3.3 Work, Energy & Power Flashcards

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1
Q

what is work done? and what is the equation?

A

The work done is when a force of 1N moves its point of application 1m in the direction of force, W = F x d (where d is the distance moved by the force)
WORK DONE = THE TRANSFER OF ENERGY

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2
Q

what is work done measured in?

A

joules (unit of energy)

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3
Q

what is the definition of the joule?

A

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

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4
Q

what formula is used for work done when the force is acting at an angle to the direction it’s moving in?

A

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

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5
Q

what is a closed system?

A

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

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6
Q

what is the principle of conservation of energy?

A

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

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7
Q

what is energy also known as simply?

A

the capacity to do work

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8
Q

energy can be transferred from one form to another, what is the energy transfer occurring in a car engine?

A

chemical energy stored in the fuel to kinetic energy

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9
Q

energy can be transferred from one form to another, what energy is gained when a cyclist rides up a mountain?

A

gravitational potential energy

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10
Q

name some types of energy forms…

A

kinetic, gravitational potential, thermal, elastic potential, chemical, nuclear, electrical, sound, light

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11
Q

what is kinetic energy?

A

the energy of an object due to its movement

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12
Q

what is the formula for kinetic energy?

A

KE = 0.5mv^2

where m is the mass of the object and v is the velocity

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13
Q

how do your derive the formula for kinetic energy from first principles?

A

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

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14
Q

what is an objects gravitational energy?

A

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

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15
Q

what is the formula for gravitational potential energy in a uniform gravitational field?

A

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

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16
Q

how do you derive the equation for gravitational potential energy from first principles?

A

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

17
Q

what is a common example of an object that involves an energy transfer between kinetic and gravitational potential energy?

A

a swinging pendulum

18
Q

because of conservation of energy what can we say about the energy transfers in a swinging pendulum? (if drag forces are ignored)

A

loss in potential energy = gain in kinetic energy

and vice versa

19
Q

what is an equation that enables us to work out the velocity of an object that has fallen from rest through a height Δh?

A

V = (2gΔh)^0.5
comes from KE = GPE
0.5mv^2 = mgΔh
the m’s cancel and rearrange to get V = (2gΔh)^0.5

20
Q

what is the formula for power?

A

power = work done / time

it is defined as the rate of doing work or transferring energy

21
Q

what is power measured in?

A

watts, w

or joules per second

22
Q

what unit of power is commonly used day to day?

A

kilowatts, kW which is equal to 1000W

23
Q

what is another equation for mechanical power?

the power output of a moving object that is subject to a force

A

P = F x V
f is equal to the force
v is equal to the velocity of the object

24
Q

how can you derive P = Fv from first principles?

A
work done = force x distance
power = work done / time
P = Fd / t
distance / time = speed
P = Fv
25
Q

If the force and motion of the object are in different directions what formula should you use for power?

A

P = Fvcosϴ

26
Q

can you ever have an 100% energy transfer?

A

NO, energy transfers always involve losses (usually as heat or friction), nothing can ever be 100% efficient

27
Q

what is equation for efficiency?

A

(useful output energy / total input energy) x 100

28
Q

what is efficiency a measure of?

A

it is a measure of how well a device converts the energy you put in into the energy you want it to give out

29
Q

what is the common type of diagram used in represent efficiency/power problem?

A

sankey diagrams (one with arrows)

30
Q

name a device where the small energy lost is actually useful..

A

heaters

31
Q

What happens when you do work

A

Energy is transferred

Work done = energy transferred