4: WORK AND ENERGY
BIG IDEA:† Energy
is transferred when a force moves an object.
4.1: Work is the use of force to move an object.
is necessary to do work
∑ Work is the use of force to
move an object some distance.†
do work only when you exert a force on an object and move it.†
a page is not work. ††Turning the page is.†
Work =Force x Distance
When you multiply a force in newtons times a distance in meters, the product is one newton-meter, or the joule (J).
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?††††
that are moving can do work
You do work on objects, but objects also
do work .
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.†
4.2:† Energy is transferred
when work is done.
- 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.†
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
- 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.†
Gravitational Potential Energy
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.†
Potential Energy= mass x gravitational
acceleration x height
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/s2
Energy= ††††††mass x velocity2 †
- Notice that
velocity is squared while mass is not.†
That is because increasing the velocity of an object has a greater
effect on the objectís kinetic energy than increasing the mass.†
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?
Calculating Mechanical Energy
- Mechanical energy is the
energy possessed by an object due to its motion or positionóin other words
it is the objectís combined potential energy and kinetic energy.
Energy = Potential Energy + Kinetic Energy
- ME= PE+KE
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.
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
- Each of
these terms is also a combination of potential and kinetic energy.
an object has due to the motion of its molecules.
energy-† The energy
stored in chemical bonds that hold chemical compounds together.† If these bonds are broken, then energy
is released or absorbed (fireworks).
energy-† The potential
energy stored in the nucleus of an atom.†
The source of the Sunís energy is nuclear energy.
energy-† the energy
associated with electrical and magnetic interactions.† Examples are electrical energy and
radiant energy (the energy carried by light).†