Lower Secondary Science

[fruittypie]

excellent! Thanks for sharing

[fruittypie]

Vivian22:

Dr.Daniel:

[quote=\"Vivian22\"]Hi,


Can someone now help me with this Chemistry question? How do we derive at the answer? Thanks in advance! http://i44.tinypic.com/35n4py8.png\">

The winner is B. Nitrogen needs three more electrons to form a complete outer shell and I see that symbol Q has three lines coming from it. Each one of those lines represents a bond or shared electron. Symbol R has 4 lines connected to it, meaning it has four bonds. Elements that do that are in the Carbon, Silicon group (vertical column) , so R could be Silicon. And symbol V has a single bond, so it could be Hydrogen, which needs one more electron to complete the valence shell.

Choice A is no good because there is no way Q is Fl. Fl just needs one bond to get to the noble gas (full valence shell) configuration. Also there is no way V is Si because it is only forming one bond. Si will form 4.

Choice C is no good because Q is not Oxygen. Oxygen will have two bonds for two shared electrons.

And D is no good because Q is not Si for reasons stated above.

I just received a neat chemistry model kit and sometimes have students put together some molecules in class. Its great because each element has a different color and there are a certain number of holes drilled in wooden spheres so that you know how many bonds each element makes. You connect the wooden spheres with little wooden rods. Also the holes are drilled at the correct bond angles so that you can see the proper shape of each molecule.

Thank you! That was a great explanation :goodpost:[/quote]Are you a student? Do you mind to email the papers to me?

Thanks

[kitty2]

Dr Daniel,can you help with these questions? :thankyou:


A block of wood is brought to the planet Mars.
1) state if there will be a change in mass and explain why?
2)state if there will be a change in weight and explain why?
3) find the weight,given that mass is 500g.

[Dr.033430Daniel]

kitty2:

Dr Daniel,can you help with these questions? :thankyou:


.

A block of wood is brought to the planet Mars.
1) state if there will be a change in mass and explain why?

There is no change in mass. Mass is the amount of matter in a substance. It is how much material is present. So no matter where you are: on Earth, the Moon, Mars, even in space far away from any planet or star, you always have mass. 500 grams is always 500 grams.

2)state if there will be a change in weight and explain why?

Weight in physics refers to the force of gravity acting on a mass. Weight is expressed in the SI units of Newtons. On the earth, the gravitational field strength is 10N/kg (the value used in secondary schools preparing for the O-Level), meaning every 1kg has a force of 10N acting on it. On mars, the gravitational field strength is less, only 3.7 N/kg. So that means that on Mars, you can have the same amount of mass, but it weighs less.

3) find the weight,given that mass is 500g.

You take the mass in kg times the gravitational field strength on Mars. The mass is 0.5kg and the gravitational field strength on Mars is 3.7N/kg, so multiplying those together you get a weight of 1.85N.

I always tell the students that it is possible that someone sitting in school today may walk on Mars. I hope the USA gets off their butts and takes the lead on this, but if they do not, someone else will. And the trip to Mars will need scientists of all kinds. Engineers to build the space ship, chemists to help the astronauts breathe on the way there, biologists to help grow plants on Martian soil, the list goes on. My Father played a small part in the Apollo moon landing project by designing a pressure gage that is sitting on the moon right now, so I always try to inspire students to think about the potential of a manned mission to mars.

[kitty2]

Dr.Daniel:

kitty2:

Dr Daniel,can you help with these questions? :thankyou:


.

A block of wood is brought to the planet Mars.
1) state if there will be a change in mass and explain why?

There is no change in mass. Mass is the amount of matter in a substance. It is how much material is present. So no matter where you are: on Earth, the Moon, Mars, even in space far away from any planet or star, you always have mass. 500 grams is always 500 grams.

2)state if there will be a change in weight and explain why?

Weight in physics refers to the force of gravity acting on a mass. Weight is expressed in the SI units of Newtons. On the earth, the gravitational field strength is 10N/kg (the value used in secondary schools preparing for the O-Level), meaning every 1kg has a force of 10N acting on it. On mars, the gravitational field strength is less, only 3.7 N/kg. So that means that on Mars, you can have the same amount of mass, but it weighs less.

3) find the weight,given that mass is 500g.

You take the mass in kg times the gravitational field strength on Mars. The mass is 0.5kg and the gravitational field strength on Mars is 3.7N/kg, so multiplying those together you get a weight of 1.85N.

I always tell the students that it is possible that someone sitting in school today may walk on Mars. I hope the USA gets off their butts and takes the lead on this, but if they do not, someone else will. And the trip to Mars will need scientists of all kinds. Engineers to build the space ship, chemists to help the astronauts breathe on the way there, biologists to help grow plants on Martian soil, the list goes on. My Father played a small part in the Apollo moon landing project by designing a pressure gage that is sitting on the moon right now, so I always try to inspire students to think about the potential of a manned mission to mars.

:thankyou: Dr Daniel :salute:

[Crester]

Hi, need help in this Physics Ques.

1)a small cork is fixed with wax to a metal plate. An electric heater is placed close to thè plate. After some time, the wax melts and the cork drops off.
How does the heat reach the wax?
A) by conduction only
B) by conduction and convection
C) Radiation and conduction
D) Radiation and convection

Thanks.

[Dr.033430Daniel]

Crester:

Hi, need help in this Physics Ques.

1)a small cork is fixed with wax to a metal plate. An electric heater is placed close to thè plate. After some time, the wax melts and the cork drops off.
How does the heat reach the wax?
A) by conduction only
B) by conduction and convection
C) Radiation and conduction
D) Radiation and convection

Thanks.

The question is vague, but the answer I think they are looking for is C. I am picturing a plate that is vertical with wax on the plate. I picture the electric heater to the side of the plate, not directly underneath. For this type of question , a drawing is really needed in the question in order to make it clear. So radiation is how the heat gets from the heater to the metal plate. Since it is a metal plate, that is a tipoff that there must be conduction from the plate to the wax. I suppose when I look at this, conduction and radiation are a must and convection only if the heater is underneath the plate. So only choice C fits.

[Crester]

Dr.Daniel:

Crester:

Hi, need help in this Physics Ques.

1)a small cork is fixed with wax to a metal plate. An electric heater is placed close to thè plate. After some time, the wax melts and the cork drops off.
How does the heat reach the wax?
A) by conduction only
B) by conduction and convection
C) Radiation and conduction
D) Radiation and convection

Thanks.

The question is vague, but the answer I think they are looking for is C. I am picturing a plate that is vertical with wax on the plate. I picture the electric heater to the side of the plate, not directly underneath. For this type of question , a drawing is really needed in the question in order to make it clear. So radiation is how the heat gets from the heater to the metal plate. Since it is a metal plate, that is a tipoff that there must be conduction from the plate to the wax. I suppose when I look at this, conduction and radiation are a must and convection only if the heater is underneath the plate. So only choice C fits.

Thanks!

[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!

[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.