<p>This was an MC question on the mid-term and I’m not so sure about it.</p>
<p>An object floats in water but sinks in oil. The object is half submerged when it floats in water. If oil is poured on the top of the object without spilling over, does the object…</p>
<p>a. rise
b. sink
c. stay at the same level</p>
<p>I said rise since the average density of the object/oil will decrease when oil is poured over it. However, practically, it should be at about the same level. You can’t pour that much oil over the top of an object to have it rise substantially and the oil also adds height to the object which would negate any perceived rising (since a little addition in height would increase the percentage that the object is above the water).</p>
<p>I’m not entirely sure, but I’d say stay at the same level. Since the oil would float on the water, the object would sink in the oil, and float in the water, threby remaining in between the two liquids, essentially staying where it is.</p>
<p>The object would sink a bit. While this could be figured out intuitively…here’s the formal reason:</p>
<p>Remember, an object displaces its own weight in liquid (In other words, the buyoant force on a floating object equals its weight). Because adding the oil would increase the mass or weight of the object, it would displace more water. In order to displace more water, a larger amount of volume of the object would need to be submerged…and thusly the object would sink a bit.</p>