CP PHYSICS
ENERGY NOTES
Work is the transfer of energy by mechanical means. This energy transfer results
from motion.
Two types of energy:
Kinetic energy (KE)
Potential energy (PE)
Kinetic energy
 energy of moving objects
 measure in joules (J)
 change in kinetic energy of an object is equal to the net work done on it
 faster an object moves the more kinetic energy it has
 equation:
KE = ½ mv2
Potential energy
 energy stored in an object because of its state or position
 gravitational potential energy depends on an object’s height above the
surface of the Earth
 potential energy is usually measure from the surface of the Earth of the
floor of a room
 the higher an object is above the surface of Earth, the greater its potential
energy is
 equation:
PE = mgh
Conservation of Energy
 Law of conservation of energy states that within a closed, isolated system,
energy can change form but the total amount of energy is constant.
 Example: Man diving from high dive into a pool. At top of high dive the
man has 100% potential energy but his kinetic energy is 0%. As he dives,
his potential energy decreases and his kinetic energy increases. At the
very bottom of the dive, just as he enters the pool, his potential energy has
decreased to 0% and his kinetic energy is 100%. Therefore, as the man
falls, his potential energy is converted into kinetic energy. At each point in
his path down into the pool the potential and kinetic energies add up to
100%.
Example Calculations
1.
What is the kinetic energy of a 6.8 kg bowling ball rolling with a
velocity of 1.8 m/s?
m = 6.8 kg
v = 1.8 m/s
KE = ?
KE = ½ mv2
= ½ (6.8)(1.82)
KE = 11.02 J
CP PHYSICS, ENERGY NOTES, page 2
Example Calculations (cont’d)
2.
What is the kinetic energy of a 468 N girl who runs around a track
with a velocity of 0.55 m/s?
FW = 468 N
m = 468/9.8 = 47.76 kg
v = 0.55 m/s
KE = ?
3.
A 2.2 kg book is lifted from the floor and placed on a bookshelf 1.75 m
from the floor. What is the book’s gravitational potential energy?
m = 2.2 kg
h = 1.75 m
g = 9.8 m/s2
PE = ?
4.
PE = mgh
= FWh
= (524)(8.5)
PE = 4454 J
What is the change in kinetic energy of a 3.5 kg object which has it
velocity increase from 12 m/s to 25 m/s?
m = 3.5 kg
v1 = 12 m/s
v2 = 25 m/s
KE2 = ½ mv2
= ½ (3.5)(25)2
KE2 = 1094 J
6.
PE = mgh
= (2.2)(9.8)(1.75)
PE = 37.73 J
A 524 N boy stands on a high dive platform that is 8.5 m above the
surface of the pool. What is the boy’s potential energy?
FW = 524 N
h = 8.5 m
PE = ?
5.
KE = ½ mv2
= ½ (47.76)(0.552)
KE = 7.22 J
KE1 = ½ mv2
= ½ (3.5)(12)2
KE1 = 252 J
∆KE = KE2 – KE1
= 1094 – 252
∆KE = 842 J
A 1500 kg car is driven over the edge of a cliff during a movie stunt.
What is its kinetic energy after it has dropped 8.0 m?
m = 1500 kg
h = 8.0 m
g = 9.8 m/s2
KE = PE = mgh
= (1500)(9.8)(8.0)
KE = 117,600 J