work done in raising a stone of mass 4kg , specific gravity 4 , immersed in water from a depth of 3m to 1m below the surface is-
a)58.8J b)49.6J c)11.7J d)97.3J
THE ANSWER IS (a)58.8J ............ but how???
2007-03-06 00:20:51 · 5 answers · asked by Anonymous in Science & Mathematics ➔ Physics
This is a 2 part problem.
The work done against gravity is simply the downward force multiplied by the distance raised.
If the stone wasn't under water, the downward force would simply be mass * g
Because it is under water, the downward force will be partly offset by the upthrust from the water
Under water the stone displaces its own volume of water.
The upthrust is equal to the weight of water displaced by the stone. Since the stone has a s.g. of 4 the weight of the water displaced will be a quarter the weight of the stone.
Hence the net downward force will be mg - (mg)/4
Taking g as 9.81
force = 3 * 9.81 Newtons
If you are raising it from 3 m below to 1 m below, you are raising it 2 metres
Work done = 3 * 9.81 * 2 = 58.86Joules
Good luck with the test.
2007-03-06 00:47:38 · answer #1 · answered by lunchtime_browser 7 · 1⤊ 0⤋
The mass is 4 kg. The weight would be 4 x 9.81 = 39.24 N
The specific gravity is 4. Hence the upthrust due to buoyancy would be 39.24/4 = 9.81 N
The net downward force would be 39.24 - 9.81 = 29.43 N
To raise the stone against this force through 2 m, the work done would be 29.43 x 2 = 58.86 J
Simple, isn't t?
2007-03-06 08:49:57 · answer #2 · answered by Bharat 4 · 2⤊ 0⤋
Specific Gravity = 4 which means that density of stone = 4000kg/m^3
The volume of stone = mass/density
= 4/4000
=0.001m^3
This is also the volume of water displaced. SO the mass of water displaced = volume x density
= 0.001 x1000
= 1Kg
So the upthrust on the stone will be this mass x g
= 1 x 9.8
=9.8N
and weight of stone = 4 x 9.8
= 39.2N
The net weight of the stone = 39.2 - 9.8
= 29.4 N
Work done = this force x distance
= 29.4 x 2
= 58.8J
2007-03-06 08:44:36 · answer #3 · answered by Southpaw 5 · 2⤊ 0⤋
Take of the gravity and work it out 4kg multiply by 3m and multiply by 4 for gravity
2007-03-06 08:27:05 · answer #4 · answered by Anonymous · 0⤊ 1⤋
pls check out this link
http://tutor4physics.com/unitsofwork.htm
hope it helps in guiding u.....
All the very best!
also check out this link
http://sify.com/news_info/education/ollessons/fullstory.php?id=13362129&page=2
I think this may be more relevant and hopefully helpful.....
:-)
2007-03-06 08:38:57 · answer #5 · answered by mari 2 · 1⤊ 0⤋