Upthrust is a force that exists in liquids like water and pushes objects upward against gravity. It is why boats and other objects float, as the upthrust from the water balances out the downward force of gravity. The shape and size of an object affects how much upthrust it receives, with larger, flatter surfaces receiving more upthrust. This explains why people and objects are buoyant and float in water.

2. What’s upthrust?
• Upthrust is a force that only 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.
It pushes UP!
That’s why it’s
called UPthrust!

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

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. • The Dead Sea, in Israel, 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.)
Baby Float

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

12. Calculating Upthrust
Upthrust =
Weight of object in air
Minus
Weight of object in water

13. What other force exists
is water?

14. Water Resistance
Water resistance is a type of friction which
slows you down.

15. What forces are acting on this
powerboat?
Gravity
Upthrust
Water
Resistance
Propulsion

16. Now its your turn!
• Draw a picture of a submarine either on, or
underwater and label all the forces acting on it.
• Don’t forget to add and label arrows to show the
strength and direction of the forces.
• Work out the upthrust for each of the following
things placed in water:

17. Now its your and my favourite
Bill Nye.
• Bill Nye