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SOS Classroom

Grades 6-8 Physics: Surface Tension

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Surface tension

Building a fast boat.

Materials and Equipment

  • transparency
  • scissors
  • several cotton balls, cotton swabs or small pieces of sponge
  • marker
  • ruler
  • stop watch, accurate to 0.01 s
  • postal weighing scale
  • glue or tape
  • liquid detergent
  • soap
  • toothpaste
  • water
  • thermometer (optional)
  • a few small pieces of wood to make a straight channel for your raft

Experimental Procedure

  1. Exploring the existence of surface tension
    1. Think of a shape for your raft and draw the shape onto the transparency.
      • The raft should be symmetric.
      • You will need a slot should at the end of the raft in order to insert a piece of cotton or sponge for the storage of “fuel.”
    2. Cut out the raft and insert the cotton or sponge in the slot.
    3. Prepare fresh water in a bathtub or basin.
    4. Put the raft onto the water surface and let it float.
    5. Put a drop of detergent onto the cotton or sponge at the end of the ship.
    6. Observe the motion.

    From your background research, you should know that detergents decrease the surface tension of water. How can this help to explain your results?

  2. What other substances can work as a surface-tension motor for your raft?
    1. Prepare fresh water in a bathtub or basin.
    2. Change the cotton or sponge of the raft.
    3. Put the raft onto the water surface and let it float.
    4. Put a drop of soap or toothpaste or some other substance onto the cotton or sponge at the end of the ship.
    5. Observe the motion and compare to the first raft.

  3. Exploring the effect of shape on the motion of your raft
    1. Design another shape of a ship projection and draw the shape onto the transparency.
      • You may want to try an asymmetric shape.
      • You may want to experiment with different sizes, or different basic shapes.
    2. Cut out the raft and insert the cotton or sponge at the end of the raft.
    3. Prepare fresh water in a bathtub or basin.
    4. Put the raft onto the water surface and let it float.
    5. Put a drop of detergent onto the cotton or sponge at the end of the ship.
    6. Observe the motion and compare to the other rafts.

  4. Measurement of Net Surface Tension
    1. Prepare a new raft with a shape similar to the one below:
      Example raft drawing.
      The length of the raft is quite flexible but for the first trial, it is suggested to be 3–5 cm long, and a width approximately half of this length.
    2. Weigh the raft.
    3. Prepare the a channel for your raft, marked with a predetermined distance (see drawing, below):
      Top view and end view drawings of channel for measuring acceleration and speed of raft.
      The length of the channel is suggested to be about 25 cm long with the width of the channel just slightly larger than the width of the raft. The height is about 3 cm. If you make bigger boats, you’ll need to adjust the dimensions accordingly.
    4. Prepare fresh water in a bathtub or basin.
    5. Put the raft at one end of the channel and put a drop of detergent onto the cotton or sponge at the end of the ship.
    6. Measure the time required for the raft to travel inside the channel between the predetermined marks.
    7. Calculate the acceleration and the kinetic energy imparted to the boat.
    8. Increase the mass of the raft by adding more pieces of transparency (record the new weight).
    9. Repeat the measurement and calculations for several weights of the ship.
    10. Compare the measured values.

Source