Upthrust
Forces and Motion | Properties of Matter

Measuring buoyancy forces

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

The purpose of this activity is to make measurements to find the size of buoyancy forces and to start to think about what the upthrust force depends upon.

What to Prepare

  • A top pan balance calibrated in newtons
  • A spring balance calibrated in newtons (a newtonmeter)
  • A bowl or large beaker of water and an object (perhaps a wooden block) to weigh
  • Copies of sheet to support practical work (see below)
  • Copies of sheet to support homework (see below)

What Happens During this Activity

As a preliminary activity, to encourage thinking, the object should be weighed on the top pan balance and pupils invited to describe the forces acting on the object (pull of gravity and a support force exerted by the balance). The balance is then replaced by a pupil's hand and pupils invited to describe the forces as the object rests on their hand (gravity force and the support exerted by the hand).

The object is then transferred to the newtonmeter and these processes repeated. The object, still suspended from the newtonmeter, is lowered into the water. The reduced reading on the meter is noted. Stop and pause before the reading reaches zero. Now invite pupils, in small groups, to describe the forces acting on the wooden block. A support sheet with a sequence of sketches is provided to help focus thinking.

Upthrust
is a special case of Force
appears in the relation Upthrust = Weight of Displaced Liquid
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