1. Upthrust

2. It pushes UP! That’s why it’s called Upthrust!
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!

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

4. What would happen if the forces weren’t balanced?

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

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

7. You can even read a newspaper!
The Dead Sea is the saltiest sea on earth. It’s REALLY easy to float on it!
You can even read a newspaper!

8. 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?

9. 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.)

10. 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.

11. 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

12. 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
0.71N

13. Volume of the object:
Mass displaced:
Upthrust :
Resultant force:

14. a)calculate the upthrust on the sphere b) which way will it move
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
561132N
Go up

15. 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
U= 34000N
U=weight balloon + weight of hot air
Density of hot air = 1.1

16. 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?
V=5.1x10-2m
The above volume is less than the volume displaced by the less dense water in the pool