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Physics: Work and Kinetic Energy

Terms in this set (69)

What is the formula for Work done in a STRAIGHT line with CONSTANT Force?
dW=Fd (dot product)
dW= derivative of Work
F= force
d= displacement
What is the formula for the total work done on an object with CONSTANT force?
W= F∆x
∆x= change in position
What is the formula for finding total work done by a VARYING force?
W= ∫Fdx
1 joule equals what?
(1 Newton)(1 meter)
If the force is not in the same direction as displacement, what is the formula for work?
W= Fd (cosθ)

where θ is the angle between the force and the displacement
Does the perpendicular component of force (i.e. Fsinθ) have any impact on the forward direction of an object?
no
If θ = 0 then what is the formula for work?
W=Fd
Is work a scalar or vector quantity?
Scalar
Work can be written in both of these forms
W= F•d
W= |F|*|d| cosθ
What are the three situations in which the direction of force affects work?
1) Force has a component in SAME direction of displacement= Positive Work

2) Force has component that is in the opposite direction of displacement= Negative work

3) Force is perpendicular to displacement= NO work is being done
What is the situation in which no work is being done but a force is present and there is displacement?
When the force is perpendicular to the displacement there is no work
A constant force can do what type of work on an object?
positive, negative or zero work depending on angle between force and displacement
Can there be a force acting on an object AND no work being done?
Yes, if the object isn't moving then no work is done
ex: holding up a barbell
Is work being done while holding a book and walking with constant velocity?
No because the force of you holding up the book is only in the upwards direction and not in the direction of displacement of the book
When a ball on a string moves in uniform circular motion, what is the work done by the string on the ball?
Zero
Why is there no work done by a string on a ball while the ball is traveling in uniform circular motion?
The string imparts no force in the direction of the balls displacement
When one body does work on another body, what is the amount of work done by the second body on the first one?
equal amount of work but negative
When does a force do negative work?
When it acts in the OPPOSITE direction of motion
When is there NO work done even if a force is present?
When the force is PERPENDICULAR to the displacement of the object
When total Work is positive, what is the speed of an object?
object speeds up
When total Work is negative what is the speed of an object?
Object slows down
When total work is 0, what is the objects speed?
either 0 or the objects speed is constant
What is the formula for kinetic energy?
Is kinetic energy a scalar or vector?
scalar, depends on only mass and speed of object! NOT THE DIRECTION OF TRAVEL
Can kinetic energy be negative?
no
When is kinetic energy = 0?
When an object is at rest
Does kinetic energy depend on the direction of travel?
No, only the mass and speed
What is the formula for the total work done on an object?
K2-K1 (final kinetic energy- initial kinetic energy)
What are the units for kinetic energy and work?
Joules
What other units is the joule equivalent to?
N•m and (kg•m^2/s^2)
What is the work energy theorem?
When is the work energy theorem useful?
When we want to understand how the speed of an object changes irregardless of the time it took to undergo that change
does kinetic energy use mass or weight?
MASS
What is the m value in kinetic energy formula?
an objects MASS
when objects are at rest, the total work done to an object to make it move must equal what?
the final kinetic energy of an object because the initial kinetic energy was zero
Maximizing the distance over which a force acts does what?
Lessens the amount of force felt
example: moving your hand back when you catch a ball, you maximize the distance the ball has to work so as to lessen the force you feel
Kinetic energy is equal to the total amount of work done to accelerate an object from rest
...
What are the two ways to calculate net work?
1) Calculate work done by each force and sum these individual forces
or
2) Calculate net force and compute work done by net force
If an object moves with constant velocity, what is the net work being done on it?
Zero because the net force acting on it is zero and thus the work must also be zero
A force that exerts positive work does what to speed?
Increases speed
A force that exerts negative work does what to speed?
Decreases it
What units are kinetic energy in?
Joules (N•m)= kg•m^2/s^2
What are conservative forces?
Those that are path independent and do not dissipate energy i.e. forces that are involved in reversible energy transfers
ex: gravity, spring force
What are non conservative forces?
Paths in which energy is lost and cannot be regained in the system, the energy transfer is not reversible
ex: kinetic friction
Conservative forces can be seen as this relationship
kinetic energy ---> potential energy ---> kinetic E
Non conservative forces can be seen as this
kinetic energy----> sound, heat : not easily recovered
When conservative forces perform NEGATIVE work (removing kinetic energy from an object) the energy is stored in what form?
Potential energy
Potential energy can only be defined for what type of forces?
Conservative forces
What is the formula for work done by a conservative force?
W= -∆U
U= potential energy
Work done by a conservative force is independent of what?
The path taken by an object moving between two points and only depends on the final and initial locations
When the starting and ending points of motion are the same for conservative forces , the net work must always be what?
Zero
What are the two primary conservative forces dealt with in physics 1?
gravity and spring forces
What is the formula for gravitational potential energy?
mgh

h= height
What is the formula for work done by gravity?
W= -∆(mgh)

h= height
only the difference in potential energy is important, the actual magnitude of potential energy without a reference point is meaningless
...
Can you increase an objects potential energy without increasing its kinetic energy?
Yes; even if no work is being done ( kinetic energy is zero) an objects potential energy can still increase
Ex: lifting an object with constant velocity upwards
What is Hooke's law?
x is define as the springs displacement from its equilibrium
What are the units of the spring constant?
k= N/m (Newton/meter)
What is the formula for work done by potential energy?
W=-∆U
What is the formula for work done by gravitational energy?
Ugravitational= mgy
W= -∆U
What is the formula for elastic potential energy?
Wspring= -∆U= -∆(1/2kx^2)
What is Mechanical energy?
Emech= KE + U
∆Emech= ∆(KE+U)= ∆KE + ∆U= Work + ∆U = 0
If nonconservative forces are ABSENT, what is the formula for change in Mechanical Energy?
∆Emech= W + ∆U= 0
What is the change in mechanical energy equal to when ONLY conservative forces are acting?
0
What does the area under a force vs.position graph represent?
Work done
What is the formula for the amount of force needed to stretch a spring?
F=kx
What are the two ways mechanical energy can be calculated?
1) ∆Emech= Wnon-conservative= Fdx
2) ∆Emech= ∆(KE+U)
What does constant speed mean about the work done?
Net work is zero because ∆KE is zero
Net Work can be calculated two ways, what are they?
1) Wnet= ∆KE
2) Wnet = ∑W
∑W= sum of each work done on system
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