Solving Electrical Resistances & Calculating Work & Mass: Help Needed!

[ritwik06]

1. Three resistances of value 1,2 and 5 have to be connected in such a way that all are used once. Which value cannot be obtained?
1) 17/3 2) 17/4 3) 17/6 4)17/7

I know formulas for parallel and interconnected resistance. My teacher tells me that by looking at the options I can solve this. Pleasse help. I am not able to make out.

2. In stretching a spring through 4 cm, 2 J work is done. If it compressed by 6 cm then amount of work will be?

According to me its 3j but my book gives it 4.5 J. How come?

3. A wooden cube (densit 0.60 g/cc) just dips inside the water if pressed with a force of 10 N. What must be the size of cube? (g= 10m/s^2)

I have no idea.

Please help

 

[radou]

You should show some efforts in order to get help.

Hint: regarding 2), what is the relation between work and the change of potential energy in a spring?

 

[ritwik06]

You should show some efforts in order to get help.

Hint: regarding 2), what is the relation between work and the change of potential energy in a spring?

I am sorry sir. I have problems. I am a student of class 9th. I take part in all the olympiads to learn new things. I have nothing to read except my own class textbooks. I just use the internet. Just imagine as to how can I think about something about which I don't have the slightest idea. I need help from you all (if its not a burden). Please try solving my questions and tell me about the topics and books or sites in which I could find relevant material.

regards

 

[radou]

First of all, you don't have to call me 'sir', not that I don't appreciate your politeness though.

Second, for number 2), you should find this link useful: http://hyperphysics.phy-astr.gsu.edu/hbase/pespr.html" .

 

[ritwik06]

First of all, you don't have to call me 'sir', not that I don't appreciate your politeness though.

Second, for number 2), you should find this link useful: http://hyperphysics.phy-astr.gsu.edu/hbase/pespr.html" .

Thank you very very much for your help. You gave me exactly what I needed. Thanks a million times!