Download Notes 23, Energy

Notes 23: Energy (or the ability to do work)
Objective 1: Define kinetic energy.
1. Kinetic Energy (KE) - the energy an object has because it is moving; a scalar quantity.
 2  mv
KE  1
2
 s
KE  kg  m
2
 kg  m
s2
 m  N  m  Joule
2. KE depends on speed and mass.
3. The Work-Kinetic Energy Theorem
“The net work done on an object is equal to the change in the kinetic energy of the object.”
Objective 2: Describe the concepts of “gravitational potential energy” and “elastic potential
energy.”
4. Potential Energy (U or PE) - the amount of energy “stored” in a system due to its position or
configuration, i.e., the ability to do work at some time in the future; a scalar quantity.
5. Types of Potential Energy - gravitational, elastic (spring), nuclear, electrical, chemical;
6. Gravitational Potential Energy (Ug) - potential energy held by an object due to its height
above ground level.
U g  mass  gravity  height  mgh
energy units  kg  m
s2
 m  N  m  Joules
7. Elastic (spring) Potential Energy (Ue) - the energy stored in a spring or other elastic material
due to it being either stretched or compressed;
U e  1  spring  k   displacement  1 kx 2
2
2
energy units  N  m2  N  m  Joules
m
where k = spring constant
and x = spring displacement
Objective 3: State the principle of “conservation of energy.”
8.
The total energy of a system remains constant in spite of being transformed from one form to
another.
Total E = KE + U + Q (heat) + W = a constant
9.
Energy conservation occurs even when acceleration varies.
10.
Mechanical energy is not conserved in the presence of friction.
Objective 4: List different forms of energy and describe examples of the transformation of
energy from one form to another.
11.
Different forms of Energy:
a. Mechanical Energy
-- Kinetic Energy (KE) – mechanical energy of motion;
-- Potential Energy (U) – mechanical energy of position or condition
-- Rotational Energy – mechanical energy due to spin;
b. Wave Energy
c. Heat or Thermal Energy (Q)
d. Mass Energy – energy holding the nuclei of atoms together
e. Chemical Energy
f.
Sound Energy
g. Light Energy
12.
Energy Transformation
a. Conservative forces
1) A force is conservative if the total work it does on a
particle is zero when the particle moves around any closed
path returning to its initial position.
2) Gravity is an example of a conservative force. It can
transform energy back and forth from Ug and KE.
3) Springs can transform energy back and froth from Ue
and KE.
b. Non-conservative forces. Frictional forces almost always cause heat (Q) and vibration
(sound), two other forms of energy, to be generated. As heat and noise is generated, KE
energy is lost.
c. Example (assuming no frictional forces):
1. A sledgehammer (10 kg) object is raised 10 m above the ground by 980 J of work;
2. The Ug has been increased by 980 J by being raised 10 m;
3. Allowed to fall to the ground, the hammer achieves a v2 of 14 m/s and a KE of 980 J;
4. Upon striking a stake in the ground, the sledgehammer can do 980 J of work on the
stake, driving it into the ground a certain distance;
NOTICE: the energy involved is always 980 J.
d. Example – Bobsled run
1.
2.
2.
All potential energy at the top.
All kinetic energy at the bottom.
The total energy remains constant.