Edexcel A Level Physics:复习笔记4.2 Upthrust

Atmospheric pressure at sea level has a value of 100 kPa. The density of sea water is 1020 kg m-3.

At what depth in the sea would the total pressure be 250 kPa?

A. 20 m               B. 9.5 m               C. 18 m          D. 15 m

Icebergs typically float with a large volume of ice beneath the water. Ice has a density of 917 kg m-3 and a volume of Vi.

The density of seawater is 1020 kg m-3.What fraction of the iceberg is above the water?

A. 0.10 Vi          B. 0.90 Vi          C. 0.97 Vi          D. 0.20 Vi

Don't get confused by the two step process to find upthrust.

• Step 1: You need the volume of the submerged object, but only because you want to know how much fluid was displaced
• Step 2: What you really want to know is the weight of the displaced fluid.

A couple of familiar equations will help;

• m = ρV to get mass (and that's the V from step 1 out of the way),

then

• W = mg to get weight

If you are feeling particularly mathematical, you can combine your equations, so that W = ρVg