Page 119 - Physics

Sinking and fl oating

Solution
Given,

Upthrust  weight of balloon and fabric weight of hydrogen greatestweight  (i)
Upthrust !weight of theairdisplaced = volumeof theair×density×gravity

7
3
-3
(1.0× 10 × 10 −6 m × 1.25 kg/m × 10 N/kg) ==10×1.25×10 =125 N
Weight of hydrogen  Density of Hydrogen volume g 
−6
3
7
3
Weight of hydrogen = 0.90 kg / m ×1.0 ×10 ×10 m ×10N / kg = 9N
Ther efore, by substituting these values in equation (i)
125 N = 80 N + 9 N + greatest weight
Hence, the greatest added weight 125 N 89 N 36 N  

Submarines Ships
Submarines are made of steel and other A ship is made up of steel which is denser
dense materials with a lot of space fi lled than water. One would expect the ship to
with air because of the space the average sink in water due to the high density of steel.
density of the submarine is lower than However, the ship has a large volume with
that of seawater so it fl oats. If we wish to a hollow space. Large volume allows the
submerge (sink) the submarine, seawater is ship to displace a large volume of water
allowed to fl ow into its ballast tanks until the leading to a greater upthrust. Furthermore,
average density of the submarine becomes the hollow space makes the average density
greater than that of the seawater. To make the of the ship to be less than the density of the
submarine resurface (fl oat), air is forced into seawater. Therefore, the ship fl oats on water.
the ballast tanks pushing out the water until When the ship is loaded, its mass increases,
the submarine’s average density is again less resulting to the increase of average density.
than that of the seawater. Figure 5.12 shows This causes the ship to move lower into
a submarine fl oating on water. the water. Putting more load into the ship
will make the ship to move deeper into the
water. If the load is increased beyond a
certain limit, the ship will sink completely.
Therefore, the amount of load in the ship
must be controlled to avoid overloading.
It is for this reason ships are marked with
a special line which shows the maximum
loading that the ship can take. This line is
known as the Plimsoll lines, shown in Figure
Figure 5.12: Submarine 5.13.

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Physics Form 1 Final.indd 113 16/10/2024 20:56