CBSE Class 7 Science Chapter 08 Measurement Of Time And Motion MCQs Set 03

Answer: (D) Movement of stars across the sky

Explanation:
The chapter specifically identifies the rising and setting of the Sun, phases of the Moon, and changing seasons as natural cycles that ancient peoples used for timekeeping and calendar development. The movement of stars, while astronomically important, is not mentioned in the text as a foundation for early time measurement methods.

Answer: (A) 2 seconds

Explanation:
Time period equals total time divided by number of oscillations. With 20 seconds for 10 oscillations, the time period is 20 ÷ 10 = 2 seconds. This calculation directly mirrors the method taught in Activity 8.2 of the chapter.

Answer: (A) Time intervals as short as 2 seconds

Explanation:
The chapter explicitly states that the Samrat Yantra's shadow falls on a scale marked to measure time intervals as short as 2 seconds, making it exceptionally precise for a sundial despite measuring local solar time rather than standard time.

Answer: (B) The flow rate decreased as water levels dropped

Explanation:
• Earlier flowing-out water clocks had a fundamental flaw: as water levels dropped, the flow rate decreased, making them inaccurate
• The sinking bowl design (Ghatika-yantra) avoided this problem by maintaining consistent filling rates
• This improvement made it reliable enough to be used at Buddhist monasteries, royal palaces, and town squares for continuous timekeeping

Answer: (C) To account for small variations and obtain a more reliable average value

Explanation:
Repeating measurements multiple times allows students to identify and average out small variations caused by experimental imprecision, resulting in a more accurate determination of the time period. The chapter emphasizes that the time period is 'almost the same every time,' but acknowledges these minor differences require repeated trials.

Answer: (C) Multiply 3.6 by 1000 and divide by 900

Explanation:
Speed in m/s requires distance in metres and time in seconds. Converting: (3.6 km × 1000 m/km) ÷ (15 min × 60 s/min) = 3600 m ÷ 900 s. This conversion method is demonstrated in Example 8.1 of the chapter.

Answer: (B) Uniform motion covers equal distances in equal time intervals

Explanation:
Uniform linear motion is explicitly defined as motion along a straight line with constant speed, which mathematically means the object covers equal distances in equal intervals of time. This is the defining feature illustrated by Train X in Table 8.3 of the chapter.

Answer: (B) Train Y, because the distances covered in successive 10-minute intervals vary

Explanation:
Train Y covers: 20 km, 15 km, 15 km, 25 km, 20 km, and 25 km in successive 10-minute intervals. These unequal distances in equal time intervals define non-uniform motion. Train X consistently covers 20 km per interval, confirming uniform motion—demonstrating the distinction made clear in the chapter's analysis.

Answer: (C) 100 km

Explanation:
The chapter states: Total distance covered = Speed × Total time taken. Therefore, distance = 50 km/h × 2 h = 100 km. This is the exact calculation method presented in Example 8.2.

Answer: (B) They both rely on repeating periodic processes that occur at fixed intervals

Explanation:
• Pendulum clocks use the constant oscillation period of a pendulum
• Ghatika-yantra bowls filled at consistent rates
• Both exploit naturally repeating cycles to mark equal time intervals
• This principle—'periodically repeating processes'—is foundational to all clocks, old or modern, as stated in the chapter's discussion of modern time measurement.

Answer: (B) That the time for each swing of a suspended lamp remained constant regardless of the amplitude

Explanation:
The chapter explicitly states that Galileo observed a lamp suspended from a church ceiling swinging back and forth, and using his pulse to measure time, he found 'the lamp took the same time for each swing.' This consistency led him to conclude that oscillation time depends on pendulum length, which Huygens then exploited in his mechanical clock.

Answer: (B) The runner has an average speed of 8 m/s, though the actual motion may not be perfectly uniform

Explanation:
Speed is calculated as 400 m ÷ 50 s = 8 m/s. The chapter emphasizes that even though we calculate speed this way, actual running involves varying speeds (acceleration, deceleration), so we call this the 'average speed.' The chapter notes that perfectly uniform motion is rare in real life, which is why average speed must be used.

Answer: (C) Hourglass

Explanation:
Figure 8.3 and the accompanying text clearly identify the hourglass as the device where 'time was measured on the basis of the flow of sand from one bulb to another.' Sundials use shadows, water clocks use flowing water, and candle clocks use burning marks.

Answer: (C) To make the water level easily visible as it drips down

Explanation:
The chapter explicitly states: 'You may add a few drops of ink or colour to make the water level easily visible.' This enhancement allows students to clearly observe and mark the water levels at one-minute intervals, enabling them to calibrate their water clock for timing measurements.

Answer: (B) 4 hours

Explanation:
Applying the formula: Time = 360 km ÷ 90 km/h = 4 hours. This calculation is identical to Example 8.3 presented in the chapter, illustrating the three-way relationship between speed, distance, and time.

Answer: (A) To give athletes a fair comparison based on actual performance differences

Explanation:
When athletes' times are extremely close, only precise measurement to fractions of a second can distinguish the actual faster performer. Without such precision, truly faster sprinters might be ranked below slower competitors, making the competition unfair. This connects to the 'Science and Society' section describing how sports timekeeping devices measure to one-hundredth or one-thousandth of a second.

Answer: (B) Sundial

Explanation:
In a sundial, time is determined by the changing position of the shadow of an object cast by the light of the Sun during the day.

Answer: (C) The time period remained almost the same every time

Explanation:
The activity demonstrates that the time period of a simple pendulum of a given length is constant at a place, which is why students observed the time period to be almost the same every time they repeated the measurement.

Answer: (C) Uniform linear motion

Explanation:
Train X covers equal distances (20 km) in equal intervals of time (10 minutes), which is the defining characteristic of uniform linear motion as explained in the chapter.

Answer: (A) Train A with a speed of 60 km/h

Explanation:
Train A: 120 km ÷ 2 h = 60 km/h; Train B: 150 km ÷ 2.5 h = 60 km/h. Both trains have the same speed of 60 km/h, demonstrating that equal speed can result from different distance-time combinations.

Answer: (C) Real objects constantly change their speed due to various factors in everyday life

Explanation:
The chapter explicitly states that uniform linear motion is an idealisation and that in everyday life, objects seldom move with constant speed over long distances or for long intervals of time, which is why average speeds must be used for practical calculations.