Question

Which one of the following would a hydrogen balloon find easiest to lift?
A. 1 kg of steel
B. 1 kg of water
C. 1 kg of lightly packed feathers
D. All the same

Answer 1

Recall the principle of floatation. The upward force acting on the given objects is the buoyant force. The buoyant force depends on the volume of the object. If the volume of two objects is the same, then and then only the two objects will reach the ground at the same time. Note that the buoyant force is independent of the mass of the object.

Complete Step by Step Answer:
We can see the above question is based on the principle of floatation. When we dip an object in the liquid, the liquid resists the sinking of that object. The resistance force provided by the liquid is known as buoyant force. We know that the direction of the buoyant force is always upward. We have the expression for the buoyant force acting on the object is,
${F_B} = {\rho _l}Vg$
Here, ${\rho _l}$ is the density of the liquid, V is the volume of the object and g is the acceleration due to gravity.

From the above equation, we can infer that the buoyant force depends on the volume of the object. In the case of lightly packed feathers, since the density of the feathers is very small, the volume of the bunch of these feathers would be very large as compared to the volume of steel and water. Therefore, the buoyant force acting on the lightly packed feathers is greater as compared to steel and water. So, the balloon will lift the lightly packed feathers easily since the additional upwards force (buoyant force) is acting on the feathers.

So, the correct answer is option C.

Note: There is a misconception that the 1 kg of feathers bunch and 1 kg of iron ball will fall at the same time on the ground if dropped from the same height. Due to the larger volume of 1 kg feathers, the upward buoyant force acting on the feathers is greater than the buoyant force acting on the iron ball and therefore, the iron ball will reach the ground before the bunch of feathers.