Gartley222
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Need help from the experts! please help me solve this. Thank you
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uberteknik
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(Original post by Gartley222)
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Need help from the experts! please help me solve this. Thank you
In the first question, the conservation law means that the kinetic energy cannot be lost, it must be converted into another form of energy. The problem is to work out what causes drag, and what the kinetic energy of the object converted into.

In the second question, you need to again look at the energy conversion process, and realise that energy is lost during the pumping process, electrical energy conversion, evaporation etc. as the water fills the reservoir. Similarly, when the reservoir potential energy is released, losses will occur as it first converts to kinetic and then to electrical energy. The problem is to work out how the energy losses accrue and where those losses occur. Then make a statement about the efficiency of the system which limits the useful energy which can be stored and retrieved in this way.
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Physics Enemy
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(Original post by Gartley222)
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1) Car exerts force on air it pushes against, by N3 experiences drag force ∝ v^2. Early on: faster car => higher drag, more work done by car on air, linear KE (car) -> thermal (air). Later: slower car => less drag => lower energy transfer rate.

Also friction of road against tyres: rotational KE (wheels) -> thermal (road). By the time car comes to rest, all KE transferred to heating impacted air and road.

2) Spare elec during off-peak periods is used to spin (reversible) generators, in turn driving the (reversible) water turbines. This pumps water through height h, from lower to upper reservoir.

Elec energy -> rotational KE transfer is quite inefficient due to elec impedence. Also energy losses by fluid resistance ∝ speed of the water.

Losses are mostly conversions to thermal energy in the generators, turbines and moving water. Pump storage hydroelec ~ 70-80% efficient.
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Gartley222
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(Original post by uberteknik)
In the first question, the conservation law means that the kinetic energy cannot be lost, it must be converted into another form of energy. The problem is to work out what causes drag, and what the kinetic energy of the object converted into.

In the second question, you need to again look at the energy conversion process, and realise that energy is lost during the pumping process, electrical energy conversion, evaporation etc. as the water fills the reservoir. Similarly, when the reservoir potential energy is released, losses will occur as it first converts to kinetic and then to electrical energy. The problem is to work out how the energy losses accrue and where those losses occur. Then make a statement about the efficiency of the system which limits the useful energy which can be stored and retrieved in this way.
Thank you so much!
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Gartley222
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(Original post by Physics Enemy)
1) Car exerts force on air it pushes against, by N3 experiences drag force ∝ v^2. Early on; faster car => higher drag, more work done by car on air, linear KE (car) -> thermal (air). Work done by air pressure => car heats up, as thermal (air) -> thermal (car).

As car decelerates, drag force dec, rates of energy transfer dec. Note friction of road against tyres, rotational KE (wheels) -> thermal (road). By N3, road transfers thermal to tyres, heats them up.

2) Spare elec during off-peak periods is used to spin (reversible) generators, in turn driving the (reversible) water turbines. This pumps water through height h, from lower to upper reservoir.

Elec energy -> rotational KE transfer is quite inefficient due to elec impedence. Also energy losses by fluid resistance ∝ speed of the water.

Losses are mostly conversions to thermal energy in the generators, turbines and moving water. Pump storage hydroelec ~ 70-80% efficient.
(Original post by Physics Enemy)
1) Car exerts force on air it pushes against, by N3 experiences drag force ∝ v^2. Early on; faster car => higher drag, more work done by car on air, linear KE (car) -> thermal (air). Work done by air pressure => car heats up, as thermal (air) -> thermal (car).

As car decelerates, drag force dec, rates of energy transfer dec. Note friction of road against tyres, rotational KE (wheels) -> thermal (road). By N3, road transfers thermal to tyres, heats them up.

2) Spare elec during off-peak periods is used to spin (reversible) generators, in turn driving the (reversible) water turbines. This pumps water through height h, from lower to upper reservoir.

Elec energy -> rotational KE transfer is quite inefficient due to elec impedence. Also energy losses by fluid resistance ∝ speed of the water.

Losses are mostly conversions to thermal energy in the generators, turbines and moving water. Pump storage hydroelec ~ 70-80% efficient.
THank you very much!
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