Lower Secondary Science

[Dr.033430Daniel]

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.

[PhysicsTeacher]

CLLIU:

First Lesson science yesterday night turn up to be a big disappointment. Accordingly to be daughter, the tutor keep refering to her book. Also, teaches wrongly few things. Drawing 2 incident ray for light topics, instead of 1 etc.. Always on mobile phone..

Wow Lau..I immediate called to terminate.. Such a shame for a Tuition centre to have these type of resources. It create bad credibility for the centre who has multiple outlets..I wouldnt try a 2nd time, and wouldnt recommend my friends or colleagues.

Shouldn't there industry be better regulated ???

It is waste of our time and money..

This is exactly how incompetent physics teachers that ruin student's interest in Physics. Physics is a very interesting subject and easy high scorer, if it is properly taught. It is never about drills and drills. To learn physics must learn concepts first!

[eyktay]

FOR SECONDARY 1:

1) Without tuition, what are some recommended websites, guidebooks or enrichment materials to recommend?

2) If walking down the Tuition aisle…any recommended centres or teachers?

MTIA!

[eyktay]

FOR SECONDARY 1:

1) Without tuition, what are some recommended websites, guidebooks or enrichment materials to recommend?

2) If walking down the Tuition aisle…any recommended centres or teachers?

MTIA!

[eyktay]

FOR SECONDARY 1:

1) Without tuition, what are some recommended websites, guidebooks or enrichment materials to recommend?

2) If walking down the Tuition aisle…any recommended centres or teachers?

MTIA!

[Dr.033430Daniel]

eyktay:

FOR SECONDARY 1:

1) Without tuition, what are some recommended websites, guidebooks or enrichment materials to recommend?

2) If walking down the Tuition aisle.......any recommended centres or teachers?

MTIA!

For materials available in Popular, the best book I have found is Lower Secondary Science, Conceptual Examples by G.S. Cheng. The questions are stated more clearly and properly vs most of the other books I have found.

For tuition, I offer Sec 1 classes and we have 2 great groups of students this year. And we have one more Sec 1 Holiday Program available last week of June. Just PM me for details.

[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: