Q4(a). Name any one ancient instrument used to measure the time.

Answer. One ancient instrument used to measure time is the sundial.

(b) When do we observe a curved line on distance-time graph

Answer. We observe a curved line on a distance-time graph when an object is moving with non-uniform (changing) speed.

(c) What is a pendulum bob resting position called?

 Answer. The resting position of a pendulum’s bob is called the equilibrium position. In this position, the forces acting on the bob (gravity and the tension in the string) are balanced. The bob is neither accelerating upwards nor downwards.

(d) What is the SI unit of time?

Answer. The SI unit of time is the second. It’s denoted by the symbol “s” or “sec”.

Q5.

• State the factors on which the time period of a simple pendulum depends

Answer. The time period of a simple pendulum depends on two main factors:

1. Length (l) of the pendulum: The time period is directly proportional to the square root of the length. A longer pendulum swings back and forth slower than a shorter one.
2. Acceleration due to gravity (g): The time period is inversely proportional to the square root of the acceleration due to gravity. This means in places with weaker gravity (like the moon), a pendulum with the same length would have a longer period.

Note: In a simple pendulum, the mass of the bob (as long as it’s small compared to the supporting string) does not affect the time period.

b.  Explain how time was measured when pendulum clock was not available

Answer./ Before the invention of the pendulum clock, people relied on a variety of ingenious methods to track the passage of time. Here’s a glimpse into those fascinating timekeepers:

• Sundials: These ancient devices used the sun’s position in the sky to tell time. A pointed object, called a gnomon, cast a shadow on a marked surface, indicating the hour based on the shadow’s position. Sundials were great for daytime use in sunny locations, but not so helpful at night or on cloudy days.
• Water Clocks (Clepsydras): These timekeepers relied on the steady flow of water. They typically consisted of two vessels, with water slowly dripping from the top one to the bottom. Markings on the bottom vessel indicated the passage of time as the water level rose. Water clocks were more reliable than sundials in some ways, but they needed refilling and weren’t very precise.
• Sand Clocks (Hourglasses): These portable devices used sand flowing through a narrow constriction to measure time. Once all the sand had trickled down, a set amount of time (usually an hour) had passed. Sand clocks were convenient for short durations, but they needed to be constantly flipped to reuse.
• Candles: Marking candles with increments of time was another method. As the candle burned down at a relatively constant rate, the markings indicated the elapsed time. This method, however, wasn’t very accurate and wasn’ t ideal for longer durations.
• Natural Observations: People also relied on natural phenomena to gauge time. The position of stars at night, the changing of seasons, or even the crowing of roosters could provide a rough sense of the day or night.

These timekeeping methods served various purposes in the past. Farmers used them to manage their crops, travelers relied on them for navigation, and societies needed them for scheduling daily activities. While not as precise as our modern clocks, these ingenious inventions played a crucial role in measuring time before the pendulum’s steady swing revolutionized timekeeping.

C. What are the characteristics of the distance-time graph or a stationary object? Justify your answer

Answer. The distance-time graph of a stationary object is a flat line parallel to the time axis. This is because the object isn’t moving, so its distance remains constant over time. Imagine a horizontal line on a graph – that’s the signature of a motionless object!