Grade 7 Science: Unit 3: Structural Strength & Stability
1. Why is the choice of materials important when designing a new product?
The choice of materials can affect the strength of the product, its ease of manufacture, and its recyclability.
2. Why is the amount of energy needed in manufacturing important to consider when creating a product?
The amount of energy needed to manufacture a product relates directly to its final cost. A manufacturer must ensure that enough energy is available from the energy provider and consider whether the energy source is compatible with the manufacturing process, e.g., voltage range, electrical vs. thermal or mechanical.
3. Reflecting: Make a list of things to consider before buying a product. Rank your criteria in order of importance. Do you always follow your criteria? Explain.
Factors to be considered may include: need, cost, quality, safety, recycled content and recyclability, brand association, peer pressure, effectiveness of marketing campaign, colour, size, shape, etc.
3.2 Product Testing: Page (138-139) Pg. 1
1) Why are prototypes tested before they are mass-produced?
Prototypes are built and tested to ensure that a product will act as intended. They often generate ideas for product refinement and further application.
2) How does testing help to improve the products we use?
Testing can help designers improve product safety and ergonomics, generate ideas for improvement and see how products may fail under normal conditions, not laboratory simulations.
3) Reflecting: Testing a prototype can yield unexpected results. Car designers discovered they should weaken, rather than strengthen, part of the car body to improve safety. Why would an unexpected result be important? What things should you consider if the results of your tests are unexpected?
Unexpected results give manufacturers an objective view of their product, which may result in improvements. They want to ensure that any unexpected result is repeatable, not just as isolated circumstance. If it�s a negative result, the product must be modified; if positive, the possible uses of the product can be expanded or added to marketing strategies.
3.4 Stability: Page (142-145) Pg. 1b
1) Un your own words, explain centre of gravity and how it affects the stability of a structure.
Centre of gravity is the point around which an object is stable. An object will balance around its centre of gravity.
2)
a) What are the support points and support base of the structures in figure 8?
A: the base is the live from 2 to 3; support points are all the points between 2 and 3.
B: base and support point are both at 6
C: base is the line from 9 to 10; support points are all the points between 9 and 10.
D: base and support point are both at 13.
b) Which of the structures in fig. 8 are stable? Explain your evaluation.
Figure A is the most stable because its centre of gravity is balanced over the base, its base is wider than any other, it has more than one support point, and the endpoints of the base are almost the same distance from the centre of gravity.
3) The towers used by radio broadcasters are tall and slender. Using the term centre of gravity, describe three ways you could make a radio tower more stable without decreasing its height.
To make the tower more stable, lower the centre of gravity. This can be achieved by:
a) Adding a mass low down on the tower (perhaps by using thicker materials on the lower parts of the tower),
b) Increasing the size of the base,
c) Changing the shape of the base to a larger square
4) Giraffes have long, heavy necks, but they do not tip over, even when they run or drink. Giraffes do not bend their knees when they drink from ponds, and they keep their necks straight (fig. 9). Speculate on how the giraffe�s structure prevents it from tipping over.
The giraffe actually forms a tripod-wide base, with widely separated support points and even distribution of mass over these points.
3.5: Stabilizing the Tower of Pisa (page 146-147) Pg. 4
a) The lean of the tower was a problem. The people of Pisa decided to solve it by curving the tower. Was this the best solution? What other solutions can you think of?
Curving the tower was not the best solution, since the tower continued to tilt. Possible other solutions include support beams installed to brace the tower, or cables attached to large masses sunk into the ground on the side opposite the lean.
b) Speculate of the position of the tower�s centre of gravity. What happens to the stability of the tower as its lean increases?
The centre of gravity is higher than it should be. The tower becomes less stable as the lean increases.
c) In 1990, the tower was closed to the public. Why do you think this was done?
Constant movement of visitors inside the tower would contribute to its instability in the same way that a ferry may become unstable if all the passengers suddenly rush to one side.
d) If there were a way to straighten the tower, is there any reason to believe it would stay straight? Explain.
As long as the soil conditions remain the same, the tower will likely continue to lean.
e) Why would removing some of the soil from under the north side of the tower reduce the lean?
As the remaining soil settled to fill in the cavity produced on the north side, the tower would shift in that direction.
f) No one wants to completely straighten the tower, especially the people of Pisa. Suggest some reasons why.
Suggestions include the fact that the tower is a major tourist attraction because of its lean. If it were straight, people would not be as interested in coming to see it.
Understanding Concepts 1:The stones used in the foundation of the Leaning Tower are thicker than the stones used higher up. How does this affect the stability of the tower?
The stones in the base are larger to accommodate the mass of the building�s upper structure. The large mass of the base produced a low centre of gravity to counterbalance the mass of the upper structure of the tower.
