Physics ( Pressure )

• dbowie

  18 Jul 07, 18:49

  Three cylindrical blocks X,Y,Z are made of different densities 2.00g/cm^3, 5.00g/cm^3 and 10.00g/cm^3. If the sides of each block resting on a level surface has an area of 2cm^2, 4cm^2 and 6cm^2 and the height of block X is 10cm, calculate the
  
  ( i ) weight of block X,
  
  ( ii ) pressure exerted by block X on the surface,
  
  ( iii ) heights of block Y and Z if pressure exerted by each block on the surface is the same.
  
  ( Take g=10m/s^2 )
• unclebutcher

  18 Jul 07, 21:27

  Originally posted by dbowie:

  Three cylindrical blocks X,Y,Z are made of different densities 2.00g/cm^3, 5.00g/cm^3 and 10.00g/cm^3. If the sides of each block resting on a level surface has an area of 2cm^2, 4cm^2 and 6cm^2 and the height of block X is 10cm, calculate the
  
  ( i ) weight of block X,
  
  ( ii ) pressure exerted by block X on the surface,
  
  ( iii ) heights of block Y and Z if pressure exerted by each block on the surface is the same.
  
  ( Take g=10m/s^2 )

  i.) Weight X: (10cm)(2cm^2)*(2.00gcm^-3)*(10Nkg^-1)/1000(to convert G to KG)=0.4N
  
  ii.) P=F/A
  
  P= Weight/base area
  = (0.4N)/(2/(100^2)m^2)
  = 4000Pa
  
  iii.) P=F/A
  F=mg
  P=mg/A
  m=(base area)(height)
  Thus
  P=(base area)(height)(g)/A
  
  for Y
  
  P= (4)(height)(10)/4
  
  for Z
  
  P= (6)(height)(10)/6
  
  cross multiply.
  
  im not sure if im right cuz im doing it mentally without pen and paper,and im really shagged now.sorry if answer is wrong. eagle help check? i think iii.) is wrong
• starmoonsun

  19 Jul 07, 15:32

  Originally posted by unclebutcher:

  i.) Weight X: (10cm)(2cm^2)*(2.00gcm^-3)*(10Nkg^-1)/1000(to convert G to KG)=0.4N
  
  ii.) P=F/A
  
  P= Weight/base area
  = (0.4N)/(2/(100^2)m^2)
  = 4000Pa
  
  iii.) P=F/A
  F=mg
  P=mg/A
  m=(base area)(height)
  Thus
  P=(base area)(height)(g)/A
  
  for Y
  
  P= (4)(height)(10)/4
  
  for Z
  
  P= (6)(height)(10)/6
  
  cross multiply.
  
  im not sure if im right cuz im doing it mentally without pen and paper,and im really shagged now.sorry if answer is wrong. eagle help check? i think iii.) is wrong

  huh i dun understand part 2.
  
• eagle

  19 Jul 07, 17:36

  Originally posted by starmoonsun:

  huh i dun understand part 2.
  

  part 2 he's converting the 2cm^2 into m^2 because when we use the equations, we need to bear in mind to convert everything to SI Units
• eagle

  19 Jul 07, 17:42

  Originally posted by dbowie:

  Three cylindrical blocks X,Y,Z are made of different densities 2.00g/cm^3, 5.00g/cm^3 and 10.00g/cm^3. If the sides of each block resting on a level surface has an area of 2cm^2, 4cm^2 and 6cm^2 and the height of block X is 10cm, calculate the
  
  ( i ) weight of block X,
  
  ( ii ) pressure exerted by block X on the surface,
  
  ( iii ) heights of block Y and Z if pressure exerted by each block on the surface is the same.
  
  ( Take g=10m/s^2 )

  (i) Unclebutcher is correct
  
  (ii) Unclebutcher's workings correct, but careless
  area of block X = 2 cm^2 = 2/10000 m^2
  Hence, pressure = 0.4 / (2/10000) = 2000Pa
  
  (iii)
  Block Y:
  Weight = 2000Pa * (4/10000) = 0.8N = 80g
  Hence, 5 * 4 * h = 80
  h = 4 cm
  
  Block Z:
  Weight = 2000Pa * (6/10000) = 1.2N = 120g
  Hence, 10 * 6 * h = 120
  h = 2 cm
  
  There's a slightly shorter way to do because they are cylinders, but it will be damn confusing, and does not explain the concept of pressure properly...