Here are the correct answers for the questions based on the image provided:

7. A passenger in a bus jerks forward when the driver suddenly applies the brakes because of:
Answer: D. Inertia

8. When two similar poles of two bar magnets are brought nearer to each other:
Answer: C. Repulsion occurs

9. Determine the frequency of the wave shown in figure 1:
The wave completes one full cycle in 0.4 seconds. Frequency $f$ is given by:
$f = \frac{1}{T}$ Where $T$ is the period of the wave.
$f = \frac{1}{0.4} = 2.5 \, \text{Hz}$
Answer: B. 2.5 Hz

10. A cyclist going round a curve leans inward in order to:
Answer: A. Provide the necessary centripetal force

11. A density bottle has a mass of 5g when empty, 70g when full of water, and 74g when filled with the liquid. What is the relative density of the liquid?
Relative density $\text{RD}$ is given by:
$\text{RD} = \frac{\text{Mass of liquid}}{\text{Mass of an equal volume of water}}$ Mass of the liquid = $74 - 5 = 69 \, \text{g}$
Mass of water = $70 - 5 = 65 \, \text{g}$
$\text{RD} = \frac{69}{65} \approx 1.06$
Answer: B. 1.06

12. Which of the following is not an electromagnetic wave?
Answer: A. Sound waves

Let me know if you need detailed explanations for any of these!

Five related questions to expand knowledge:

1. Why does inertia cause passengers to jerk forward when brakes are applied?
2. How can the properties of magnets explain attraction and repulsion between poles?
3. What are other ways to calculate frequency if the wave's period is not given?
4. What factors influence a cyclist’s need to lean inward while turning?
5. How is relative density applied in practical scenarios like shipbuilding or hydrometry?

Tip: To solve wave problems quickly, always remember the relationship between frequency, period, and wavelength!