Discipline: PHYSICS

Key Stage: 3 or 4

Key idea: FORCES

TITLE:

Finding the shape which allows an object to move through water easily

AIM:

Develop investigative skills whilst exploring the interaction of forces on the motion of objects in water

BACKGROUND

When steam ships were first being designed, it became more important to have a hull shape that cut through water easily. Up until then the free supply of energy from the wind had been used to create the force needed to move sailing ships. Steam engines required coal, which cost money and had to be stored at various ports all over the world. To reduce the costs, ships had to be designed to use the least amount of fuel. To this end a special lab was built by the Royal Navy at Haslar, near Portsmouth. This was the first lab to consist of a large water tank and to use models of hull shapes to investigate how they moved through water.
When an object moves, friction opposes its motion. In water it is harder to move because water is denser than air. The opposing force exerted by water is often called drag. To accelerate a ship the engines must create a force that exceeds this drag and to keep it moving the engines must match the drag force.

ACTIVITY

• Set up a piece of guttering at least 1 metre in length, ensuring that the end stops are in place. Clamp a pulley to the edge of a bench so that the top of the pulley is just above the height of the end stop. Full the guttering with water.
• Collect a wooden block [5cm x 10cm approx.] and attach a piece of string to the centre of the front edge using a drawing pin. The string must be as long as the tank and then hang over the pulley as shown.
• Cut strips of plastic from drinks bottles to make some different shaped fronts and pin these to the block of wood.
• Add a mass to the end of the string and time how long each shape takes to be pulled the whole length of the tank
  

QUESTIONS/TASKS

1. Consider how you will plan to make your test fair, reliable and accurate.
2. Predict which shape will move most easily and give a scientific reason why. Use a force arrow diagram.
3. Try adding shapes to the rear of the ship as well. Could these improve the performance?
4. TOBI [see http://www.soc.soton.ac.uk/OED/index.php?page=tb] is not streamlined. Can you explain why?

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