Transcript Mechanical Energy - Mrs. Procee's Online Classroom

Mechanical Energy
Section 2.4
Objectives
• define mechanical energy
• solve problems calculating mechanical
energy, potential and kinetic energy
• describe the law of conservation of energy
and relate to energy conversion examples
Mechanical Energy
• Energy due to motion and the position of the
object
• Includes both kinetic and potential energy
Mechanical energy = kinetic energy + potential
energy
Em = Ek + Ep
Practice
• Grandin Ghosts football quarterback throws
a 0.500 kg football towards the end zone in a
straight line. At the height of 9.5 m above
the Earth, it has a speed of 18.0 m/s. What
is the total mechanical energy of the
football?
Practice
• A Grandin Ghost Marching Band Color
Guard member is practicing tossing her flag
up in the air. At the top of the throw, the
0.650 kg flag has a total mechanical energy
of 40.0 J and is moving at 2.60 m/s.
Calculate the gravitational potential of the
flag.
Practice
• Nigel, the pelican, is flying horizontally at 8.00 m/s,
carrying Nemo from the dentist’s office back to the
ocean. If Nemo has a mass of 300 g, and Nigel is
30.0 m above the ground, calculate the mechanical
energy.
Practice
• Rapunzel drops her 2800 g worth of hair down to
the prince from her tower. When her hair is 50.0 m
above the ground, it has a total mechanical energy
of 3.65 x 103 J. Calculate the kinetic energy of her
hair.
Law of Conservation of
Energy
• What does this law mean?
• Total amount of energy in a given situation
remains constant
• Energy converted from one form to another
• Total energy does not change
• Kinetic energy converted to potential and vice
versa, but total mechanical E remains constant
Em = Ek + Ep
Law of Conservation of
Energy
• Example: Energy Conservation – Weighed Spring
http://www.learnalberta.ca/Search.aspx?lang=en&search=energy+conser
vation+weighted+spring&grade=&subject=&audience=&language=&form
at=&type=
Law of Conservation of
Energy
Rearranging formulas
• When potential energy = kinetic energy, then
solve for the following quantities:
• Height
• velocity
Practice
• Ms Sinclair decides that as an end of the
semester joke, she will drop a 10.0 kg water
balloon on her students. If the water balloon
is dropped from the third floor stairwell all
the way to the bottom floor, a distance of
12.0 m, what will be the speed of the balloon
just before it hits the floor?
Practice
• The Cullens and wolves are preparing to battle the
oncoming newborn vampire army. Jasper jumps
vertically in the air to avoid a clash between Jacob
and a newborn. If he has a mass of 65.0 kg and
jumps with a initial speed of 14.3 m/s, how high
will he rise?
Practice
• Merry and Pippin are travelling through Fangorn
forest with Treebeard, the Ent. Merry is eating an
apple when he suddenly drops it while laughing at a
hobbit joke that Pippin made. If Merry is 11.0 m
from the ground, what velocity does the apple reach
just before it hits the ground?
Pendulum
• How does a pendulum work?
• Draw out the movement of the pendulum:
where would it have maximum gravitational
potential energy? Maximum kinetic energy?
How does it demonstrate law of
conservation of energy?
Practice
• A pendulum consists of a 500 g metal ball
suspended on a 50.0 cm string. The ball is
pulled horizontally and up a total vertical
distance of 10.0 cm. It is then released. At
the bottom of the arc, the mechanical energy
of the ball was determined to be 0.491 J.
What was the speed of the ball at the bottom
of its arc?
What do we look for in a
question?
• What are key words I should look for when solving
a problem?
• How can I tell the difference between:
•
•
•
•
Kinetic energy?
Potential energy?
Mechanical energy?
Conservation of energy?
Practice
• A 60.0 kg athlete jumps vertically upward from the
ground to a height of 0.910 m above the ground.
What was the athlete’s initial vertical speed?
• A baseball with a mass of 300 g has a kinetic energy
of 304 J. Calculate the speed of the baseball.
Practice
• Standing on level ground, a person with a mass of
55.0 kg jumps straight up into the air to a position
where the person has gained 800J of gravitational
potential energy. How high did the person leap?
• A construction worker drops a 2.00 kg hammer from
a roof. When the hammer is 50.0 m above the
ground, it has a total mechanical energy of 1.88 x
103 J. Calculate the kinetic energy of the hammer.