IGCSE/GCSE/O & A Level/IB/University Student Forum
Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: Igcseboy on May 04, 2010, 09:50:41 am
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Fast modern passenger and car ferries are often twin hull boats, propelled using water jets instead of conventional propellers. A pumping system throws water out backwards at high speed. One such ferry is the Stena Lynx II, which sails between Fishguard in Wales and Rosslare in Ireland.
Some data about the Stena Lynx II:
Resistive forces at cruising speed = 8.0 x 10^5 N
Mass of water per second ejected by the jets = 60000kgs 1
Show that water must be thrown backwards at about 13 ms-1 while the boat is maintaining cruising speed.
The pumping system converts energy which is expended in two main ways: working against resistive forces and giving kinetic energy to the water thrown backwards.
Calculate the power expended against the resistive forces at the cruising speed of 20 m s 1.
Power = ……………………………………
Calculate the rate at which the water in the jets gains kinetic energy.
Rate=………...............................
Hence show that the overall efficiency of the pumping system at cruising speed is about 40%.
Fuel consumption of pumping system at cruising speed= 1.6 litres/second
1 litre of fuel provides 3.4 x 107 J of energy when burnt
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Could you please post the original paper from which you got this from....
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i guess its edexcel salters horners jan 2001
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Sorry, I only do AS Physics, I dont know how to solve that question....
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same here i also do only AS physics but was practicing some of these papers i guess its not a big deal if i cannot do it as it is not of much concern for us!!!! :-\
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force=rate of change of momentum
800000=60000*v
v=800000/60000=13.3m/s
P=Fv=800000*20=1600000W
water gains KE at rate1/2mv^2=1/2*60000*13.333^2=5333333W
effieciency=useful energy out/useful energy in
=(5333333+16000000)/(1.6*34000000)=21333333/54400000=0.4
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so it was related with momentum????
because i dnt have tht in my sylabus@!!!!
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F=ma is in your syllbus - ma is rate of change of momentum
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Sorry, I only do AS Physics, I dont know how to solve that question....
Wot board are you doing? Cos I do AS Physics, and that's quite a common question.. And @Igcseboy, it's newton's second law of motion- the rate of change of momentum of a body is proportional to the resultant force on the body, and acts in the direction of the force.
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ya ya i got it got confused but now am good
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I am doing Edexcel, and we do not have momentum until A2. We have further momentum as well in that topic.