Class exercise 3b – Volume of irregular objects

Aim: To determine the volume of a stone and ping pong ball using the displacement method

Apparatus:

  • Ping pong ball
  • Sinker (a stone)
  • String and Masking Tape
  • Displacement can
  • Measuring cylinder

Procedure:

1. Elevate the displacement can.  Place the measuring cylinder on the bench to collect water that would overflow from the spout. (In our case, we placed the measuring cylinder in the sink and the displacement can on the bench)

Evernote Camera Roll 20130121 173301

2. Fill up the displacement can until water overflows from the spout.  Wait till the water stops flowing out from the displacement can, and pour away all the water in the measuring cylinder.

3. Sketch the experimental set-up:

Evernote Camera Roll 20130121 182432

4. Tie the stone with a string, and gently lower the stone into the displacement can.

5. Note the volume of water collected in the measuring cylinder, V.

6. Use the same method to determine the volume of the ping pong ball:

  1. Tie the ping pong ball and the stone together using the same piece of string
  2. Prepare a separate, larger container (We used another displacement can) to collect the displaced water as it exceeds the capacity of the measuring cylinder
  3. Lower the two objects into the displacement can, ensuring that the ping pong ball is trapped below the stone when in the can (so that it does not float and its entire volume can be measured).
  4. Pour the displaced water into separate measuring cylinders to measure its volume.
Our first (failed) attempt. We later wrapped the ping pong ball and the sinker together to stop it from floating.

Our first (failed) attempt. We later bound the ping pong ball and the sinker together with the string, with the ball at the bottom, to stop it from floating.

Results

1st attempt 2nd attempt
Volume of sinker, V / cm3 25.5 25.5
Volume of ping pong ball and sinker / cm3 56 63.5
Volume of ping pong ball / cm3 30.5 38
Average volume of ping pong ball / cm3

34.25

Analysis

– List three possible causes of inaccuracy in recording the volume of the water contained in the measuring cylinder.

  1. Wetness of stone
  2. Volume of string
  3. Masking tape on stone (left over from previous experiments)
  4. Water left in measuring cylinder after 1st measurement
  5. Water left in the spout of the displacement can

– Suggest how the possible causes mentioned can be minimized so that measurements can be more accurate.

  1. Dry the stone with a cloth after each attempt
  2. Lower the entire length of string into the water when measuring the volumes of the ball and stone. Also, use the same string for measuring both volumes.
  3. Use a new, dry measuring cylinder for each measurement.
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