CHAPTER
4: WORK AND ENERGY

**THE
BIG IDEA: **Energy
is transferred when a force moves an object.

**SECTION
4.1: **Work is the use of force to move an object.

*Force
is necessary to do work*

· Work is the use of force to
move an object some distance.

· You
do work only when you exert a force on an object and move it.

· Reading
a page is not work. Turning the page is.

*Calculating
work*

·
Work =Force *x* Distance

·
W=Fd

·
When you multiply a force in newtons times a distance in meters, the product is one newton-meter, or the joule (J).

*Use
the formula for work to solve the following problem:*

*How much work is being done if a
person pushes a cart with a force of 70N for 2m? *

*W=140J*

*Objects
that are moving can do work*

·
You do work on objects, but objects also
do work .

·
Read
the second paragraph on page 118 of your book.

·
Throughout history, people have taken
advantage of the capability of objects in motion to do work. Waterwheels and Windmills are examples.

__________________________________________________________________

**SECTION
4.2: **Energy is transferred
when work is done.

*Work
transfers energy*

- Energy is the ability of
a person or an object to do work or to cause a change.
- When you do
work on an object, some of your energy is transferred to the object.
- You can
think of work as the transfer
of energy. In fact, both
work and energy are measured in the same unit, the joule.

*Work
changes potential and kinetic energy*

- Kinetic energy is the
energy of motion.
- Any moving
object has some kinetic energy. The
faster an object moves, the more kinetic energy it has.
- Potential energy
is stored energy, or the energy an object has due to its position or
shape.
- If you are
holding a ball above the ground, the ball has potential energy. The higher you hold the ball, the more
potential energy it has.

*Calculating
Gravitational Potential Energy*

- Potential
energy caused by gravity is called gravitational potential energy.
- This must
be taken into account when spacecraft are launched and when roller
coasters are designed.

Gravitational
Potential Energy= mass *x *gravitational
acceleration *x *height

GPE=mgh

- Calculate
the GPE:

If
you lift a 2kg box of toys to the top shelf of your closet, which is 3m high,
how much gravitational potential energy will the box of toys have?

*Hint: gravitational acceleration= 9.8m/s ^{2}*

*GPE=58.8J*

*Calculating
Kinetic Energy*

- Kinetic
Energy=
__mass__*x*velocity^{2 }

2

- Notice that
velocity is squared while mass is not.
That is because increasing the velocity of an object has a greater
effect on the object’s kinetic energy than increasing the mass.
- Calculate
the kinetic energy:

A grasshopper with a mass of 0.002kg jumps up at a
speed of 15m/s. What is the kinetic
energy of the grasshopper?

*KE=0.225J*

*Calculating Mechanical Energy*

- Mechanical energy is the
energy possessed by an object due to its motion or position—in other words
it is the object’s combined potential energy and kinetic energy.
- Mechanical
Energy = Potential Energy + Kinetic Energy
- ME= PE+KE
- For
example, a skateboarder has a potential energy of 200J due to his position
at the top of a hill and a kinetic energy of 100J due to his motion. His total mechanical energy is 300J.

*The
total amount of energy is constant*

- Energy is
transferred when work is done. No
matter how energy is transferred or transformed, all of the energy is
still present somewhere in one form or another. This is known as the conservation of energy.
- See the example of the in-line skater on page
126.

*Forms
of energy*

- Each of
these terms is also a combination of potential and kinetic energy.
**Thermal energy**- Energy an object has due to the motion of its molecules.**Chemical energy**- The energy stored in chemical bonds that hold chemical compounds together. If these bonds are broken, then energy is released or absorbed (fireworks).**Nuclear energy**- The potential energy stored in the nucleus of an atom. The source of the Sun’s energy is nuclear energy.**Electromagnetic energy**- the energy associated with electrical and magnetic interactions. Examples are electrical energy and radiant energy (the energy carried by light).