# Talk

## Help Science Teachers (or clever bods!) - Can you tell me how to work out these answers for dd1?

(8 Posts)
Polgara2 Mon 18-Jul-11 08:46:22

DD1 is stuck on these questions and I have zilcho knowledge of Physics!!! So anyone who can point us in the right direction would be greatly appreciated!

2 Water waves can be made by vibrating a wooden bar up and down in a tray of water.The bar moves up and down at a frequency of 5 hertz.
(Then diagram showing wave length at 48cm)
2 (a) Calculate the speed, in cm/s, of the water waves.
Write down the equation you use and then show clearly how you work out your answer.

4 (b) Each day the average European family uses 100 kg of hot water.
To kill bacteria, the water going into the tank at 20 °C must be heated to 60 °C.
Calculate the energy needed to increase the temperature of 100 kg of water by 40 °C.
Specific heat capacity of water = 4200 J/kg °C.
Write down the equation you use, and then show clearly how you work out your answer

7 The picture shows a food processor, which is used to grate, shred, liquidise and mix food.
The table gives some information about the food processor.
Energy input Electrical
Useful energy output Kinetic
Power rating 1200 watts
Efficiency 0.8
7 (a) The food processor is used for a total of 30 minutes a day.
Calculate the cost of the energy wasted by the food processor each day.
Electricity costs 15 p per kilowatt-hour.
Write down the equations you use, and then show clearly how you work out your answer.

Thank you very much

Makeminealarge Mon 18-Jul-11 08:53:34

http://tutor4physics.com/workenergy.htm look up power and energy equations for the last two questions the first question relates to speed. hope this helps? if you are still stuck i could work it out for you, but your DD should have the equations in her notes xx

pickledsiblings Mon 18-Jul-11 09:06:23

First question relies on an understanding that frequency in hertz means the number of cycles (waves) per second. So, in this case there are 5 waves, each 48cm long produced every second. Hopefully DD can work out the answer now .

I'll leave the rest for someone else.

Polgara2 Mon 18-Jul-11 09:11:59

Hmm so if speed = wavelength x frequency that makes it 48 x 5? Or am I being thick?

MindtheGappp Mon 18-Jul-11 16:35:22

2) v = f x lamda
= 5/s x 48cm
= 240 cm/s

4) delta H = m x c x delta T
= 100kg x 4200 J/kg degC x 40 degC
= 16.8 MJ

7) E = P x t
= 1.2 kW x 0.5 hr
= 0.6 kWh

Wasted energy = 0.2 x 0.6kWh
= 0.12kWh

Cost of wasted energy = 15p/kWh x 0.12kWh
= 1.8p

Polgara2 Mon 18-Jul-11 23:30:56

Thank you MindtheGappp - that was very helpful! We have more or less understood most of that .

I have looked at that page makeminealarge has suggested and it may as well be in russian or something . I clearly don't get physics!

DD1 says she only has the equation for the first question not the others (but we couldn't work out how to utilise it - we do now so thank you! )

Her teacher has given them some information but hasn't gone through any of it with them so she is a bit in the dark. I have also just realised that these are GCSE questions and she is only in y9 so I am not too concerned that she was scratching her head a bit at first!

Thanks all

sammie0782 Tue 19-Jul-11 11:14:27

Specific heat capacity is usually A Level - and chemistry rather than physics! (I tutor these subjects so am quite sure on this!)

Not sure what her teacher is trying to acheive...!

MindtheGappp Tue 19-Jul-11 12:09:49

Specific heat is definitely in AS Physics.

It is also used in KS3 and 4 when teaching about electrical energy and efficiency. It's a standard Y9 practical. Specific heat is not explained - it a case of giving the equation and the values of c for water, aluminium, copper etc. A clever kid might question why the values are different.

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