Download PE = KE

Work and Energy
How do we calculate kinetic
and potential energy?
 Key Concepts
 Sample Problems
 Practice worksheets
Main ideas
•An object that has energy can do
work.
•Work is done when energy is
transferred from one object to
another or changed from one form
to another.
ENERGY A BODY HAS
BECAUSE OF ITS POSITION
OR CONDITION
P.E.= Ep = mgh
G.P.E. = weight x height
EXAMPLE
What is the p.e. of a book of
mass 1.2kg, resting on a shelf
2 m above the ground?
p.e. = mgh
p.e. = 1.2kg x 10m/s/s x 2m
p.e. = 12.0 N x 2 m
p.e. = 24 N-meter or Joules
SAMPLE PROBLEM:
A hiker weighing
680 N climbs 40 m up
a hill. Calculate the
gravitational potential
energy gained at the
top of the climb.
Ep= mgh
P.E.= mgh
Ep = 680 N x 40 m
= 27,200 N-m
or 27.2 KJ
KINETIC ENERGY
2
(E k) = ½ mv
KE
=
2
mv
2
http://www.youtube.com/watch?v=B
SWl_Zj-CZs&feature=related
Tutorial- Calculating PE and KE
POTENTIAL
ENERGY
ENERGY A BODY HAS
BECAUSE OF ITS POSITION
OR CONDITION
P.E.= Ep = mgh
G.P.E. = weight x height
POTENTIAL ENERGY
Ep or p.e. is the potential energy an
object has because of its height.
“The higher an object is lifted, the
greater its p.e.”
“The greater the mass of an object
the greater its p.e.”
1. Calculate the p.e. of a 40 kg iron
bar raised 1 m. (g = 10N/kg)
2. Calculate the p.e. of a 65 kg
boulder resting at 22 m above the
road.
3. A 200 g jar falls from a height 1.5
m onto the floor. Calculate the p.e.
of the jar before the fall.
ANSWERS
1. Calculate the p.e. of a 40 kg iron
bar raised 1 m. 400 N-m or 400J
2. Calculate the p.e. of a 65 kg
boulder resting at 22 m above
the road. 14,300 N-m or 14.3 KJ
3. A 200 g jar falls from a height 1.5
m onto the floor. Calculate the
p.e. of the jar before the fall. 3 J
KINETIC ENERGY
• Kinetic energy Ek or k.e. is the
energy that an object has because it
is moving
• The unit of measure for k.e. is joule
• The greater the mass of a moving
object, the greater the k.e. of the
object
What is the Ek of an
arrow weighing 0.025 kg
traveling at 25m/s?
What is the Ek of an arrow weighing
0.025 kg traveling at 25m/s?
Answer:
k.e. = ½ mv2
= ½ x 0.025kg x 25m/s2
= ½ x 0.025kg x 625m/s
= (0.5) 15.625
= 7.8125 Nm
but 1 Nm = 1J
= 7.8125 J
Work and Energy
How do we calculate kinetic
and potential energy?
 Key Concepts
 Sample Problems
 Practice worksheets
Potential Energy = Weight x Height (P.E. = w x h)
or Ep= mgh
Kinetic Energy = ½ Mass x Velocity2 (K.E.= 1/2mv2)
weight = mg
Work= F x d
Units:
Energy = joules (J)
Weight = newtons (N)
Mass = kilograms (kg)
Velocity = m/s
Work =
joules
Height = meters (m)
Gravity constant = 9.8m/sec/sec
1km = 1000m
1kg= 1000g
1hr= 3,600sec
SUMMARY OF CONCEPTS:
GPE = w x h or
GPE = mgh
•If h is doubled, the GPE is doubled.
If H is tripled, the GPE is also tripled.
•If speed (velocity) is doubled (2x), the Kinetic
Energy quadruples (increases 4x or 22).
If speed (velocity) is tripled (3X), the KE increases
9x or 32.
CONSERVATION OF ENERGY
http://www.youtube.com/watch?v=o
_5oYuDY2qM&NR=1
Intro
Law of CONSERVATION
OF ENERGY
“Energy cannot be
created nor destroyed.
Energy is converted
from one form to
another.”
PE
=
KE
WORK
P.E
.
K.E
.
WORK AND ENERGY
1. PE
= KE
2. Work = Energy
3.
Work = PE gained = KE
Work = KE