle- the pressure in a fluid decreases as the fluid’s
velocity increases.
This allows a plane to fly, if the air moving over the wing is going faster
than the air going below it the pressure below will push the plane up.
-This upward force is called lift.
Bernoulli’s principle also explains why a curve ball curves or a two
seam fastball tails.
Independent researchAnother principle of fluid dynamics is Pascal’s Principle-. Hydraulics work because
of the properties it describes.