Question

Ram is being weighed by the spring balance in two different situations. First when he was fully immersed in water and the second time when he was partially immersed in water, then
A. Reading will be more in the first case.
B. Reading will be more in the second case.
C. Reading would be the same in both cases.
D. Reading will depend on the experimental setup.

Answer 1

We will solve this problem with the help of buoyant force and its application. In both the cases the setup is the same and the only difference is the man fully immersed in water and partially immersed in water. We also see that the question clearly states that Ram is being measured by the spring balance.

Complete answer:
Note that Ram is being measured by the spring balance is told in the question, but also in option D it is said that the reading will depend on the experimental setup, thus we can get actually two answers from the given conditions.Let’s observe the first case: Ram is being measured

If Ram is completely immersed in water, he will experience more buoyant force as more volume is then immersed in the water and the formula of Buoyant force being, F = $\rho gV$ , where the terms represent density of fluid, acceleration due to gravity and volume of immersed body respectively. Thus, we can say that in the first case when the body is completely immersed, the value of buoyant force experienced by the body is higher.

Now buoyant for buoyant force experienced is in direction opposite to gravitational weight, thus the value of Effective weight measured by the spring balance is W – F, where W is gravitational weight and F is buoyancy force.Now, we can understand why the body weighs less when completely immersed than when partially immersed due to phenomenon of Buoyant force.

Hence the correct option is B.

Note: The main mistakes students make is they do not see whether the questions ask about the buoyant force or the measured weight and answer the question. Remember that direction buoyant force is opposite to that of gravitational. These questions can also ask about the volume of the immersed part by providing the mass of the body immersed.