The key factor that the acceleration principle emphasizes as a determinant of investment is changes in technology and capital durability. This principle suggests that investment is driven by changes in the level of output or income, which in turn is influenced by changes in the level of investment. In other words, investment is determined by the rate of change in income or output.

Explanation:

Changes in technology:
- Technological advancements play a crucial role in determining investment decisions.
- New technologies often lead to increased productivity and efficiency, which can result in higher profits for businesses.
- In order to take advantage of these opportunities, firms may need to invest in new equipment, machinery, or infrastructure.
- Thus, changes in technology can stimulate investment as businesses seek to upgrade their operations and stay competitive in the market.

Capital durability:
- The durability or lifespan of capital goods is another important factor that influences investment decisions.
- When businesses invest in capital goods, such as machinery or equipment, they typically expect these assets to generate returns over a certain period of time.
- The longer the expected lifespan or durability of these capital goods, the more likely businesses are to invest in them.
- This is because longer-lasting capital goods provide a longer period for businesses to recoup their initial investment and generate profits.
- On the other hand, if capital goods have a short lifespan, businesses may be hesitant to invest in them as they may not generate sufficient returns within the expected timeframe.

Overall, the acceleration principle highlights the importance of changes in technology and capital durability as determinants of investment. Technological advancements can create new opportunities and drive investment as businesses seek to adopt new technologies and improve their productivity. Additionally, the durability of capital goods influences investment decisions, as businesses consider the expected lifespan of these assets and the potential returns they can generate.