3. Energy: Main Ideas
Energy is the ability to do work.
The law of conservation of energy says that energy can
be neither created nor destroyed. Energy can change
from one form to another, but total energy always
remains unchanged.
Work and energy are closely related. When work is
done on a system, the system gains potential or kinetic
energy. When a system has energy, that energy can be
used to do work.
Units of energy and work are the same.
SI = Newton Meters or Joules
English = foot-pounds.
4. Potential vs. Kinetic Energy
Potential Energy = Stored Energy
Kinetic Energy = Energy in motion
If a force changes and object’s vertical position on
earth, the object is given potential energy.
Gravitational potential energy is the energy
gained because of a change in an object’s
vertical position.
An unbalanced force changes and object’s motion
and causes an increase or decrease in it’s
speed. Kinetic energy is the energy something
has because of it’s speed.
In many systems, energy changes back and forth
between potential and kinetic energy. If there is
resistance, some kinetic energy is changed to
heat energy.
6. Newton’s First Law
Newton’s First Law (Law of Inertia): Every
object will remain at rest, or will continue to
move in a straight line with constant speed,
unless the object is acted on by a non-zero
net force.
Inertia: The property of an object to resist
changes in it’s motion (an objects mass tells
you how much inertia that it has).
8. Potential Energy (Mechanical and
Fluid) : Main Ideas
If an object of weight (w) is raised a
vertical distance (h), the gravitational
potential energy (Ep ) is equal to w * h.
Elastic potential energy equals the work
done to stretch or compress elastic
objects such as springs.
9. Energy (Mechanical and Fluid):
Main Ideas Continued
Energy = Ability to work
Potential Energy Kinetic Energy
Ep = wh (English) Ek = ½ mv2
Ep = mgh (SI) Ek = ½ Iω2
Note: (wh) = (mgh) or Nm = kg*m*m/sec2
Therefore Nm = kg *m2/sec2 = J
Fluid
Ep = weight x height (Eng) * Weight = ρw * V ρw = 62.416 lbs/ft3
Ep = mgh (SI) * m = ρ * v ρ = density = 1000 kg/m3
10. Potential Energy Problems
W = ________lbs
W = ________N
h = ________ft
h = ________m
Student X has just finished climbing to the top of a tower to drop
water balloons on unsuspecting passersby. If Student X has weight
of 205 lbs (including water balloons) and the tower has a height of
40 ft how much Gravitational Potential Energy-in Joules-was added
to this system??? W = _____ J
11. Elastic Potential Energy
Springs have a constant (k) which indicates how flexible (stiff) the
spring is. This constant can be found using the fomula: k = f/d
k = spring constant
f = force required to stretch or compress the srping
d = distance the spring is displaced
After you know the spring constant (k), you can then calculate the
elastic potential energy using the formula:
13. Kinetic Energy Problems
Kinetic Energy is mass in motion
(Ek = ½ mv2)
Two Foot-ball players crash into each
other during a game. Player A is the
water boy who has a mass of 50 kg (110
lbs or 490 N) but is running at a speed of
4 m/sec. This player runs directly in to
Player B who is the defensive tackle that
has a mass of 100 kg (220 lbs or 980 N)
and is moving at 2m/sec.
Who has the greatest kinetic energy
when they crash?
If you are playing baseball, is it better to
use a bat twice as big as normal, or to
swing the bat two times faster than
normal?
14. Kinetic Examples (cont.)
Given: (Ek=1/2mv2)
A wrecking ball that weighs one ton moves with
a speed of 30 ft/sec just before it slams into the
side of a building.
note: acceleration due to
gravity is 32 ft/s2
Find:
Kinetic energy of wrecking ball when it hits the
wall
15. Potential Energy in a Fluid System
When a water pump lifts water from a reservoir to a water tower, the work
done is changed to potential energy. The formula used to calculate this fluid
potential energy is the same as the formula for mechanical.
17. Newton’s First Law
Newton’s First Law (Law of Inertia): Every
object will remain at rest, or will continue to
move in a straight line with constant speed,
unless the object is acted on by a net force.
Inertia: The property of an object to resist
changes in it’s motion (an objects mass tells
you how much inertia that it has).
18. Moment of Inertia (Spinning Mass):
A property that describes an objects resistance to change in
rotational motion. The moment of inertia is determined by:
1. Location of Axis
2. Mass
3. How the mass is distributed
