Download energy a body has because of its position or

ENERGY A BODY HAS
BECAUSE OF ITS POSITION
OR CONDITION
P.E.= Ep = mgh
G.P.E. = weight x height
KINETIC ENERGY
2
(E k) = ½ mv
KE
=
2
mv
2
POTENTIAL
ENERGY
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 1200 g, 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.= Ep= mgh
Ep = 680 N x 40 m
= 27,200 N-m
or 27.2 KJ
1. Calculate the p.e. of a 40 kg iron
bar raised 1 m. (g = 10m/s/s)
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
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.”
the gain in p.e. = work done
Ep = W
gain in p.e. = work done
Ep = W
Remember: W = F x d where
the lifting force is m x g
(F= ma)
(m = mass, g = acceleration
due to gravity)
Putting these formulae together:
Gain in Potential energy= work done lifting
Ep = W
= lifting force x distance lifted (height)
Ep = F x d (h)
= weight force x distance lifted
Ep = Fg x d (h)
(g = 10N/kg)
= mass of object x g x distance lifted
Ep = mgh
KINETIC ENERGY
2
(E k) = ½ mv
KE
=
2
mv
2
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 speed the greater the
k.e. of the object
• The greater the mass of a moving
object, the greater the k.e. of the
object
What is the Ek of an arrow
weighing 25 gm traveling at
25m/s?
What is the Ek of an arrow weighing 25
gm traveling at 25m/s?
Answer:
k.e. = ½ mv2
= ½ x .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
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
Units:
Energy = joules (J)
Weight = newtons (N)
Mass = kilograms (kg)
Velocity = m/s
Height = meters (m)
Work = joules
Gravity constant = 9.8m/sec/sec
1km = 1000m
1kg= 1000g
1hr= 3,600sec
Energy cannot be created
nor destroyed. Energy is
converted from one form to
another.