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Boats & Jelly

You will need:

  • Different toy boats, each with a different hull shape
  • Jelly crystals, enough for 2 litres
  • Newton spring balance from school
  • Large flat platter and a measuring scale
  • Small screw hooks and weights (metal nuts will do)


1. Make at the jelly a few hours beforehand. Once the jelly is set, scrape it out to cover the platter.

2. Weigh each boat.

3. Place a boat at the end of the platter and gently pull it through the jelly using the spring scale. Take a note of the force required (in Newtons).

4. Repeat step 3 for each boat. Are there any differences? Try to explain your answer.

5. To make it more of a fair test, try to even the weight of the boats using the metal nuts.

Make sure that the weight is evenly distributed along the boat. Repeat the experiment.

6. Work out how could you make this an even fairer test?
For example; pulling from the same point of the boat, at the same angle, at the same speed, through the same amount of jelly... Should you use blocks with different shaped wedges attached instead?
boat jelly

Why Does This Happen?

An object is set into motion when it is pushed or pulled, i.e. subjected to a force.
Force is measured in Newtons, which expresses the amount of energy being applied over a given distance and time. To pull the boat through the jelly required a certain amount of force to keep it moving. The greater the force applied, the greater the spring stretched within the spring scale.

Each boat would have had a different angle on it's bow to cut through the jelly. If the boat had an acute angled bow (i.e. 'sharp'), the boat would have neatly cut through the jelly and would have required less force to keep it moving. If the the boat was shaped like a barge, with a blunt front end, a larger force would need to have been applied to keep the boat moving at the same speed.

Ice breakers in polar regions are designed to slowly rise up onto ice, thereby allowing the weight of the boat and the shape of the bow (an inclined plane) to break the ice. This process isn't performed at great speed as the ship can still be damaged. Dense pack ice cannot be negotiated as the ice floes are too thick, however thin ice of a few metres thick can be broken. Commercial shipping lanes within the Arctic circle are regularly cleared by ice breakers.

Specifications on the Yamal, a nuclear powered Russian ice breaker.
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Reference: Kane, J. W. Sternheim, M. M. (1988). Physics. 3rd ed. John Wiley & Sons, New York.

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