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12/22/12
1104 Period 2 Solutions: Forces and Energy
2.1 What Are the Four Fundamental Forces?
1) Defining the fundamental forces: Based on your class activities and your instructor’s
explanation, give examples of each of the fundamental forces.
a) Gravitational Force
An attractive force between all objects. The gravitational force causes
objects to roll downhill, causes dropped objects to fall toward the Earth,
and holds the Earth in an orbit around the Sun. Example: an unsupported
object falls to the ground.
b) Electromagnetic Force
An attractive force between particles of opposite charge, and a repulsive
force between particles with like charge (both positive or both negative).
The electromagnetic force holds atoms together and causes charged
objects to attract or repel one another. Example: when the negatively
charged styrofoam ring is repelled by the negatively charged rod, the ring
floats above the rod.
c) Strong Nuclear Force
An attractive force that holds protons and neutrons together in the nuclei
of atoms. The strong force acts only when particles are close together.
Example: all matter consisting of atoms.
d)
Weak Nuclear Force
The force responsible for radioactive decay of atomic nuclei, such as the
decay that produces radon gas. Example: decay of radioactive samples.
e)
Group discussion question: Which of the fundamental force(s) is/are responsible for
the force of friction?
The electromagnetic force between the atoms in an object and the atoms
in the surface the object moves across is responsible for the force of
friction. Gravitational force pressing an object down onto a surface can
increase the frictional force between the object and the surface.
2.2 Forces, Work, and Energy
2)
The units of work and energy: Work is done when a force moves an object. Energy
is required to do work.
a) Work and energy are measured in units of joules.
1 joule = 1 kg m2/s2
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Force is measured in units of newtons.
1 newton = 1 kg m/s
Explain why a joule can also be called a newton meter.
Write the units of energy as 1 kg m2/s2 = 1 kg (m/s2) m = 1 N m
___W = F x D__ The unit of work and energy (joule) is the product of units
of force (newton) times units of distance (meter).
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b)
3)
If you did one joule of work, how much energy did you exert to do this work?
(Assume no energy was wasted.)
_1 joule _
A truck does work: In this activity, we measure the work done against the force of
friction by a toy truck pulling a block connected to a spring scale.
Once the truck and block are moving at a constant velocity, how many newtons of force
does the truck exert on the block?
_____________
How many meters did the truck pull the block?
_____________
Calculate the amount of work the truck did to pull the block that distance.
use W = F D
_________
Place 300 grams (0.3 kilograms) of mass on top of the block. How many newtons of
force does the truck exert on the moving block with 0.3 kg of mass on it? _________
How much work must the truck do to move the block with 0.3 kg of mass the same
distance as in question 2?
use W = F D
___________
Why does the truck do more work when mass is added?
Adding mass to the block increases the force of friction between the block
and the floor. The truck must exert a greater force to overcome the force
of friction between the block and the floor.
2.3 How are Forms of Energy Defined?
4) Forms of energy: Your instructor will discuss the forms of energy that we will study in
Physics 1104. Give an example of each form of energy listed below.
a)
Kinetic Energy ( also called Mechanical Energy of Motion)
Energy exhibited by objects in motion. Example: any moving
object.
b)
Thermal Energy
Unorganized energy of motion exhibited by vibrations of atoms and
molecules. Example: rub hands together.
c)
Sound Energy
Organized energy of motion exhibited by vibrations of atoms and
molecules. Example: any sound.
d)
Electrical Energy
Energy resulting from the forces between charged particles.
Example: Electrical energy from a hand-cranked generator lights a
bulb.
e)
Magnetic Energy
Energy resulting from the forces between magnets, causing the
magnets to attract or repel. Example: one magnet floats above
another magnet.
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f)
Radiant Energy
Energy resulting from the vibrations of charges, such as radio
waves, microwaves, light rays, and X-rays. Example: radiant energy
from a floodlight shining on a solar cell powers the free play radio.
g)
Gravitational Potential Energy
Energy stored in raised objects, which have the potential to fall.
Examples: water wheel or any raised object.
h)
Strain Potential Energy
Energy stored in stretched or compressed objects, such as springs.
Examples: music box and wind-up toys.
i)
Electrical Potential Energy
Energy stored in the separation of static (not moving) electric
charges. Example: capacitor
j)
Chemical Potential Energy
Energy available in the chemical bonds between atoms or molecules.
Example: battery
k)
Nuclear Energy
Energy available in the nucleus of atoms that can decay. Examples:
smoke detector, the Sun
2.4 What Happens When Energy Is Converted from One Form to Another?
5)
a)
Energy conversions:
Observe the demonstration of an exercise bicycle connected to large light bulbs. List
the energy conversions that occur when the bulbs light.
chemical potential energy
energy, and sound energy
kinetic energy
radiant energy
electrical energy, thermal
Was any energy wasted as the bicyclist generated electricity? _yes__ Explain.
Thermal energy was wasted from friction between the moving parts of the
bicycle and by heat emitted from the incandescent bulbs. Sound energy
was also wasted.
b)
c)
Observe the demonstration of the waterwheel. List the energy conversions that
occur when water in the raised bucket lifts the plastic bottle.
Gravitational potential energy of the water decreases as it flows downhill
kinetic, thermal, and sound energy turning the waterwheel and
gears
gravitational potential energy increase of the bottle as it is lifted
Connect a hand-cranked generator to a solar cell. Shine the spotlight onto the solar
cell. List the energy conversions that take place.
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Electrical
radiant, thermal
electrical
kinetic, thermal and sound
List any forms of wasted energy: thermal and sound
d)
Group Discussion Question: How is the Law of Conservation of Energy illustrated by
these energy conversions?
In each of the energy conversions, the total amount of energy into the
conversion process equals the total energy out: the sum of the useful
energy plus the wasted energy output.
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