Centre of gravity

Centre of gravity

Pencil and paper
Overview
Using this Resource
Connecting to the Curriculum
Marking Student Responses
Working with Students
Further Resources
This task is about the centre of gravity.
The centre of gravity is the point in an object about which its weight is evenly balanced. Show how this concept might relate to the following situations.
a)
The placing of 18 empty and 6 filled water containers on a small truck. Shade the filled containers to show where you would place them so the load was most stable.
 

 
b) The design of a plastic duck for use in a bath so that it doesn't tip over easily. Draw your design, labelling the feature(s) that keep it from tipping.

  

 

 

 

 

 

 

 

 

 

 

 

 
Task administration: 
This task can be completed with pencil and paper.
Level:
5
Curriculum info: 
Science, Knowledge, Physical World
Keywords: 
centre of gravity, balance, technology, model construction, drawing
Description of task: 
Task: complete 2 diagrams. Assessment focus: centre of gravity.
Curriculum Links: 
Science capabilities
The capabilities focus is brought about by the conversations you have and the questions you ask.
 
Capability: Interpret representations
This resource provides opportunities to discuss the advantages and disadvantages of using diagrams to communicate science information.
Science capabilities: 
Interpret representations
Answers/responses: 
 

Y10 (11/2003)
a)    The filled water containers should be in the lowest rows on the truck. 1 correct - moderate
b)    Any two of:

  • Flotation/buoyancy feature.
  • Relevant comment about shape, (e.g., balancing the size of the head with the rest of the body and/or with the size of the tail, a broad base).
  • Recognition that the duck needs to be heavier at the bottom, (e.g., with some water or sand inside the duck).
  
2 correct - very difficult
or
1 correct - difficult
Diagnostic and formative information: 

Question b) is a deceptively simple task that can give information about students' ability to read instructions, and to translate a theoretical idea into a simple, relevant, practical application. The stem of the question clearly indicated that the design was to focus on the centre of gravity so that the duck did not tip over. However 21 percent of the trial group labeled their design to show only flotation/buoyancy features. If you wanted to really emphasise the relevance aspect you could disallow such responses, but this would make the task even harder than it already seems to be.

Not all of the 26 percent who labeled a shape feature specifically linked this to its impact on the centre of gravity. Compare, for example the vague "wings will balance the duck" with the explicit "tail same as head weight for balance and at same height".

One common error was to draw and label a small weight inside the duck's tail. The following two images illustrate the difference between clearly representing a stable weight distribution at the base and drawing essentially the same solution, but in a way that we interpreted as potentially creating instability if the round shapes were free to roll around inside the base. (If it is not clear which of these situations a student intended, you could get them to explain their drawing orally.)

 

Although we did not mark for this, there is an interesting systems balance to be achieved between weighting the base for stability and keeping the duck buoyant and afloat.

  

Next steps
Students could carry out their own investigations into either of the situations assessed to developtheir understanding of stability and centre of gravity. Again this is something you may wish to discuss with the students.