The Water Pushes Back

Why it matters

Try holding a beach ball underwater. It fights you like something alive. Nothing is alive — that's just deep water pressing harder than shallow water, adding up to a shove.

The idea in plain words

The buoyant bookkeeping. Tap the terms.

upthrust = weight of displaced liquid

A 15 N ball displaces 20 N of water when fully pushed under. Apparent weight?

−5 N: the water wins by 5 N — release it and up it goes

• newton: upthrust (buoyancy) — the net upward force a fluid exerts on anything in it
• newton: apparent weight — true weight − upthrust: why rocks feel lighter underwater
• the knobs: depends on — submerged VOLUME and liquid DENSITY — not on the object's weight
• apparent = 15 − 20

Predict first

Why does water push submerged things UP?

P = hρg grows with depth, so the bottom face gets pushed up harder than the top gets pushed down. The difference IS the upthrust.

• pressure below the object is greater than pressure above it — correct
• water hates intruders
• the object's air content lifts it

What you do

Push the ball under, watch upthrust grow with the submerged fraction — until it out-muscles the weight.

Check yourself

A stone feels lighter under water because…

Its true weight is untouched; the spring balance just shares the job with the upthrust.

Upthrust on a fully submerged object depends on…

The displaced liquid doesn't care what displaced it — only how MUCH.

The same ball is pushed under water and then under mercury. The upthrust is…

Same displaced volume, but each litre of mercury weighs 13.6× more — iron floats ON mercury.

• upthrust supports part of its weight — correct
• gravity is weaker under water
• the stone absorbs water
• its volume and the liquid's density — correct
• its weight
• its colour
• far larger in mercury (13.6× denser) — correct
• the same
• larger in water