Work and Energy Notes (Etkina 6 & Knight 8.3 & 10) Flashcards
work is equivalent to changes in the system’s
energy
higher above Earth: greater
gravitational potential energy
faster an object is moving: greater
kinetic energy
elastic potential energy is the energy associated with an elastic object’s
degree of stretch
greater stretch/compression: greater
elastic potential energy
energy associated with both temperature and structure is called
internal energy
when the external force is in the direction of the object’s displacement, the external force does …, causing the system to …
positive work; gain energy
if the external force points opposite a system object’s displacement, the external force does …, causing the system’s energy to …
negative work; decrease
in the external force points perpendicular to a system object’s displacement, the external force does … work on the system, causing … to its energy
zero; no change
work is a … physical quantity. F is always …, distance (d) is always …. Only … can negate the value for work
scalar; positive; positive; the angle
if there is no work done on the system, the energy of the system should be
constant
energy is a … quantity: it is constant in an … system and changes as a result of … done on a … system
conserved; isolated; work; nonisolated
law of conservation of energy: the sum of the initial energies of a system plus the work done on the system by external forces equals the sum of the
final energies of the system
it is preferable to have a … system so that the changes occurring can be included as … within the system rather than as the … done by external forces
larger; energy changes; work
the gravitational potential energy of an object-Earth system is Ug =
mgy
y is the position of the object with respect to the … of a … coordinate system
zero; vertical
if earth is not included in the system, earth would be an … doing … on the system. The choice of system, though, … affect the result of analysis
external object; positive work; does not
when you stretch or compress an elastic spring-like object, you have to pull or push harder the more the object is stretched or compressed. the force is not …
constant
the coefficient of proportionality, k, in the equation F= kx, is called the ….
spring constant
… springs have larger values of k
stiffer
for k = 1 N/m, to stretch a spring by 1.0 m we would have to exert a
1.0 N force
Hooke’s law: if any object causes a spring to stretch/compress, the spring exerts an … on that object. The spring constant k is measured in N/m and is a measure of the …. of the spring
X or l is the … that the object has been stretched/compressed
F =
elastic force; stiffness; distance; kx
elastic potential energy equation: Us =
1/2kx^2
a change in the internal energy can be caused by
friction
change in internal (thermal) energy =
friction force * d
for change in internal energy, friction force is …, increase in internal is shared between the … and the …
average; moving object; surface
the rate at which conversion of energy occurs is called
power
the power of a process is the amount of some type of … converted into a different type divided by the … in which the process occurred. if the process involves external forces doing work, then power can also be defined as the magnitude of the … done on or by the system divided by the … needed for that work to be done
energy; time interval; work; time interval
power is defined in …, which is …
watts; 1 joule/second
1 horsepower (hp) = … W
746
a force that restored a system to an equilibrium position is called a …. systems that exhibit such restoring forces are called …
restoring force; elastic
when no forces act on a spring to compress or extend it, it will rest at its …
equilibrium length
spring force is proportional to the … of the end of the spring –> … relationship
displacement; linear
if k is large, it takes a … pull to cause a significant stretch, and we call the spring a “…” spring. If k is small, we can stretch the spring with …. force, and we call it a “…” spring
large; stiff; very little; soft
energy of one kind can be … into energy of another kind
transformed
energy transformations are changes of energy … the system from one form to another
within
an exchange of energy between system and environment is called an …. Two primary processes: … and …
energy transfer; work; heat
work: … transfer of energy to or from a system by …/… it
mechanical; pushing; pulling
heat: … transfer of energy from the environment to the system (or vice versa) because of a … between the two
nonmechanical; temperature difference
no work is done on an …. system, and therefore, the total energy of such a system is …
isolated; conserved
in order for energy to be transferred as work, the system must undergo a … during the time that the force is applied
displacement
the larger the displacement, the … the work done. the stronger the force, the … the work done
greater; greater
the … is the unit of all forms of energy
joule
work is a … quantity, having a magnitude but no …
scalar; direction
a force does the greatest possible amount of work on an object when the force points in the … as the object’s …
same direction; displacement
the sign of work is determined entirely by the … between the … and the …
angle theta; force; displacement
the total work is the sum of the work done by each …. and represents the total … to the system from the environment or from the system to the environment
force; energy transfer
if the object undergoes no displacement while the force acts, … work is done. a force … to the displacement does no work. if the part of the object on which the force acts undergoes …, no work is done
no; perpendicular; no displacement
translational kinetic energy: kinetic energy of an object moving along a … or some other …
line; path
the kinetic energy of a rolling object is always … than that of a nonrotating object moving at the same speed
greater
stored energy –> … energy –> has the … to be converted into other forms of energy
potential; potential
interaction forces that can store useful energy (e.g. gravitational/ elastic potential energy) are called … forces. mechanical energy of a system is conserved when only … act
conservative; conservative forces
nonconservative force example:
friction
only … in potential energy are significant, when it comes to analyzing potential energy
changes
gravitational potential energy depends only on the … of an object and not on the … the object took to get to that position
height; path
spring force is not a … force, so … is used
constant; average
the sum of the kinetic and potential energies is called the … of the system
mechanical energy
the mechanical energy is conserved for an … system without …
isolated; friction
a collision in which mechanical energy is conserved is called a … collision. the collision must obey two conservation laws: conservation of … and conservation of …. Although the energy is transformed into potential energy during the collision, the mechanical energy before and after the collision is purely ….
perfectly elastic; momentum; mechanical energy; kinetic energy
