An object requires a force of 30 N to hold it under water. The object weighs 90 N in air. Its density is nearest:
A) $850 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
B) $800 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
C) $750 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
D) $700 \mathrm {~kg} / \mathrm { m } ^ { 3 }$

Question

An object requires a force of 30 N to hold it under water. The object weighs 90 N in air. Its density is nearest: 
A) $850 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
B) $800 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
C) $750 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
D) $700 \mathrm {~kg} / \mathrm { m } ^ { 3 }$

Quizplus · Accepted Answer

$750 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
A free-body diagram would show $30 \mathrm {~N}$ and $90 \mathrm {~N}$ acting down and the buoyant force $F _ { B }$ acting up (see Eq. 2.4.36):
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11edd9e5_4b3e_379b_bca1_4150e353eb26_TB8827_11

An Object Requires a Force of 30 N to Hold 850 kg/m3850 \mathrm {~kg} / \mathrm { m } ^ { 3 }850 kg/m3

An Object Requires a Force of 30 N to Hold $850 \mathrm {~kg} / \mathrm { m } ^ { 3 }$