Lower Secondary Science

[jetstar]

Dr.Daniel:

jetstar:

Hi Dr Daniel,


Can i check with you on the following question:

A rope is made to swing up and down. A student does this by holding one end of the rope with his hand, and letting the other end swing freely. The wave produced has a speed of 2m/s, a frequency of 4Hz, and a wavelength of 0.5m.

I would like to ask how we can increase the wavelength to 1m?

Do we decrease the frequency of 2Hz? (In other words, assume constant speed, and increase the frequency, using the formula velocity = frequency x wavelength)

Or do we increase the speed to 4m/s? (In other words, assume constant frequency.)

Thank you!

Decrease the frequency. The speed will be constant.

I’m picturing a person with a long rope moving their hand up and down and the wave moves parallel to the ground. Maybe the other end is attached to a wall along a vertical cable so the other end is free to move up and down. If this is not the correct picture, then let me know.

The speed of a wave on a rope or string is determined by the mass per unit length of the string and the material of the string. Consider for example a guitar string. The speed of the wave is determined by how thick the string is and what it is made of. If you look at a 6 string guitar, you will see strings of different thickness and sometimes strings of different material. This gives the instrument player a range of notes that can be played.

You can also change the velocity on a string by changing the tension. So you tune a guitar by turning a screw that changes the tension. This works best if both ends of the string are secured, but it is more difficult if one end of the string or rope is free to move. Also, in a rope it is much harder to change the tension than in a string. So with one end free to move, you would be stuck with one tension and one velocity. So to change the wavelength you would change the frequency with which you move your hand.

By the way, there will still be certain resonant frequencies for a standing wave with an antinode at both ends.

Thank you for the very concise and clear explanation, Dr Daniel. My child understood it so much better now. You are a great teacher!

[Vivian22]

Hi, I need help with this question:


Given that the refractive index for glass is 1.5 and the refractive index of water is 1, what is the critical angle when a light ray travels from glass to air?

Thanks in advance

[Dr.033430Daniel]

Vivian22:

Hi, I need help with this question:


Given that the refractive index for glass is 1.5 and the refractive index of water is 1, what is the critical angle when a light ray travels from glass to air?

Thanks in advance

Hi Vivian – I always suggest that students start with the general form of Snell’s Law which is

ni (sin i) = nr (sin r)

where ni is the index of refration on the incident side and sin i is the sin of the incident angle with respect to the normal, nr is the index on the refracted side and sin r is the sin of the refracted angle. The refraction is into air with index 1. The refracted angle is 90 degrees (this is the definition of critical angle). The sin of 90 degrees is 1, so the right side of the equation just becomes 1.

So the equation becomes

ni (sin i) = 1

We are given that ni = 1.5 so

sin i = 1/1.5

i = 42 degrees

The critical angle is the incident angle, so this is our answer.

For more questions using Snell’s Law, I have a few posted here....

http://physicsandcalculus.com/2012/snells-law-problems

[Vivian22]

Thank you very much Dr. Daniel :goodpost:

[Vivian22]

Hi, I need help with a few Chemistry questions


1) Which of the following shows the number f atoms present in 6 dm^3 of ethene at room temperature and 1 atmosphere?
A) 1.5 x 10^23
B) 3 x 10^23
C) 6 x 10^23
D) 9 x 10^23

2) In 10g of the following halogens, which has the most number of particles?
A) chlorine
B) bromine
C) fluorine
D) iodine

3) An organic compound P reacts with sodium oxide to give a compound with formula C3H5O2Na. WHich of the following is compound P?
A) ethanol
B) propanol
C) ethanoic acid
D) propanoic acid.

Thanks

[Dr.033430Daniel]

Vivian22:

Hi, I need help with a few Chemistry questions


2) In 10g of the following halogens, which has the most number of particles?
A) chlorine
B) bromine
C) fluorine
D) iodine

Thanks

C) Flourine. If you look at the periodic table of the elements you can see the atomic mass of each element. Flourine has the lowest atomic mass for any element on this list, so 10g will contain more particles.

[Dr.033430Daniel]

[quote="Vivian22"]Hi, I need help with a few Chemistry questions


1) Which of the following shows the number f atoms present in 6 dm^3 of ethene at room temperature and 1 atmosphere?
A) 1.5 x 10^23
B) 3 x 10^23
C) 6 x 10^23
D) 9 x 10^23


A.\tEthene has a molecular weight of 28 grams per mole. It has a density of 1.18 kg/m3.
6dm3 = 0.006m3. Multiply that by the density and you get 7.08 grams. Divide this by 28 and you have 0.25 moles. A mole is 6.02 X 10 ^23 atoms. 0.25 moles is 1.5 x 10^23 answer A.

[Chan09]

Are there any recommended sec1 or 2 science guide books that would be useful as I find it hard to know which ones to use as the sec schools vary as I am doing o levels in 2016? The ones I see are from MC or GLM though not sure if school notes are sufficient for lower sec? Tks

[Vivian22]

Thanks again Dr Daniel

[milkbun]

Chan09:

Are there any recommended sec1 or 2 science guide books that would be useful as I find it hard to know which ones to use as the sec schools vary as I am doing o levels in 2016? The ones I see are from MC or GLM though not sure if school notes are sufficient for lower sec? Tks

Do share with me if there's any good ones cos my boy going sec 1 next year.

Anyway, his tutor provides very good notes and materials...so dont really need to worry...but just being kaisu here...tt's why i'm a kaisu parent.

:evil: