4.14

4.14

10 June 2011

11:53

·         4.14 describe power as the rate of transfer of energy or the rate of doing work

"Rate" just means "divided by time" (see Entrance Activity)

 

So

 

Power = Energy / Time

 

or

 

Power = Work done / Time

4.15

4.15

10 June 2011

11:54

·         4.15 use the relationship between power, work done (energy transferred) and time taken:

          power = work done            P = Wd

                       time taken                    t

P = Wd / t

P = E / t

 

P = Power (Watts, W)

Wd = Work Done (Joules, J)

E = Energy Transferred (Joules, J)

t = Time (s)

Untitled

PFY P 120

 

F) power is the rate of doing work

Power (in watts) = work done (in watts) / time (in seconds)

G) 1 watt is a rate of working of one joule per second

13) a girl does 1000 joules of work in 5 seconds. What power does she develop? 1000/5= 5000W

14) a man lifts a weight of 300N through a vertical height of 2m in 6 seconds. What power does he develop? 300 x 2 = 600W

16) a crane lifts a load of 3000N through a vertical height of 10m in 4 seconds. What is its rate of working  in a) watts b) kilowatts  

a) 3000 x 10 = 30,000W

b) 30kW

4.13

4.13

10 June 2011

11:53

·         4.13 understand how conservation of energy produces a link between gravitational potential energy, kinetic energy and work

<<CoE.ppt>>

Inserted from: <file://\\BPSFILES\Staff Shared Folders\Secondary\Science\Science Resources\Physics\2 IGCSE\.P1-P8 IGCSE Resources\P4 Energy Sources & Energy Transfer\P4 Resources\MABA P4\4B\4B2 GPE, KE and CoE (2)\CoE.ppt>

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CoE questions

19. a) 500N/kg

b) GPE = mgh

     GPE = 50 x 10 x 4 = 2000J

c) KE = ½ m v2

     KE = 0.5 x 50 x 2000 =  50000m/s2

Q3. A) GPE = mgh

            GPE = 35 x 10 x 30 = 10,500J

B) KE = ½mv²

     But KE (bottom) = GPE (top)

        10,500 = ½ x 35 x v²

        v² = 600

        v = 24.5m/s

C) because some of it got wasted by heat through friction

 

PFY p.121

 

Collins, p.91

4.10

1. a) joules

    weight

     distance

    b) force

       meter

       weight

    d) energy

2. 5N times 2m = 10J

4. 200J

weight and distance 

Q1. 50 000 / 4 000 = 12.5m    

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4.10

10 June 2011

11:51

·         4.10 understand that work done is equal to energy transferred

PFY p.112

Collins p. 91

 

 

 

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4.9

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4.9

10 June 2011

11:38

·         4.9 recall and use the relationship between work, force and distance moved in the direction of the force:

       work done = force × distance moved         

Wd = F × d

 

 

 

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