One possibility is a landmark.
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WAY 1: multiplying are and the depth of an object WAY 2: dividing weight by the density of an object
to feflect an object so the reflected object is the right way up
Since WE cannot see the object... we cannot help you !
The 1 way you can tell an object is in motion is by having a focus point that does not move and watch and see if the object you are looking for is moving. Another way to notice where the object is now, then later, check and see if the object has been moving. lastly, if you are a scientist, you can use telescopes, computers, and the latest gadgets to tell you if an object is in motion.
If the object is irregular the best way is to measure the volume of water it displaces when you immerse it completely in water.