Download Energy - Hudson Falls Central School District

Student Responsibilities
•
Be willing and ready to learn
•
When I am talking, you are not
•
Do not hinder the learning of
your classmates
Energy
What is Energy?
 Energy
is the ability to do work.
What is Work?
• Work
is done when a force causes a
change in distance
Work
 Work
can be expressed as an equation
W=F d
W – Work (Joules, J or Newton-meter,
N m)
F – Force (Newtons, N)
d – Distance (meters, m)
Sample Problem

A man pushes a box across a floor with a force
of 20 N for 10 m. How much work does the
man exert?
Work vs. Impulse
 Impulse
is a force exerted over an
amount of time
 Work is a force exerted over a distance
 What
about how much work is done
over time?
Power
Power is the rate at which work is done
(how fast work is done)
 Equation

P=W
t
P – Power (Watts, W or Joules/second)
W - Work (Joules, J or Newton-meter, N m)
t – time (seconds)
Horsepower
 When
the steam engine first came out,
people wanted to know how they
compared to horses.
 James Watt expressed the power of his
steam engines in horsepower, or the
rate at which horses could do work.
 745 Watts = 1 Horsepower
Sample Problems

If a machine does 5000 J of work in 10 sec, what
is the power?
Energy
Energy is the ability to do work
 Energy is measured in Joules
 When work is done, energy is used.


Work, Power and Energy are scalar
quantities
Energy Comes in Different
Forms!
Nuclear
Not
Electrical
Every
Chemical
Cat
Heat
Hates
Mechanical
Mice
Potential Energy
Energy stored by an object because of its
position or its condition
 Gravitational Potential Energy
 Energy stored as a result of its height.

Gravitational Potential
Energy
PEgravity = mgh
m – mass (kilograms)
g – acceleration due to gravity (m/s2 )
h – height (meters)
Note – Relationship between Work and PE
Which has more PE?
Sample Problem

What is the PE of a 200 kg piano hoisted 10
meters above the ground?
Elastic Potential Energy
Elastic PE is found in a stretched or
compressed elastic object (springs)
 If a spring is not stretched or compressed,
then there is no elastic potential energy

Kinetic Energy
Kinetic Energy is the energy of motion
 Tells how much work is done to bring the
object from rest to its present speed

KE = ½ mv2
m – mass (kg)
v – velocity (m/s)
KE – Kinetic Energy (Joules)
Sample Problem

Find the K.E. of a 4.0 kg cart moving at 50 m/s.
Conservation of Energy
 Energy
cannot be created or
destroyed. It can be transformed
from one form into another, but
the total amount of energy
never changes.