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
object, so the wind has
Work is done when
a FORCE moves an OBJECT some
When an object or organism does work on an object, some of its
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
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
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
KINETIC ENERGY ALSO INCREASES AS
would have to do more work on the bowling ball to
give it a
So KINETIC ENERGY depends on both MASS AND VELOCITY
FORMULA to calculate KINETIC ENERGY IS:
Kinetic Energy= Mass
at the formula. Do changes in velocity and mass have the SAME
Kinetic Energy? No, changing the velocity will have a greater
effect on Kinetic
Energy than changing its mass. This is because velocity is
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
when you transfer energy to an object, you change its position or
For example: you lift a book up to a shelf, or you compress a spring to
wind a toy.
Kinetic Energy, which is the energy of motion, Potential Energy is stored.
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
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
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
and Potential Energy involve objects being moved or physically
Both kinetic energy and potential energy have a variety of different
of the major forms of energy are :
MECHANICAL ENERGY- This is
energy associated with the motion or position of an
object. Mechanical energy can occur as kinetic energy OR
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 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.
RELATIONSHIP AMONG VISIBLE LIGHT, THE EM SPECTRUM AND
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
Energy Conversion and Conservation
change from one form of energy to another is called ENERGY CONVERSION or
an ENERGY TRANSFORMATION.
form of energy can be converted
into any other form
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.
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!)
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
Temperature is a measure of the
average Kinetic Energy of the individual particles in the object.
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
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
0ºC (+273) = 273 K
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)
Transfer always occurs with heat moving to substances with less
heat. The heat flows from the warmer substance to the colder one.
is heat transferred?
are three ways that heat can move. Conduction, Convection and
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
CURRENTS- formed when hot things rise and cooler things
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.
There's no such thing as cold.
Cold is just the absence of heat.
(So, cold doesn't seep into
CONDUCTORS AND INSULATORS
Conductor- a material that
conducts heat well.
Metals are good conductors of
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
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.
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.
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
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.
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.
IS THE DISTANCE AN OBJECT TRAVELS IN ONE UNIT OF TIME.
is a type of RATE.. A RATE tells you the amount of something that
happens, or changes, in one unit of time.
SPEED = Distance
if the speed that an object travels doesn't change, it's
moving at a CONSTANT SPEED
Most things don't travel at a constant speed. To find
Average Speed, just divide TOTAL DISTANCE BY TOTAL TIME
It's not just another word for speed.
is speed in a given direction.
is the RATE AT WHICH VELOCITY CHANGES.
science, ACCELERATION refers to INCREASING speed, DECREASING speed, or
CHANGING DIRECTION. It doesn't must mean "speeding up".
Acceleration = Final Velocity - Initial Velocity
FORCE IS A PUSH OR A PULL
The Force is Strong in this one...
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
When the forces are UNBALANCED, movement can happen.
Forces can ADD together and combine. Forces can work in opposite directions and
If one force is greater than the other, the
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.
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 = Acceleration
The momentum of an object is the product of its mass and velocity
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.
high velocity also produces a large momentum, even when mass is
Example: Bullet shot from rifle.
Small kid running
fast into a larger kid.
Small fast-moving fist hitting your chin.