Download 5.2 Physics section 5.2

Physics Section 5.2
Define and apply forms of mechanical energy.
Energy is the ability to do
work.
Kinetic energy is the energy
of an object due its motion.
The kinetic energy of a
moving object is directly
proportional to its mass and
the square of its velocity
Forms of mechanical energy
1. Kinetic energy
2. Potential Energy
a. Gravitational
b. Elastic
KE = ½ mv2
KE = kinetic energy (J)
m = mass (kg)
v= velocity (m/s)
The unit of energy is the Joule.
Example:
Find the energy of a rock that has a mass of .25 kg and a velocity
of 35 m/s. What happens to its energy if the mass is doubled?
What happens to its energy if its velocity is doubled?
problem
How fast would a 65 g bullet have to travel to
have the same amount of kinetic energy as a
140 g bullet with a velocity of 180 m/s?
Work – Kinetic Energy Theorem
The net work done by all the forces acting on an object
is equal to the change in the objects kinetic energy.
Wnet = ∆KE
Wnet = ½ mvf2 - ½ mvi2
m = mass (kg)
vf = final velocity (m/s)
vi = initial velocity (m/s)
problem
A net force of 56 N is applied to a stationary object.
For what distance must the force be applied in order
for the object to have a kinetic energy of 420 J?
problem
A force of 75 N is applied to a 15 kg object causing it
to accelerate. How far must it travel to have a
velocity of 5.0 m/s?
Potential Energy is the energy associated with an object
because of its position, shape, or condition of the object.
Types of potential energy
1. Gravitational – due to its position in a gravitational
field
2. Elastic – due to a deformation of an elastic object
The gravitational potential
energy of an object is
directly proportional to its
mass, height, and the
acceleration due to gravity.
PEg = mgh
PEg = gravitational potential energy(J)
m = mass (kg)
g = acceleration (m/s2)
h = height (m)
problem
A 55 kg object is raised 2.5 m above the surface of
the Earth. Find its gravitational potential energy.
Elastic Potential Energy
PEelastic = ½ k·x2
PEelastic = elastic potential energy(J)
k = spring constant (N/m)
x = distance stretched or
compressed (m)
http://uk.youtube.com/watch?v=z8bVx43qlHo
problem
A spring with a force constant of 18 N/m has a
relaxed length of .75 m. The spring is stretched to a
length of 1.00 m. Find its elastic potential energy.
problem
A man has a mass of 75 kg and is attached to an
unstretched bungee cord of length 18 m. He jumps
off a 85 m high bridge and stops 22 m above the
ground. If the spring constant of the bungee cord is
65 N/m, what is his total potential energy when he
stops falling?
assignment
• Page 172
• Problems 1 - 4