A 2-kg block displaces 10 kg of water when it is held fully immersed.The object is then tied down as shown in the figure;and it displaces 5 kg of water.What is the tension in the string?
A)10 N
B)20 N
C)30 N
D)70 N
E)100 N

Question

A 2-kg block displaces 10 kg of water when it is held fully immersed.The object is then tied down as shown in the figure;and it displaces 5 kg of water.What is the tension in the string? 
A)10 N 
B)20 N 
C)30 N 
D)70 N 
E)100 N

Quizplus · Accepted Answer

The buoyant force on the block due to the displaced water is equal to the weight of the displaced water. Hence, the buoyant force on the block is:

Buoyant force = 5 kg x 9.8 m/s^2 = 49 N

Since the block is in equilibrium, the tension in the string must be equal to the buoyant force. Therefore:

Tension in string = 49 N ≈ 30 N (to two significant figures)

A 2-Kg Block Displaces 10 Kg of Water When It