Energy Work Power

Textbook Chp 6 (pp 104 – 122)

What is Energy?O In biology, we say “the sun is the

source of energy for all life on Earth”O What does this statement mean?O Where does the Energy in the sun

come from??

EnergyO The concept of energy is confusing

for two reasons:O (i) the way we use the English word

“energy” may not be the same way to use the word in Physics

O (ii) even when we use the word in Physics, energy can take many different forms

EnergyO Definition: Energy is the capacity to do

workO Qn: so….what is work? O Ans: we will get to that second half of this

chapterO Unit: Joules (J)O Principle of Conservation of Energy (CoE):

O Energy cannot be created or destroyed, but can be converted from one form to another.

O The total energy in an isolated system is constant.

Forms of EnergyO There are 9 forms of energy which

are in your syllabus:O 3 types of Potential Energy

(Chemical, Gravitational, Elastic)O Kinetic Energy (i.e. “movement

energy”)O Light, Thermal, Electrical, Nuclear,

Sound Energy

Potential Energy (PE)O Potential Energy is sometimes referred

to as “stored” energy. (note: cannot use “stored energy” in O levels)

O Energy which is doing nothing right now, but is able to do work later

O A battery contains chemical PE (you will learn more in A level chemistry), so do fossil fuels and food (e.g. bio SPA)

O A stretched rubber band contains elastic PE

O An object lifted up contains gravitational PE

Half TimeO Brainiac Pendulum Demo: http://

www.youtube.com/watch?v=ZbCLAZSPLNkO Bill Nye Part 1:O http://

www.youtube.com/watch?v=zTXW9aRO23YO Bill Nye Part 2:O http://

www.youtube.com/watch?v=Vfcsq6ylRzgO Bill Nye Part 3:

http://www.youtube.com/watch?v=XkDG02nzfSk

Energy Conversion WS

Did You Know: CaloriesO 1 calorie is 4.184 J O The recommended calorie intake is

about 2000 kilo-calories per day (actual recommendation varies by size, gender and age)

O How many joules of energy is that?

Closed and Open Systems

O Recall the definition of CoE includes the statement: The total energy in an isolated system is constant.

O What is an “isolated system”?O An isolated or closed system is a

system of interacting objects where no energy is added or taken from the system

Case Study: PendulumO [applet]O Assume no air resistance and no

frictionO Note that system is closed – no

energy is added or taken from the pendulum

O Total Energy is constant, even though GPE and KE are changing over time

O What will happen if there is air resistance? Is the system still closed?

Work DoneO When a force pushes an object and

the object moves in the direction of the force, it is said that work is done by that force.

O Work done has units of Joules (J), same as energy

O Formula: O Work = F x Distance (same

direction as force)

Worked Example

O If a force of 3 N pushes an object for a distance of 5 m what is the work done by the 3 N force?

O Ans: Work = FdO = (3)(5) = 15.0 J (3 s.f.)

3 N

GPE and KEOFormula for GPE:

OGPE = mghOFormula for KE

OKE = ½ mv2

Worked Example

O An object of 5 kg, initially at rest, slides down the slope as shown above. Assuming no energy loss, what is the final velocity of the object?

O Method: by the conservation of energy, loss in GPE = gain in KE

O mgh = ½ mv2

O (5)(10)(5) = (0.5)(5)v2

O Solve for v. O Ans: v = 10.0 ms-1 (3 s.f)

5 m

Remember Taman?O The trek component of Taman (just

before the canopy walk) was a 300 m vertical climb (all my stairs).

O If you weigh 50 kg, how much energy did you spend?

O I weigh 120 kg. how much energy did I spend?

A note about EquationsO How do you know what units to pluck

in when doing equations?O As long as all units on the right side

of the equation are S.I. units, the left side of the equation will also be in S.I. units.

O In fact, this convenience is precisely what S.I. units were created in the first place.

PowerO In Physics, power is a measure of how much

work can be done in an interval of time.O If I can do a lot of work in a short amount of

time, I am more powerfulO Equation: Power = Work Done / TimeO Units: Watts (W)O Sometimes it is difficult to calculate the

Work Done directly. We may have calculate the work done indirectly using the GPE and KE equations.

Worked Example 1

O A 3 N force pushes an object for 5 m in 5 s. What is the power produced by the force?

O Power = Work Done / timeO = (force x dist) / timeO = (3)(5)/5O = 3.00 W (3 s.f.)

3 N

Work Example 2O An 10 kg object was pushed up a smooth

slope in 50 s. Assuming that the final velocity of the object is 0, what is the power produced?

5 m

Work Example 2O Cannot calculate work done directly

(we don’t know force, and don’t know distance)

O But we know that the work done results in gain in GPE of object

O Work done = gain in GPE = mghO Power = mgh/tO = (10)(10)(5)/(50)O = 10.0 W (3 s.f.)

Half TimeO Energy Playa rap

http://www.youtube.com/watch?v=P2W1VQ2bAi8

Useful Work / Energy / Power

O In reality when we do work, only a fraction of our efforts going into completing what we achieve to do. The rest of our efforts is “wasted” overcoming friction, heat lost to the surroundings, etc.

O The work which actually accomplishes our intended outcome is called the “useful” work.

O Same for “useful energy” or “useful power”

Work Example 1O A 3N block pushes a block for 5 m

over 10 s. If the friction force exerted on the block is 1 N, O (i) what is the total work done on the

block?O (ii) what is the useful work done on

the block?O (iii) what is the useful power?

Work Example 2O A 2000 kg car accelerates from 0 to

10 ms-1 in 10 seconds.O (i) what is the final KE of the car?O (ii) what is the useful power of the

car?O (iii) would you expect the actual

power of the car’s engine to be greater than or the same as your answer in (ii)? Why or why not?

EfficiencyO Efficiency measures the percentage of

usefulness. O Units: Efficiency is a percentage (%)O There are three possible equations for

efficiency but they are actually all the same:O (Useful Work / Total Work Done) x 100%O (Useful Energy Output / Total Energy Input) x

100%O (Useful Power Output / Total Power Input) x

100%

Worked Example 3O Refer to the box in Work Example 1O What is the efficiency of the force

pushing the box?

Worked Example 4O Refer to the car in Worked Example

2.O If the total power input to the car’s

engine is 15 kW, what is the efficiency of the car’s engine?

Combination QuestionsO A crane lifts a 1000 kg object up a height

of 20 metres.O (i) If the crane took 30 s to do so, what is

the useful power of the crane?O (ii) if the efficiency of the crane is 50%,

what is the total input energy supplied to the crane?

O (iii) if the object is accidentally dropped, determine the velocity of the object just before it hits the ground (ignore air resistance).

SummaryO 9 types of EnergyO Law of Conservation of EnergyO Conversion of Energy (qualitative)O Work Done O GPE and KE EquationsO PowerO Useful Work / Energy / PowerO Efficiency

Now: 10 min pop quiz!!MUAHAHAHAHA =P