a. To find the buoyancy when the object has been fully immersed in oil, we can use the principle of buoyancy which states that the buoyant force on an object is equal to the weight of the fluid displaced by the object.

• Buoyant force = Weight of fluid displaced

Let W be the weight of the object and V be the volume of the object.

When the object is fully immersed in oil, the buoyant force is equal to the weight of the oil displaced by the object.

• Buoyant force = Weight of oil displaced = Density of oil x Volume of object x g

where g is the acceleration due to gravity and Density of oil is given as the weight of the oil per unit volume.

• Weight of the object - Buoyant force = Weight of the object in oil
• W - Density of oil x V x g = 40N
• Buoyant force = Density of oil x V x g = W - 40N = 60N - 40N = 20N

Therefore, the buoyancy when the object has been fully immersed in oil is 20N.

b. To find the buoyancy when the object has been only 1/3 immersed in the oil, we need to find the volume of the object that is submerged in the oil.

Let V' be the volume of the object that is submerged in oil when it is 1/3 immersed.

• Then, V' = (1/3)V

Buoyant force = Weight of oil displaced = Density of oil x Volume of object submerged x g

• Buoyant force = Density of oil x V' x g
• Buoyant force = Density of oil x (1/3)V x g
• Weight of the object - Buoyant force = Weight of the object in oil

• W - Density of oil x (1/3)V x g = 40N
• Buoyant force = Density of oil x (1/3)V x g = W - 40N
• Buoyant force = Density of oil x V x g - 40N
• Buoyant force = 20N - (Density of oil x (2/3)V x g)

Therefore, the buoyancy when the object has been only 1/3 immersed in the oil is (20N - Density of oil x (2/3)V x g).

c. To find the weight of the object when it has been only 1/3 immersed in the oil, we can use the same equation as in part b.

• Weight of the object = Buoyant force + Weight of the object in oil
• Weight of the object = Density of oil x (1/3)V x g + 40N
• Weight of the object = Density of oil x (1/3)W/g x g + 40N
• Weight of the object = (1/3)W x Density of oil + 40N

where W is the weight of the object.

Therefore, the weight of the object when it has been only 1/3 immersed in the oil is (1/3)W x Density of oil + 40N.

Pembahasan:

Hukum archimedes mengatakan bahwa benda yang dicelupkan sebagian atau seluruhnya ke dalam zat cair akan mendapatkan gaya tekan ke atas sebesar berat zat cair yang didesak benda itu.

Gaya tekan ke atas yang dialami benda dirumuskan dengan:

F = Vb · Sc

F = Vb ·  рc ·  g

Berat benda dalam zat cair dapat dirumuskan:

W' = W - F

Keterangan:

• F = gaya tekan ke atas oleh zat cair
• Sc = berat jenis zat cair
• рc = massa jenis zat cair
• g = perepatan gravitas
• W' = gaya beratatau berat benda dalam zat cair

Keadaan benda saat dicelupkan ke dalam air:

• Terapung. Benda terapung dalam zat cair jika massa jenis benda lebih kecil dari massa jenis zat cair dan gaya tekan ke atas sama dengan berat benda.
• Melayang. Benda melayang dalam zat cair jika massa jenis benda sama dengan  massa jenis zat cair dan gaya tekan ke atas sama dengan berat benda.
• Tenggelam. Benda terapung dalam zat cair jika massa jenis benda lebih besar dari massa jenis zat cair dan gaya tekan ke atas sama dengan berat benda.

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