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3.13.6

Question 110 pts

A 319 kg motorcycle is parked in a parking garage. If the car has
35,494 J of potential energy, how many meters above ground is the
car? Report your answer to 1 decimal place. Please do not include
units or the answer will be marked incorrect.

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Question 210 pts

A box sitting on the top of a hill has 252 J of potential energy.
If the hill is 279 meters above ground, what is the box’s mass?
Report your answer to two decimal places. Please do not include
units or the answer will be marked incorrect.

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Question 310 pts

How much potential energy does a 7.4 kg object gain when it is
lifted a distance of 1.4 meters? Report your answer to the nearest
whole number. Please do not include units or the answer will be
marked incorrect.

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Question 410 pts

A 30.5 kg cart has a velocity of 5 m/s. How much kinetic energy
does the object have? Report your answer to 1 decimal place. Please
do not include units or the answer will be marked incorrect.

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Question 510 pts

How fast is a 55.2 kg lion moving if it has 2,641 joules of energy?
Please report your answer to 1 decimal place. Please do not include
units or the answer will be marked incorrect.

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Question 610 pts

Object A has a mass of m with a speed of v. Object B has a mass of
m and a velocity of 2v. Compared to the kinetic energy of Object A,
the kinetic energy of Object B is

Group of answer choices

1/4 as large

4 times larger

1/2 as large

2 times larger

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Question 710 pts

If the speed of an object doubles and its mass is halved, how does
the kinetic energy change?

Group of answer choices

It is halved

It is multiplied by 4

It doubles

It is multiplied by 1/4

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Question 830 pts

A 100 kg roller coaster is at the peak of the first hill 75 m above
ground. A second later it is at the bottom of the hill.

a. Create an LOL chart for the following situation. Ignore friction
and energy loss.

b. How fast was the roller coaster moving?

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Potential and Kinetic Energies

In the previous lesson, we looked at how work and energy are
related. In this lesson, we will look at two of the most common
forms of mechanical energy.

Potential Energy

One type of energy that we encounter is known as potential energy.
Potential energy is the energy of an object or system that is based
on the location or position of the object. Remember, energy is the
ability of the object to do work. So potential energy means that
the object could potentially use its current position to do work.
For instance, an object on top of a three story building could do
work if it fell to the ground. The work done would be the force
exerted on the object by gravity times the distance the object
fell. An object with potential energy does not need to be moving or
doing work. It only has the potential to do work at some point in
the future.

For the purposes of this course, we will look at two different
forms of mechanical potential energy; gravitational potential
energy and elastic potential energy. Elastic potential energy will
be covered in the next lesson, so we will just discuss
gravitational potential energy here.

Image from Wikipedia:
http://upload.wikimedia.org/wikipedia/commons/b/be/Gravitational_field_Earth_lines_equipotentials.svg

Gravitational Potential Energy (GPE)

Gravitational potential energy is the energy an object has because
of its position inside of a gravitational field. Since gravity
always tries to move the object toward the center of the field, we
will assume the field points directly downward. An object's
gravitational potential energy is based on how far off the ground
the object is.

The height of the object is actually a relative height based off of
a reference point. In most instances, the reference point will be
the ground. Changing the reference point will change the amount of
potential energy that the object has, even though the position of
the object hasn’t actually moved. For this reason, we often do not
look at the potential energy at a specific location, but how the
potential energy changes as the position of the object changes.
Let’s look at how to define potential energy based on our
definition of work.

We know that work is force times distance.

W = f * d

Potential energy is the ability of a stationary object to do work
so the force in the work equation is the force of gravity. The
distance in the equation is equal to the height of the object (h)
Recall, Fw = mass times the force of gravity or Fw = m * g. Combing
these equations we get:

W = m * g * h

Since potential energy (PE) is the ability to do work, we can
write:

PE = m * g * h

Since the reference height is something we choose, potential energy
is relative to the object’s position. If our reference point for a
book on a desk is the ground, then the book has the potential to
fall down. The book has positive potential energy because the
object could do work by falling to the ground.

If our reference point was the top of the desk, then the object
would not have any potential energy, because it was already at the
reference point. If we chose the reference height as the ceiling of
the room, our book would have negative potential energy. Since the
book cannot fall up to the reference height by itself, energy would
be needed to do work just to get it to the ceiling.

Example Problem 1

A 1 kg book sits on a desk 1.5 m above the floor. How much
potential energy does the book have?

GPE = m * g * h

GPE = 1 kg * 9.8 m/s2 * 1.5 m = 29.4 J

Example Problem 2

A man carries a box on the top of his head. If the box has 150 J of
potential energy and is 2.5 m above ground, what is the box’s
mass?

GPE = mgh

150 = m * 9.8 * 2.5

m = 6.1 kg

Kinetic Energy

Kinetic energy is an extremely important form of mechanical energy
because it has to do with the motion of an object. Unlike
gravitational potential energy, kinetic energy is based on how fast
an object is moving. The formula for calculating the kinetic energy
that an object has is:

KE = .5 * m * v2

Notice that the speed of the object is squared. That means as an
object moves faster, the kinetic energy of the object increases
very quickly. Remember, we noted that energy was the "currency"
that we spend to do work. An object that has kinetic energy can use
that energy to accomplish something. The faster an object goes, the
more work it can do as it transforms that energy into another form.
We will talk more about these transformations below.

Example Problem 3

An 1800 kg car is traveling at 20 m/s. What is the car's kinetic
energy?

KE = .5 * m * v2

KE = .5 * 1800 kg * (20)2

KE = 360,000 J

When the amount of joules gets into the thousands, we convert the
joules to kilojoules by dividing the joules by 1000.

Here, the KE = 360 kilojoules.

Example Problem 4

How fast is a 35 kg jaguar moving if it has 1800 joules of
energy?

KE = .5 * m * v2

KE/.5*m = v2

v2 =1800/(.5)(35) = 102.9

v = 10.1 m/s

The Kinetic Potential Energy Connection

In middle school you probably learned that energy can be converted
from one form to another. For instance, light energy from the sun
is converted to chemical energy by plants. We eat the plants and
convert the chemical energy from the plant into mechanical energy
that makes our body move.

Potential energy can become kinetic energy when an object begins
moving. For instance, imagine a 700 kg car is resting at the top of
a 100 m hill. If the car begins rolling down the hill, some of the
potential energy is converted to kinetic energy. When the car
reaches the bottom of the hill, all of the potential energy has
been converted to kinetic energy. We can represent these
conversions using charts referred to as LOL charts.

LOL Charts

The LOL chart is broken into two sections. The first chart
represents the energy at the start. In this case the chart shows a
bar that is all potential energy. Since this is a qualitative
representation, we are not worried about numbers.

The second chart represents the energy at the end. This is when the
car reaches the bottom of the hill. We will discuss the circle in
the LOL chart in a later lesson. Since all of the potential energy
becomes kinetic energy, the KE bar should be the same heith as the
PE bar.

The LOL chart is an excellent representation of the change from
potential to kinetic energy. It also helps us solve problems. Since
energy is not lost just converted, we know that the the amount of
potential energy at the start is equal to the amount of kinetic
energy at the end. This means for this situation we can say that
GPE = KE. Using the equations for GPE and KE we can write:

m * g * h = .5 * m * v2

Using the numbers from above we can find out how fast the car was
moving.

700 kg * 9.8 m/s2 * 100 m = .5 * 700 kg * v2

1960 = v2

v = 44.3 m/s

The car is traveling at 44.4 m/s.

Answer #1

just do questions 5 through 10
3.13.6
Question 110 pts
A 319 kg motorcycle is parked in a parking garage. If the car has
35,494 J of potential energy, how many meters above ground is the
car? Report your answer to 1 decimal place. Please do not include
units or the answer will be marked incorrect.
Flag this Question
Question 210 pts
A box sitting on the top of a hill has 252 J of potential energy.
If the hill...

8 through 10 done please!!
3.13.6
Question 110 pts
A 319 kg motorcycle is parked in a parking garage. If the car has
35,494 J of potential energy, how many meters above ground is the
car? Report your answer to 1 decimal place. Please do not include
units or the answer will be marked incorrect.
Flag this Question
Question 210 pts
A box sitting on the top of a hill has 252 J of potential energy.
If the hill is...

do all five questions
Question 1
20 pts
Ignoring the effects of air resistance, if a ball falls freely
toward the ground, its total mechanical energy
Group of answer choices
increases
remains the same
not enough information
decreases
Flag this Question
Question 2
20 pts
A child jumps off a wall from an initial height of 16.4 m and lands
on a trampoline. Before the child springs back up into the air the
trampoline compresses 1.8 meters. The spring constant...

Hookes review
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Question 1
20 pts
A student pulls a cart across the floor with a force of 62 N. If
the cart travels 65 m, how much work does the cart do on the
student? Please do not include units or the answer will be marked
incorrect.
Flag this Question
Question 2
20 pts
Object A has a mass of m with a speed of v. Object B has a mass of
m and a velocity...

Do all five questions
Question 1
20 pts
Ignoring the effects of air resistance, if a ball falls freely
toward the ground, its total mechanical energy
Group of answer choices
increases
remains the same
not enough information
decreases
Flag this Question
Question 2
20 pts
A child jumps off a wall from an initial height of 16.4 m and
lands on a trampoline. Before the child springs back up into the
air the trampoline compresses 1.8 meters. The spring constant...

answer all 10 questions
answer all questions
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Question 4
10 pts
A rocket launcher has a spring constant of 783 N/m. What force is
required to stretch the spring a distance of 2 m? You must show
work to earn credit for your answer.
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Question 5
10 pts
A spring has a spring constant of 96 N/m. How much work is done
when the spring stretches...

1 a) Starting from rest, a 50 Kg person dives
into water from a height of 80 m above the water
surface. Air resistance is negligible. (Use g= 10 m/s2)
Calculate (A) the diver’s KE just before striking the water surface
(B) the velocity of the diver just before striking the water (C)
the work done by gravitational force on the diver (D) the power
delivered by gravitational force on the diver
b) A ball of mass m=0.2 Kg
sitting...

A. Suppose a 10kg object as a total energy of 500J. If all of
this energy is kinetic energy, how fast is the object moving? (I
got 10 m/s) Is this correct?
B. Assume now that the same object has only gravitational
potential energy. If the object is on Earth, how high up is it? (I
got 5.1 m, is this correct?)
C. According to conservation of energy, the object must have the
same TOTAL energy (kinetic plus potential) at...

this question by someone by earlier.
consider a .62 kg
basetball that is dropped from a height of 1 metee avove the
floor:
1.What form or forms
of energy does the ball have before it is dropped, if an orgin is
set on the ground? Be explicit and give a value for any energy and
what form or forms of energy does the ball have right before it
makes impact with the ground?
This is the results:
When the ball...

Question 1 (1 point)
Which is not necessary in order to do work on an object (use the
scientific definition of work)?
Question 1 options:
There must be a change in momentum.
A net force must be applied to the object.
The object must undergo a displacement.
A component of the force must be in the direction of motion.
Question 2 (1 point)
The change in gravitational potential energy for a 1.9 kg box
lifted 2.2 m is:
Question 2...

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