3.3 Work, Energy & Power

Question 1

what is work done? and what is the equation?

Answer 1

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

Question 2

what is work done measured in?

Answer 2

joules (unit of energy)

Question 3

what is the definition of the joule?

Answer 3

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

Question 4

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

Answer 4

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

Question 5

what is a closed system?

Answer 5

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

Question 6

what is the principle of conservation of energy?

Answer 6

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

Question 7

what is energy also known as simply?

Answer 7

the capacity to do work

Question 8

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

Answer 8

chemical energy stored in the fuel to kinetic energy

Question 9

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

Answer 9

gravitational potential energy

Question 10

name some types of energy forms…

Answer 10

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

Question 11

what is kinetic energy?

Answer 11

the energy of an object due to its movement

Question 12

what is the formula for kinetic energy?

Answer 12

KE = 0.5mv^2

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

Question 13

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

Answer 13

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

Question 14

what is an objects gravitational energy?

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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

Question 17

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

Answer 17

a swinging pendulum

Question 18

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

Answer 18

loss in potential energy = gain in kinetic energy

and vice versa

Question 19

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

Answer 19

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

Question 20

what is the formula for power?

Answer 20

power = work done / time

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

Question 21

what is power measured in?

Answer 21

watts, w

or joules per second

Question 22

what unit of power is commonly used day to day?

Answer 22

kilowatts, kW which is equal to 1000W

Question 23

what is another equation for power?

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

Answer 23

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

Question 24

how can you derive P = Fv from first principles?

Answer 24

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

Question 25

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

Answer 25

P = Fvcosϴ

Question 26

can you ever have an 100% energy transfer?

Answer 26

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

Question 27

what is equation for efficiency?

Answer 27

(useful output energy / total input energy) x 100

Question 28

what is efficiency a measure of?

Answer 28

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

Question 29

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

Answer 29

sankey diagrams (one with arrows)

Question 30

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

Answer 30

heaters