3.3 Work, Energy & Power Flashcards
what is work done? and what is the equation?
the work done, or energy transferred is the product of the force and the distance moved by the force in the direction of movement, W = F x d (where d is the distance moved by the force)
THE TRANSFER OF ENERGY
what is work done measured in?
joules (unit of energy)
what is the definition of the joule?
one joule is equal to the work done when a force of 1 Newton causes an object to move 1m in the direction of the force
what formula is used for work done when the force is acting at an angle to the direction it’s moving in?
W = Fxcosϴ
component of force in the same direction to force (Fcosϴ) multiplied by the distance x
YOU MUST ALWAYS FIND THE COMPONENT PARALLEL TO THE DIRECTION OF MOTION
what is a closed system?
a closed system is any system in which all the energy transfers are accounted for, energy or matter cannot enter or leave a closed system
what is the principle of conservation of energy?
the principle of conservation of energy states that the total energy of a closed system remains constant, energy can neither be created nor destroyed, it can only be transferred from one form to another
what is energy also known as simply?
the capacity to do work
energy can be transferred from one form to another, what is the energy transfer occurring in a car engine?
chemical energy stored in the fuel to kinetic energy
energy can be transferred from one form to another, what energy is gained when a cyclist rides up a mountain?
gravitational potential energy
name some types of energy forms…
kinetic, gravitational potential, thermal, elastic potential, chemical, nuclear, electrical, sound, light
what is kinetic energy?
the energy of an object due to its movement
what is the formula for kinetic energy?
KE = 0.5mv^2
where m is the mass of the object and v is the velocity
how do your derive the formula for kinetic energy from first principles?
KE = work done = force x distance
KE = ma x d
suing suvat v^2 = u^2 + 2as and therefore as = 0.5(v^2 - u^2)
substituting as for ad (as d and s are equal) gives you KE = 0.5m(v^2 - u^2)
if we consider an acceleration from rest, u = 0
therefore KE = 0.5mv^2
what is an objects gravitational energy?
gravitational potential energy (Ep) is the energy an object has due to its position in a uniform gravitational field, the greater the height of the object, the greater its gravitational potential energy
what is the formula for gravitational potential energy in a uniform gravitational field?
Ep or GPE = mgΔh
where m is the mass of the object
g is the acceleration due to gravity
and Δh is the change in height
how do you derive the equation for gravitational potential energy from first principles?
gravitational potential energy = work done to lift something = force x distance
the force required to lift the object = weight of the object, mg
and the distance moved is the change in height, Δh
hence Ep = mg x Δh or mgΔh
what is a common example of an object that involves an energy transfer between kinetic and gravitational potential energy?
a swinging pendulum
because of conservation of energy what can we say about the energy transfers in a swinging pendulum? (if drag forces are ignored)
loss in potential energy = gain in kinetic energy
and vice versa
what is an equation that enables us to work out the velocity of an object that has fallen from rest through a height Δh?
V = (2gΔh)^0.5
comes from KE = GPE
0.5mv^2 = mgΔh
the m’s cancel and rearrange to get V = (2gΔh)^0.5
what is the formula for power?
power = work done / time
it is defined as the rate of doing work or transferring energy
what is power measured in?
watts, w
or joules per second
what unit of power is commonly used day to day?
kilowatts, kW which is equal to 1000W
what is another equation for power?
the power output of a moving object that is subject to a force
P = F x V
f is equal to the force
v is equal to the velocity of the object
how can you derive P = Fv from first principles?
work done = force x distance power = work done / time P = Fd / t distance / time = speed P = Fv
If the force and motion of the object are in different directions what formula should you use for power?
P = Fvcosϴ
can you ever have an 100% energy transfer?
NO, energy transfers always involve losses (usually as heat or friction), nothing can ever be 100% efficient
what is equation for efficiency?
(useful output energy / total input energy) x 100
what is efficiency a measure of?
it is a measure of how well a device converts the energy you put in into the energy you want it to give out
what is the common type of diagram used in represent efficiency/power problem?
sankey diagrams (one with arrows)
name a device where the small energy lost is actually useful..
heaters
