Force Motion Notes


Force, Motion and Energy
7th Grade Physics!


What is Energy?  

    Energy is the ability to do work or cause change.

        Examples:  When the wind moves a leaf, or picks up a house.
In this case, the change is in the position of the
object, so the wind has energy.
            
Work is done when a FORCE moves an OBJECT some
DISTANCE.
        
When an object or organism does work on an object, some of its
ENERGY is transferred to that object.  So WORK could be thought of as THE TRANSFER OF ENERGY. When energy is transferred, the object upon which the work is done GAINS energy.

Energy is measured in units called JOULES.

One JOULE (J) is the amount of work you do when you exert a force of one NEWTON to move an object a distance of one METER.

One NEWTON is the amount of force required to accelerate one kilogram of mass at 1 meter per second.

1 N = 1kg X 1m/s2





There are two general kinds of Energy


Kinetic Energy and Potential Energy
Whether the energy is Kinetic or Potential depends on whether the energy is being TRANSFERRED OR STORED.


    KINETIC ENERGY:  This is the energy of motion.  Because the object is MOVING it is doing work, therefore it has ENERGY. The word “Kinetic” comes from the Greek word KINETOS, meaning “moving”.


        The Kinetic Energy of an object depends on both its MASS and its VELOCITY “MASS”  the amount of matter in an object “VELOCITY” speed in a given direction. “SPEED” the distance an object travels in one unit of time.


        Think about rolling a golf ball and a bowling ball so that they travel
        at the same velocity.  Which ball would you have to roll more
        forcefully?  You would have to exert a greater force on the
        bowling ball because it has a greater mass than the golf ball.

        Since energy is transferred during work, the more work you do, the
        more energy you give to the ball.  So, a bowling ball has more
        KINETIC ENERGY than a golf ball travelling at the same velocity.

        KINETIC ENERGY INCREASES AS MASS INCREASES
        KINETIC ENERGY ALSO INCREASES AS VELOCITY INCREASES
            (because you would have to do more work on the bowling ball to
            give it a greater velocity)

So KINETIC ENERGY depends on both MASS AND VELOCITY







The FORMULA to calculate KINETIC ENERGY IS:

Kinetic Energy= Mass X Velocity2
                          2


Look at the formula.  Do changes in velocity and mass have the SAME effect on
Kinetic Energy?  No, changing the velocity will have a greater effect on Kinetic
Energy than changing its mass.  This is because velocity is squared.  



Use your knowledge of the game of BASEBALL (or any other time when you hit
something with a stick...) and give an example of what we're talking about in the section above.  
(Is it better to hit the baseball with a heavy bat, or one moving quickly?)







Potential Energy


Sometimes when you transfer energy to an object, you change its position or shape. 
For example: you lift a book up to a shelf, or you compress a spring to wind a toy.


Unlike Kinetic Energy, which is the energy of motion, Potential Energy is stored.


Energy that is stored and held in readiness is called POTENTIAL ENERGY.
This type of energy has the POTENTIAL to do work. Just like you people!

Examples of Potential Energy:
    An archer gives potential energy to a bow by pulling it back.  This stored
    energy can send an arrow flying towards its target.  
    The potential energy associated with objects that can be stretched or
    compressed is called ELASTIC POTENTIAL ENERGY.

    POTENTIAL ENERGY that depends on height is GRAVITATIONAL
        POTENTIAL ENERGY.
       You might give this type of potential energy to an object when you lift it.

        The GRAVITATIONAL POTENTIAL ENERGY of an object is
            equal to the work done to lift it.



    Gravitation Potential Energy = Weight X Height
When weight is measured in NEWTONS and height is measured in METERS, the unit of gravitational potential energy is the newton-meter.  This unit is also known as the Joule (J)

Once you know the weight and height, you can calculate GRAVITATIONAL POTENTIAL ENERGY.

Example:  A hiker climbs 40 meters up  a hill, and he weighs 480 Newtons.  The hiker has gained 27,200 Joules of GRAVITATIONAL POTENTIAL ENERGY by climbing the hill.  

The greater the weight of an object, or the greater height it is lifted, the greater the gravitational potential energy.

How could the hiker have gained MORE gravitational potential energy?

If you know the MASS of an object, rather than its weight in NEWTONS, then you can still convert its mass into Newtons.

MULTIPLY THE MASS OF THE OBJECT BY THE ACCELERATION OF GRAVITY

The acceleration of gravity on Earth is (9.8m/s2)


So a second equation for finding Gravitational Potential Energy is:

Gravitational Potential Energy = MASS X GRAV. ACCELERATION X HEIGHT

THE UNITS ARE JOULES






OTHER FORMS OF ENERGY
Kinetic and Potential Energy involve objects being moved or physically changed. 
Both kinetic energy and potential energy have a variety of different forms.


Some of the major forms of energy are :
MECHANICAL ENERGY,
THERMAL ENERGY,
CHEMICAL ENERGY,
ELECTRICAL ENERGY,
ELECTROMAGNETIC ENERGY
AND
NUCLEAR ENERGY.


MECHANICAL ENERGY-  This is energy associated with the motion or position of an object.  Mechanical energy can occur as kinetic energy OR potential energy

THERMAL ENERGY-  This is the total energy of the particles in an object. When the thermal energy increases, the particles move FASTER, making it feel warm to the touch.  The particles have both potential and kinetic energy due to their arrangement and motion.

CHEMICAL ENERGY- This is potential energy stored in chemical bonds that hold chemical compounds together.  Chemical energy is stored in the foods you eat, and in a match used to light a candle, and even in the cells of your body.

ELECTRICAL ENERGY- Moving electrical charges produce electricity or electrical energy.  We rely on electrical energy from batteries and power lines to run electrical devices such as radios, TVs, lights and computers.

ELECTROMAGNETIC ENERGY-  Electromagnetic Energy is energy that travels in waves. These waves have some electrical properties and some magnetic properties.  Some examples of electromagnetic energy are: visible light, ultra violet radiation, microwaves, and infra red radiation.
EXPLAIN THE RELATIONSHIP AMONG VISIBLE LIGHT, THE EM SPECTRUM AND SIGHT (D4B)

NUCLEAR ENERGY-  This is a type of potential energy stored in the nucleus of an atom. One kind of nuclear energy occurs when a nucleus splits (nuclear fission) and another occurs when nuclei fuse (fusion).  Power plants use fission to produce energy. Fusion occurs in the sun and other stars, as well as in nuclear bombs.


Energy Conversion and Conservation
A change from one form of energy to another is called ENERGY CONVERSION or
an ENERGY TRANSFORMATION.  


Any form of energy can be converted into any other form


Examples:

Toaster:  Converts electrical energy into thermal energy.
Electric Motor: electrical energy converted to mechanical energy
Your Body-converts chemical energy in the food you eat into the mechanical energy you need to move your muscles.  Chemical energy in your body is also converted to the thermal energy your body uses to maintain its temperature. Chemical energy is also converted into the electrical energy your brain uses to think.

OFTEN, A SERIES OF ENERGY CONVERSIONS ARE NEEDED TO DO A TASK.

Example:

A match: Strike a match and the mechanical energy needed to move the match is converted to thermal energy. The thermal energy causes the match to release the stored chemical energy, which is converted to thermal energy, and then to the electromagnetic energy you see as light.

A car engine: Electrical energy produces a hot spark. The thermal energy of the spark releases the chemical energy of the fuel.  When the fuel burns, the chemical energy turns into thermal energy.  Thermal energy is converted into mechanical energy used to mvoe the car, and to electrical energy to produce more sparks



Energy cannot be created OR destroyed, but it can change form!
(This is much more important than it sounds, try
and remember it!  Trust me!)






THE LAW OF CONSERVATION OF ENERGY-  when one energy form is converted into another, NO ENERGY is destroyed in the process.  Energy cannot
be created OR destroyed.





ENERGY AND FRICTION


As the pendulum moves, it encounters friction at the pivot of the string and from the air through which it moves.  When an object experiences friction, the motion (and thus the kinetic energy) of the atoms or molecules increases.  This means the THERMAL energy increases.  So some of the mechanical energy of the moving pendulum is converted to THERMAL energy.  The pendulum slows down, but no energy was destroyed.




THERMAL ENERGY AND HEAT


Matter is made of tiny particles called atoms and molecules.   These particles are always in motion, even if the object they make up isn’t moving at all.  Since Kinetic Energy is the energy of motion, all particles of matter have KINETIC energy.  The faster the particles move, the more kinetic energy they have.  

Temperature is a measure of the average Kinetic Energy of the individual particles in the object.


TEMPERATURE SCALES:
The three common scales for measuring temperature are the Fahrenheit, Celsius, and Kelvin Scales.

Fahrenheit Scale:  In the US, the most commonly used temperature scale is the Fahrenheit scale.  On this scale, the number 32 is assigned to the temperature at which water freezes.  The number 212 is assigned to the temperature at which water boils.  The interval between these two temperatures is divided into 180 equal intervals, called DEGREES FAHRENHEIT. (ªF)



Celsius Scale-  The temperatue scale used in most of the world is the Celsius Scale.  On this scale, 0 is the number assigned to the temperature at which water freezes.  The number 100 is assigned to the temperature at which water boils.  The interval between freezing and boiling is divided into 100 equal parts, called DEGREES CELSIUS (ªC)

Kelvin Scale-  The temperature scale commonly used in physical science is the Kelvin Scale.  Units on the Kelvin scale are the same size as those on the Celsius Scale, and are called Kelvins (K).  Temperatures on the kelvin scale can be changed to Celsius degrees by adding 273.  So the freezing point of water is 273 K and the boiling point of water is 373 K.

The big deal with the number 273?  Experiments have led scientists to conclude that -273 ªC is the lowest temperature possible.  At this temperature, called Absolute Zero, no more energy can be removed from matter and molecular motion stops.  The Kelvin scale is defined so that zero on the Kelvin scale is absolute zero.

CONVERSIONS F TO C TO K
C TO F

-273ªC (+273) = 0 K (absolute zero)

0ºC (+273) = 273 K



Thermal Energy:

The total energy of all the particles in a substance is called Thermal Energy.  Even if two samples of matter are at the same temperature, they do not necessarily have the same Thermal Energy.  The more particles a substance has at a given temperature, the more thermal energy it has.


Heat- is the movement of thermal energy from a substance at a higher temperature to another at a lower temperature. (Heat is thermal energy moving from a warmer object to a cooler object)

Heat Transfer always occurs with heat moving to substances with less heat.  The heat flows from the warmer substance to the colder one.

How is heat transferred?
There are three ways that heat can move.  Conduction, Convection and Radiation

CONDUCTION-  In this process, heat is transferred from one particle of matter to another particle of matter without the  movement of the matter itself.  

CONVECTION-  In this process, heat is transferred by the movement of currents
within a fluid (a liquid or a gas).  

    CONVECTION CURRENTS- formed when hot things rise and cooler things
        sink.  
RADIATION- This is the transfer of energy by electromagnetic waves.  Radiation does not require matter to transfer the thermal energy.  This is a big difference, and an important one.




Interesting Factoid...


There's no such thing as cold.  Cold is just the absence of heat.


(So, cold doesn't seep into anything...)


CONDUCTORS AND INSULATORS


Conductor-  a material that conducts heat well.  
Metals are good conductors of heat.  

Insulator- A material that does not conduct heat well.
Wood, wool, straw, paper, cork, are all good insulators.

House Insulation:  Prevents heat from leaving the building in cold weather, and
prevents heat from entering the building in hot weather.  





States of Matter and Thermal Energy (Review)
Although the chemical composition of a substance stays the same, the arrangement of the particles that make up the matter differs from one state to another.
Solids- Particles in fixed positions, can vibrate only.  Fixed shape and volume
Liquids-Particles close together, but not held together as tightly, so they can move around.  
Gases- Particles are moving so fast that they don't even stay close together, gases expand to fill the space available.  No fixed shape or fixed volume.




FORCE AND MOTION
GRAVITY-the force of attraction between object with MASS. 
On Earth, it is the force that pulls objects towards Earth.

The LAW OF UNIVERSAL GRAVITATION states that the force of gravity acts between all objects in the universe.  ANY two objects in the universe, without exception, attract each other.

YOU are attracted to the Earth, but the Earth is also attracted to you.
 Also, every object around you are also attracted not only to the Earth, but also to YOU.  


Gravitational Force depends on MASS.  The force also depends on the DiSTANCE between the objects. 
The farther apart the objects are, the weaker the gravitational force between them.


Remember the diffenrence between MASS and WEIGHT.

Mass is a measure of the amount of matter in an object.
Weight is a measure of the force of gravity upon an object.





Motion

It's often difficult to tell if an object is in motion.  Right now, as you’re sitting in class, aside from your eyes blinking and your chest moving, and your head occasionally slamming forward on your desk, you probably would say that you are not moving.

An object is in motion when its distance from another object is changing.  Since the distance between you and me isn’t changing, you conclude that you’re not moving.

At the same time you're sitting there, you're actually moving at about 30km/sec.  At that speed, you can travel from NYC to LA in about two minutes!  You’re moving this fast because you’re sitting on planet Earth, which is orbitting the sun.  Earth moves 30 km every second in its path around the sun.

Whether or not an object is moving depends on your point of view.  

A REFERENCE POINT is a place or object used for comparison to determine if something is in motion.  An object is in motion if it changes its position relative to a reference point.


SPEED


SPEED IS THE DISTANCE AN OBJECT TRAVELS IN ONE UNIT OF TIME.


Speed is a type of RATE..  A RATE tells you the amount of something that happens, or changes, in one unit of time.
SPEED = Distance
            Time

    

Constant Speed: 
if the speed that an object travels doesn't change, it's
moving at a CONSTANT SPEED


Average Speed:
Most things don't travel at a constant speed.  To find
Average Speed, just divide TOTAL DISTANCE BY TOTAL TIME




VELOCITY-  It's not just another word for speed.


Velocity is speed in a given direction.  






ACCELERATION
Acceleration is the RATE AT WHICH VELOCITY CHANGES.


In science, ACCELERATION refers to INCREASING speed, DECREASING speed, or CHANGING DIRECTION.  It doesn't must mean "speeding up".





CALCULATING ACCELERATION

Acceleration = Final Velocity - Initial Velocity

         


Force

A FORCE IS A PUSH OR A PULL


The Force is Strong in this One...
The Force is Strong in this one...

Forces are described by not only how strong they are, but also by the direction in whcih they act.

Unbalanced Forces:  Objects may have a variety of forces acting upon them. When these forces are balanced, the object doesn’t move.
                When the forces are UNBALANCED, movement can happen.

                  Forces can ADD together and combine. Forces can work in opposite directions and subtract.
        
If one force is greater than the other, the overall force
is in the direction of the greater force.

The overall force on an object after all the forces are added
or subtracted is called the NET FORCE.


    

FORCE, MASS AND ACCELERATION ARE RELATED.
THE NET FORCE ON AN OBJECT IS EQUAL TO THE PRODUCT OF ITS ACCELERATION AND ITS MASS.

FORCE= Mass X Acceleration

ACCELERATION = Force
                            Mass

MASS = Acceleration
         Force





Momentum
The momentum of an object is the product of its mass and velocity


Momentum= Mass X Velocity

 

The more momentum an object has, the harder it is to stop.  You can catch a baseball moving at 20m/s, but you can’t stop a car moving at the same speed.  THe car has more momentum, because it has a greater mass.


A high velocity also produces a large momentum, even when mass is small. 
Example:  Bullet shot from rifle.
          Small kid running fast into a larger kid.
Small fast-moving fist hitting your chin.