Download 5.2 PE Notes

Chapter 5.2
Notes
Potential Energy
Energy that is dependent
upon height is called
potential energy.
Potential energy equation 1
= mass x gravity x height
Potential Energy = PE
PE = mgh
Potential energy
equation 2 =
weight x height
PE = wh
A ball is thrown up. At the top
of the path, how fast is the ball
moving?
0 m/s – Not moving
How much kinetic energy
does the ball have at the top
of the path?
0 – since it is not moving
 If the ball is thrown to a height of
5 meters and the ball has a mass
of 10 grams, what is the PE?
 Which equation? PE = mgh or
PE = wh
 PE = mgh
2
 PE = 10 g x 10 m/s x 5 m/s
 PE = 500 J
 If gravity is the only
force causing the
potential energy, than
the energy is called
gravitational
potential energy.
 A system can also
have potential energy
due to a spring.
Potential energy that is
stored in a spring is called
elastic potential energy.
The equilibrium of the
spring is its unstretched
position – where the spring
has no force.
 Equation for elastic potential
energy = ½ (spring constant)
2
(spring displacement)
 The spring constant is represented
by k
 The spring displacement or
distance the spring is stretched is
represented by x
2
 PEelastic = ½ kx
Spring constant of 180,000
and the spring is compressed
.15 m, what is the elastic
potential energy?
2
PEelastic = ½ kx
2
PEelastic = ½ 180,000 x .15
PEelastic = ½ 180,000 x .0225
PEelastic = 2025 J
As a mass accelerates on a
spring, the spring’s stored PE
is converted to kinetic energy.
When the mass reaches the
equilibrium position where x =
0, the Potential energy = 0 and
the kinetic energy is at the
maximum.
 The mass will continue to vibrate
back and forth, changing potential
energy to kinetic energy until the
mass stops moving.
 This process with the mass
vibrating right to left and back
again, changing kinetic to potential
and potential to kinetic is called
simple harmonic motion.
 When potential energy is converted
to kinetic energy, what forces take
away some of the energy?
 Friction, - Energy is transformed
into heat
 The law of conservation of
energy states that in an isolated
system, energy is conserved – it
cannot be created or destroyed.
Total energy 1 =
PE + KE
Total energy 2 =
2
mgh + ½ mv
 What is the total energy of a
ball that has KE of 10 and PE
of 10?
 Which Equation; TE = KE + PE
2
or TE = ½ mv +mgh?
 TE = KE + PE
 TE = 10 + 10
 TE = 20
 What is the total energy of a ball
that has a mass of 250 and a
height of 25 with a velocity of 40?
 Which Equation; TE = KE + PE or
2
TE = 1/2mv +mgh?
 TE = ½mv2 +mgh
 TE = ½ 250(40)2 + 250(10)25
 TE = ½ 250 (1600) + 250(10)25
 TE=200,000+62,500 =262,500 J