Quiz - Physics - Displacement

[Pedro Burrito]

Spoiler: Answer

b. 82.5 liters / 2.9 cubic feet

for metric - 85 / 1.03 (the weight of 1 litre of salt water)

 

[Angelo Farina]

The answer was obvious for us used to the metric system. Of course it is not so obvious in Imperial units.
However, if one of my students at my university course of Applied Physics had posted a question in these exact terms, I had sacked him immediately. kg is a measure of mass, not weight. Weight should be expressed in Newtons...
So the correct question should have been expressed as follows:
If an object having a mass of 85 kg is neutrally buoyant in salt water, what is the volume of water the object displaces?

 

[rhwestfall]

Of course it is not so obvious in Imperial units.

I tend to disagree

 

[BlueTrin]

I tend to disagree

Would you concede that it is harder ?

 

[Angelo Farina]

Would you concede that it is harder ?

In SI units it is barely simple: 85 kg means 85 liters (in fresh water).
The density of salt water is slightly larger than fresh water, hence the volume is just less than 85 liters.

 

[BlueTrin]

In SI units it is barely simple: 85 kg means 85 liters (in fresh water).
The density of salt water is slightly larger than fresh water, hence the volume is just less than 85 kg.

That’s how I found the solution as well.

I can never remember the average density of sea salt water, only that it is a bit heavier.

 

[Pedro Burrito]

I can never remember the average density of sea salt water, only that it is a bit heavier.

Constants Used For the Dive Theory Exam

 

[HKGuns]

So if I get all these right are one of you instructors going to sign this newbs DM card?

I’m running at 100% so far and with that statement likely ruined my run.

 

[Storker]

keep it simple: 187# / 64 #/cf = XXX

Keep it even simpler: 85 kg / 1 kg/liter = XXX liters. Adjust for the fact that salt water is about 3% denser than freshwater (1.03 kg/liter), and Robert is your mother's brother.

 

[Storker]

kg is a measure of mass, not weight. Weight should be expressed in Newtons...

kp. It's a nice unit, albeit not quite PC. Allow for some slack, and an answer in kg is quite OK, because a mass of 1kg has a weight of 1 kp. And the kp unit makes for easy mental math. Me, I dont much care if the answer is in kg or kp. The student has a good grasp of the relevant math. If the unit is wrong, that's secondary.

1 kp = 9.81N, BTW