4.17 Answers07 September 2011
14:45
Wednesday, September 21, 2011
Tuesday, September 20, 2011
4.15
·4.15 use the relationship between power, work done (energy transferred) and time taken:power = work done P = Wd
time taken t
P = Wd / tP = E / tP = Power (Watts, W)Wd = Work Done (Joules, J)E = Energy Transferred (Joules, J)t = Time (s)
4.14 Entrance Activity Answers
Tell the person next to you...· You can run up the stairs in the Science block in 30 seconds.· Mr. Roff can run up the stairs in the Science block in 20 seconds.· Who is the most powerful? Why?· Mr Roff. He does more work running up the stairs (because has a higher weight) in a shorted time so he is more powerful
· It takes 2 minutes to fill the 20 litre sinks in this room with water.· How fast is the sink filling?· 10 litres/min· At what rate is the sink filling?· 10 litres/min· How long would it take to fill a 10 litre sink?· 1 min
Power Questions
PFY p.120
f) Power is the rate of doing work
Power (in Watts) = work done (in joules) / 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?
P = Wd/t
P = 1000/5
P = 200 W
14) A man lifts a weight of 300N through a vertical height of 2 m in 6 seconds. What power does he develop?
Wd = F x d
Wd = 300 x 2
Wd = 600J
P = Wd/t
P = 600/6
P = 100 W
16) A crane lifts a load of 3000N through a vertical height of 10 m in 4 seconds. What is its rate of working ina) Watts
Wd = F x d
Wd = 3000 x 10
Wd = 30,000J
P = Wd/t
P = 30,000 / 4
P = 7,500 W
b) Kilowatts
7,500 / 1000 = 7.5 kW
Wednesday, September 14, 2011
4.13
·4.13 understand how conservation of energy produces a link between gravitational potential energy, kinetic energy and work<<CoE.ppt>>
CoE questions
19PFY p.121
a) W = m x g
= 50 x 10
= 500g
b) GPE = m x g x h
= 50 x 10 x 4
= 2,000J
c) Energy In = Energy Out
KE = 2,000J
Collins, p.91
a) GPE = m x g x h
= 35 x 10 x 30
= 10,500J
b) KE = 1/2 x m x (v^2)
10,500 = 1/2 x 35 x (v^2)
v = 24.5 (24.49489743)
c) The actual speed at the bottom of the hill is likely to be less than the value calculated in part b because there is friction from the snow and her sledge. The friction reduces the speed as she goes down the hill.
Wednesday, September 7, 2011
4.10
4.10
· 4.10 understand that work done is equal to energy transferred 
· 4.10 understand that work done is equal to energy transferred
PFY p.112 & Collins p. 91
1. a) The work done (measured in "joules") is equal to the "force" (in newtons) multiplied by the "distance" moved (in metres)
b) 1 joule is the work done when a force of one "newton" moves though a distance of one "metre" (in the direction of "force")
d) Work done = "energy" transferred
2. A man lifts a parcel weighting 5 newtons from the ground on to a shelf 2 metres high. How much work does he do on the parcel?
Wd = F x d
Wd = 5 x 2
Wd = 10 J
4. A girl weighing 500 N climbs 40 m vertically when walking up the stairs in an office block. How much work does she do against gravity? What are the energy transfers here?
Wd = F x d
Wd = 500 x 40
Wd = 20,000 J
food = chemical potential energy ---> walking up = kinetic energy ---> at the top = gravitational potential energy
Q1) 50,000 J of work are done as a crane lifts a load of 400 kg. How far did the crane lift the load? (gravitational field strength, g, is 10 N/kg.)
Wd = F x d
d = Wd/F
d = 50,000/400x10
d = 12.5 m
4.9
· 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
Tuesday, September 6, 2011
Plenary Questions
1. Thermal energy travels through the bottom of a pan by conduction. The energy is passed from one vibrating atom to the next. All metals are good conductors. Plastics, water and air are poor conductors (good insulators).
2. Convection currents can from when liquids and gases are heated. The cold fluid sinks and the hot fluid rises.
3. Energy can travel through empty space by infra-red rays, which can be reflected by mirros like light rays. Dull black surfaces are good radiators and good absorbers. Shiny, bright surfaces are poor radiators and poor absorbers.
4. A vacuum flask uses silvering to cut down heat transfer by radiation and uses a vacuum to cut down heat transfer by conduction and convection.
Thursday, September 1, 2011
4.8
·4.8 describe how insulation is used to reduce energy transfers from buildings and the human bodyTask 1
· Use this interactive animations to investigate how different insulation affects the rate of cooling of hot waterTask 2
· Use these interactive animations to find out about the energy transfers in a houseTask 3
· Test your knowledge with this quiz
4.7
·4.7 describe the role of convection in everyday phenomenaTask 1
· Try the animation· Extension: what is happening to the particles in the air?The particles in the air vibrates more causing the air space between them to increase and the density to decrease, therefore, rising up. The cool air goes down to replace the risen particles. This causes a convection current.Task 2
· Why is Lee tired after cycling to the Sea in the morning?
Lee is tired after cycling to the sea in the morning because the sun heats up the land, making the air heat up quickly, increasing the spaces between the particles and decreasing the density ∴ the hot air rises. The cold air from the sea replaces the space, creating a convection current. This means that Lee is always cycling against the wind making him very tired.
·Predict what will happen when Lee cycles home in the evening
Lee will still be tired when he cycles home in the evening because at night, the convection current will still be there. However, the opposite direction.
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