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Level 65

Potential Energy & Conservation of Energy


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Conservative
Force of gravity
What is the key distinction between the two?
-when a conservative force acts, the work it does is stored in the form of energy that can be released at a later time
gravity
(physics) the force of attraction between all masses in the universe
Kinetic Friction
Friction between moving surfaces
recovered later as kinetic energy
The work done be a nonconservative force cannot be...?
Instead, what occurs?
the work of a nonconservative force is converted to other forms of energy
zero total work
How much work does a conservative force do on a closed path?
Potential Energy
(mass) x (g) x (height)
kinetic energy
a running dog
a storage system for energy
In a sense, what is potential energy?
no
nitrogen monoxide
Instead, what happens to the energy?
it is dissipated into other forms of energy such as heat or sound
yes
Is velocity a vector?
conservative forces
As such, only what type of foces have the potential energy storage system?
-trick question!
What expression describes potential energy?
Why not?
because each different conservative force has a different expression for potential energy
potential energy decreases
When an object falls, what is the impact of the fall on potential energy?
potential energy increases
When an object is lifted, what is the impact of the fall on potential energy?
Joules
Potential energy is measured in units of_______.
scalar
a physical quantity that has magnitude but no direction
energy cannot be created or destroyed
What does the energy conservation theorem state?
positive work removes PE and transforms it into KE
In terms of PE and KE, what does positive work do?
negative work removes KE and transforms it into PE
In terms of PE and KE, what does negative work do?
As such, what is a new definition of work that we can employ?
work is the process of energy transfer from one form of energy to another
What specifically is gravitational PE?
stored energy an object has due to being under the gravitational influence of another body
PEg = mgh
What is the equation that describes the relationship between PEg and work?
we are free to choose
What is h set to when using this equation?
What is usually the best choice?
to set H to zero at the lowest height of interest
KE = 0 for balls A and B
You are given one ball (A) that is dropped from a low height and one ball (B) that is dropped from a high height. What is KE equal to for balls A and B prior to release?
PE b initial > PE a initial
What is the relationship between the PEs of balls A and B?
Why?
because both person a and B cover the same angle in the same time
positive work
As A and B are released and fall towards the ground, what type of work does gravity do?
What does gravity do interms of PE and KE?
Gravity takes energy from PE storage and puts it into KE
all of the energy has moved from potential energy to kinetic enervy
By the time the object has reached the bottom, where is all of the energy?
Which object has more KE at the end of the motion? Why?
ball B has more KE at the end of the motion because object B had more PE to begin with
Which object then has a greater velocity? Why?
ball B has a greater velocity because it has a greater KE
the ability to calculate time
What is the only thing that we give up when we use the energy method?
What is the definition of mechanical energy?
the sum of potential and kinetic energies of an object
mechanical energy is conserved
What happens to mechanical energy in systems involving only conservate forces?
no it does not
If a force does zero net work, does it need to be accounted for when performing calculations using the energy method?
KE decreases
When gravity does negative work on a rising object, what is the impact on KE?
KE increases
When gravity does positive work on a falling object, what is the impact on KE?
What is mechanical energy?
the sum of the potential and kinetic energies of an object
mechanical forces Change the amount of mechanical energy in a system
What impact do nonconservative forces have on the amount of mechanical energy in a system?
Provide an example
nonconservative forces might decrease mechanical energy by converting it into thermal energy
increases the total mechanical energy of a system
What impact does positive nonconservative work have on a system?
Ei < Ef
Therefore, what is the relationship between Ei and Ef?
decreases the total mechanical energy of a system
What impact does negative nonconservative work have on a system?
What impact does zero nonconservative work have on a system?
has no impact on the total mechanical energy of a system
Ei + Wnc = Ef
What is the final definition of the energy method?
Is it possible for the kinetic energy of an object to be negative? Explain.
-the kinetic energy cannot be negative, since KE is given by 1/2mv^2 and m and v^2 are always positive or zero
Is it possible for the gravitational PE of an object to be negative? Explain
the gravitational PE can be negative since any level can be chosen to be zero
If the force on an object is zero, does that mean that the PE of the system is zero?
zero force implies that the rate of Change in the PE with distance is zero--that is the PE is constant--but the value of the PE can be anything at all
If the potential energy of a system is zero, is the force zero?
-similarly, if the PE is zero, it does not mean that the force is zero
-the change in gravitational potential energy for ball 1 is equal to the change for ball 2
Ball 1 is thrown to the ground; ball 2 is dropped to the ground from rest. Assuming the balls have the same mass and are released from the same height, how does the chang…
no nonconservative work is done on the bicycle
You coast up a hill on a bicycle with decreasing speed. Your friend pedals up the hill with constant speed. Ignoring friction, does the mechanical energy of your system (you-bike-Eart) increase, decrease, or stay the same?
Does the mechanical energy of your friend's system increase, decrease or stay the same?
your friend does positive nonconservative work to maintain a constant speed while going uphill
-as the leaf falls, air resistance exerts a force on it opposite to its direction
A leaf falls to the ground with a constant speed. Is KEi + PEi greater than, less than, or the same as KEf + PEf?
ME decreases
A block of mass M1 is connected to a second block of M2. When the blocks are released from rest, they move through a distance d at which point M2 hits the floor. As b…
the tension does negative nonconservative work
Is the nonconservative work done on block to by the tension in the rope positive, negative, or zero? Why?