What’s upthrust?

• Upthrust is a force that only exists in fluids, such as lakes, oceans, swimming pools and even cups of tea!

• It is a push that always goes straight up, against the direction of gravity.

It pushes UP! That’s why it’s

called Upthrust!

Upthrust in action

This boat has gravity pulling it down, and the upthrust of the water pushing up.Because the forces are balanced, the ship floats.

Upthrust

Gravity

What would happen if the forces weren’t balanced?

More examples of upthrust

If you let go of your drinking straw, it floats up and sometimes tries to bob out of the glass!

Floating in the sea

• Salty seas have more upthrust. It’s much easier to float in a sea than in a swimming pool.

• The Dead Sea is the saltiest sea on earth. It’s REALLY easy to float on it!

You can even read a newspaper!

Shape affects upthrust.

• The larger the surface area of an object, the more the upthrust.

Imagine trying to hold this large beach ball, and this small

tennis ball, under the water. Which would

be easier?

• That’s why swimmers float on their backs, not upright in the water.

(You can stay afloat like this, but it’s harder work, called “treading water”. If you stopped moving your feet you’d sink, because the force of gravity would be stronger than the upthrust.)

Submarines• Submarines are specially designed to sail

underwater. They fill up with water which makes them weigh more. More weight = more gravity. The gravity is greater than upthrust, so the submarine sinks.

All very interesting, but so what?

Upthrust = weight of fluid displaced by the object

U < W the object will sink

U > W the object will float

Questions

• A steel sphere of diameter 2,7cm is immersed in oil of density 900kg/m3. Find the resultant force on the sphere at the moment it is released. Density of steel 8100kgm3

• Volume of the object:

• Mass displaced:

• Upthrust :

• Resultant force:

• A sphere of mass 4.2kg and volume 5.2x10-3 m3 is placed 1m below the surface of a liquid of density 1100kg/m3.

• a)calculate the upthrust on the sphere

• b) which way will it move

• c)sketch the shape of the fluid flow just as the sphere starts to move

• Calculate the density of the hot air in a balloon floating at a fixed height close to the ground. The density of the cold air is 1.4kg/m3. The total mass of the balloon’s fabric, gondola, fuel, burners and occupants is 700kg and its volume is 2500m3

Question

• The density of the water in the Dead Sea is about 1200kgm-3. Calculate the volume of water that needs to be displaced for a person of weight 600N to float in this sea.

• b) how does this compare with the volume that needs to be displaced for the person to float in a swimming pool?