IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: vanibharutham on December 06, 2009, 04:50:27 pm

Title: Physics Doubt - Help please
Post by: vanibharutham on December 06, 2009, 04:50:27 pm

A wheel is connected to a motor and turned clockwise at a constant speed of 10 revolutions per second.  Two weights of 80N and 20N are hung from a string passing over the wheel.  With the wheel turning, the system is in equilibrium.  The length of string in contact with the wheel is 0.5m.  What power must the motor produce to turn the wheel?

What is the maximum power that you could achieve from a wind turbine that has a blade radius of 7m in a wind of velocity 5ms-1?

(Take the density of air as 1kgm-3)

Any help really appreciated

Title: Re: Physics Doubt - Help please
Post by: astarmathsandphysics on December 06, 2009, 06:03:12 pm

Will answer in 2 hours

Title: Re: Physics Doubt - Help please
Post by: astarmathsandphysics on December 06, 2009, 08:24:30 pm

A wheel is connected to a motor and turned clockwise at a constant speed of 10 revolutions per second.  Two weights of 80N and 20N are hung from a string passing over the wheel.  With the wheel turning, the system is in equilibrium.  The length of string in contact with the wheel is 0.5m.  What power must the motor produce to turn the wheel?

What is the maximum power that you could achieve from a wind turbine that has a blade radius of 7m in a wind of velocity 5ms-1?

(Take the density of air as 1kgm-3)

Any help really appreciated
Ignore the tuning wheel
For 20N T-20=(20/g)a (1)
For 80N 80-T=(80/g)a (2)
(1)+(2) 60=(100/g)a so a=3g/5 and from (1)T=20+(20/g)*3g/5=32
Distance moved by wheel in 1 second=0.5*10=5 and power=5*32=160J

Power =1/2mv^2=1/2*1*5*pi*7^2*5^2

Title: Re: Physics Doubt - Help please
Post by: staceyboy3 on December 07, 2009, 09:31:47 am

Why do you use Power =1/2mv^2=1/2*1*5*pi*7^2*5^2 ...?

Where does all that come from?

Title: Re: Physics Doubt - Help please
Post by: astarmathsandphysics on December 07, 2009, 11:27:04 am

When i get home

Title: Re: Physics Doubt - Help please
Post by: astarmathsandphysics on December 07, 2009, 08:08:30 pm

see attached explanation

Title: Re: Physics Doubt - Help please
Post by: vanibharutham on December 07, 2009, 08:11:57 pm

Thank you so much astar :)

your the best

Title: Re: Physics Doubt - Help please
Post by: vanibharutham on December 07, 2009, 08:14:51 pm

and that was really stupid of me ...

i had written down all the formulae...

p = m/v etc.

but only managed to a value for area = 7pi


Thanks so much for ur tiem agen :P

Title: Re: Physics Doubt - Help please
Post by: girl_92 on December 08, 2009, 04:48:39 pm

2day i had physics practicals paper we got an electricity que most of dem got the gradient as 0.33 or so i got sum whr around 0.43
n bcoz of tht the y intercept everythin was also difff for me they got 30 nd i got 22.2
culd u temme how many marks wuld i loose the queon the wholewas for 20 marks so will i be gettin a zero for tht