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  1. 1. 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!
  2. 2. 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
  3. 3. What would happen if the forces weren’t balanced?
  4. 4. More examples of upthrust If you let go of your drinking straw, it floats up and sometimes tries to bob out of the glass!
  5. 5. Floating in the sea • Salty seas have more upthrust. It’s much easier to float in a sea than in a swimming pool.
  6. 6. • 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!
  7. 7. 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?
  8. 8. • 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
  9. 9. 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.
  10. 10. All very interesting, but so what? UPTHRUST is the reason that your objects weighed less in the water than in air!
  11. 11. Calculating Upthrust Upthrust = Weight of object in air Minus Weight of object in water
  12. 12. What other force exists is water?
  13. 13. Water Resistance Water resistance is a type of friction which slows you down.
  14. 14. What forces are acting on this powerboat? Gravity Upthrust Water Resistance Propulsion
  15. 15. 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:
  16. 16. Now its your and my favourite Bill Nye. • Bill Nye