Lower Secondary Science

[superkiasume]

Dr.Daniel:

superkiasume:

Hi, can someone help my son with the following questions:


(a) The engine of a car of mass 800kg can develop a constant 80kW of power. If its maximum speed on a flat road is 100km/h, determine the resistance to its motion.

(b) With the engine working at the same rate, find the acceleration of the car when is travelling at a speed of 50km/h, if the resistance is constant.

Thank you.

For Part a, you can use the formula that at constant velocity

Power = Force X velocity

100km/hr converts to 27.8 m/s

80,000 W = Force X 27.8m/s

So Force = 2880 Newtons.

The driving force pushing the car forward is equal to the force of friction because it is moving at constant velocity.

Usually students remember that Power = Energy / time
At constant velocity the work done to push the car is the force times the distance. Then if we divide by time we get power. So if the car travels 27.8 meters in one second, then force X 27.8m is the work - then divide by 1 second of time. So this is equivalent to saying that Power = Force X velocity when moving at constant velocity.

part (b) is a bit confusing to me. If the engine is working at the same rate, that means it is delivering the same power. But the statement that the resistance is constant is confusing. Most of the resistance to a car is air resistance which depends on speed. So it is a bit hard to decipher part (b).

Thank you, Dr Daniel

[Bookshop]

Bookshop:

Bookshop:

The differences in the yearly TYS are:

1) The answer key provided. In terms of the answers provided, SAP seems to have come a long way. The answer keys used to be littered with errors here and there. But they have improved signficantly, and now generally provide answers that are more trusted than the other publishers.
2) Pricing. Popular bookshops are offering higher discounts for EPH, as it's their in-house brand.
3) Size of the book. Shinglee and EPH books are of A4 sizes, which is the O-Level standard. Also, their pages have been specially designed so that they can be torn out from the book for separate yearly revision.

You have to decide which of the above factors is the most important.
[quote=\"hopefulparent\"]I am thinking of buying the yearly TYS for my boy. He is taking the O-levels in a few months' time.

Does anyone have any experience as to which TYS is preferable? I saw EPH, SAP, Shinglee and some other brands at Popular. Are they all the same?

Thank you.

Thank you for the advise, Bookshop.

Do you have any recommendations for Physics and Chemistry revision books, where there are notes and formulas. Do you think they will be useful for last minute revision?[/quote]
EPH Science At Your Fingertips - pocket-size notebook with summarised notes.

Smashing Exams Physics and Chemistry - complete guidebook with full notes.

[tutor76]

Its time to try another new question for budding tutor out there


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[sunnymoon]

Can someone help me with this question:

Suggest reasons why copper pots are preferable to aluminium pot for cooking acidic food?

[tutor76]

Copper is lower in the metal reactivity series ,therefore it does not react with acid ,aluminium is higher in the metal reactivity series ,will react to form aluminium salt ,bad for human health .

[tutor76]

Another set of comb chemistry for student to

try ! http://i60.tinypic.com/ehx06h.jpg\">

[tutor76]

Set 2 !


http://i62.tinypic.com/2m2uxvk.jpg\">

[Rufus poko]

hi, I urgently require a science tutor for my sec 1 boy. I sincerely appreciate a good recommendation via PM. Thank you in advance!

[tutor76]

Have already pm you.thanks

[eve74sg]

Pls advise if anyone know of any good tutor or tuition centre for Sec 1 science. Prefer central location. Thanks!