Think back to the experiment Mr Rhys-Jones did with water and weight today..

Upthrust is a force that  exists in liquids, 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

This boat has gravity pushing it down, and the upthrust of the water pushing up.

Because the forces are balanced, the ship floats.

What would happen if the forces weren't balanced?

 

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

  

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

 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 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?

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UPTHRUST is the reason that your objects weighed less in the water than in air!