Textbook Solution: Time and Motion | Class 7 Oxford Science: Chapter Notes, Worksheets & Tests PDF Download

A. Choose the correct option.

1.
Ans: (a)

2.
Ans: (a)

3.
Ans: (c)

4.
Ans: (b)

B. Fill in the blanks with the correct words.

1.
Ans: Kilometres per hour

2.
Ans: An hourglass is a type of clock.

3.
Ans: The time period of a pendulum depends on the length of the string.

4.
Ans: Metre per second is the SI unit of speed.

5.
Ans: In uniform motion, speed remains the same.

C. Match the Following.

1.
Ans:

D. Answer the following questions.

1.
Ans: Three instruments used to measure time are: a clock, a sundial, and an hourglass.

2.
Ans: Periodic motion is used to measure time by identifying an event that repeats itself at regular intervals. This event should have a constant and predictable periodic motion.
Examples of periodic motion include the to-and-fro movement of a compressed spring, the rotation and revolution of the Earth, the revolution of the moon around the Earth, and the oscillation of a pendulum. This predictable motion is used to construct various types of clocks, such as a pendulum clock.

3.
Ans: Two units of time larger than a day are: a week and a year.

4.
Ans: One clock that uses the movement of the sun to keep time is a sundial.

5.
Ans: Speed is defined as the distance travelled by an object in unit time. It refers to how fast or slow an object is moving. The SI unit of speed is metres per second (m/s).

E. Numerical Problems.

1.
Ans: A vehicle travels 25 km in 15 minutes. To calculate its speed in km/h, first convert minutes to hours. There are 60 minutes in an hour, so 15 minutes is 15/60 = 0.25 hours. Speed = distance/time = 25 km/0.25 h = 100 km/h.

2.
Ans: A commercial aircraft travels at 900 km/h. To calculate the distance it travels in 5 hours, multiply the speed by the time: distance = speed x time = 900 km/h x 5 h = 4500 km.

3.
Ans: Seismic waves travel at about 8 km/s on the Earth's crust. To calculate the distance it travels in 10 s, multiply the speed by the time: distance = speed x time = 8 km/s x 10 s = 80 km.

4.
Ans:
(a) 30 s to minutes: 30 s / 60 = 0.5 minutes
(b) 72 hours to days: 72 h / 24 = 3 days
(c) 90 km/h to m/s: 90 km/h * (1000 m/1 km) * (1 h/3600 s) = 25 m/s
(d) 75 m/s to km/h: 75 m/s * (1 km/1000 m) * (3600 s/1 h) = 270 km/h.