Question 1
Calculate the speed of a car that travels 150 metres in 10 seconds. Express it in km/h.
Steps only
150 ÷ 10 = 15 m/s; 15 × 3.6 = 54 km/h.
Answer
54 km/h
CLASS 7 · SCIENCE · CHAPTER 8
Measurement of Time and Motion
Simple worked answers to all eleven end questions, five exploratory projects, and the chapter’s four activities, pendulum investigation, train-speed task, and motion-data question.
22 worked answersComplete chapter answers
22 worked answers · short steps first, explanations when you need them
Let Us Enhance Our Learning
Textbook pages 102–104
Question 2
One runner covers 400 metres in 50 seconds and another covers it in 45 seconds. Who is faster and by how much?
Steps only
First: 8 m/s. Second: about 8.89 m/s. Difference: about 0.89 m/s.
Answer
The second runner is faster by about 0.89 m/s.
Question 3
A train travels at 25 m/s and covers 360 km. How much time does it take?
Steps only
360 km = 360,000 m; 360,000 ÷ 25 = 14,400 s = 4 h.
Answer
4 hours
Question 4
A train travels 180 km in 3 h. Find (i) its speed in km/h, (ii) its speed in m/s, and (iii) the distance it travels in 4 h at the same speed.
Steps only
180 ÷ 3 = 60 km/h; 60 ÷ 3.6 ≈ 16.67 m/s; 60 × 4 = 240 km.
Answer
60 km/h; 16.67 m/s; 240 km.
Question 5
A galloping horse can reach about 18 m/s. Compare this with a train moving at 72 km/h.
Steps only
18 m/s = 64.8 km/h; 72 − 64.8 = 7.2 km/h.
Answer
The train is faster by 7.2 km/h.
Question 6
Distinguish between uniform and non-uniform motion using a car on an empty straight highway and a car in city traffic.
Steps only
Steady speed on a straight highway: uniform. Changing speed in traffic: non-uniform.
Answer
City traffic is a common example of non-uniform motion.
Question 7
An object moves uniformly. Complete the missing table entries. Time (s): 0, 10, 20, 30, __, 50, __, 70. Distance (m): 0, 8, __, 24, 32, 40, __, 56.
Steps only
Add 8 m for every 10 s.
Answer
At 20 s: 16 m; at 60 s: 48 m.
Question 8
A car travels 60 km in the first hour, 70 km in the second, and 50 km in the third. Is its motion uniform? Find its average speed.
Steps only
Different distances in equal hours mean non-uniform motion; 180 ÷ 3 = 60 km/h.
Answer
Non-uniform; average speed = 60 km/h.
Question 9
Which is more common in daily life: uniform or non-uniform motion? Give three examples.
Steps only
Daily motion usually changes speed; examples include buses, walkers, and bicycles in traffic.
Answer
Non-uniform motion is more common.
Question 10
For times 0, 10, 20, …, 100 s, the distances are 0, 6, 10, 16, 21, 29, 35, 42, 45, 55, 60 m. Is the speed uniform? Find the average speed.
Steps only
Unequal distance increases mean non-uniform speed; 60 ÷ 100 = 0.6 m/s.
Answer
Non-uniform; average speed = 0.6 m/s.
Question 11
A vehicle travels 2 km: first 500 m at 10 m/s, next 500 m at 5 m/s. What speed is needed for the remaining distance to complete the trip in 200 s? What is the average speed for the whole trip?
Steps only
First parts take 50 + 100 = 150 s; remaining 1,000 m in 50 s needs 20 m/s. Total average: 2,000 ÷ 200 = 10 m/s.
Answer
Required speed = 20 m/s; average speed = 10 m/s.
Exploratory Projects
Textbook pages 104
Question Project 1
Try bowls of different sizes and holes of different sizes. How can you make a floating-bowl water clock sink in about 24 minutes?
Steps only
Try different bowl and hole sizes, time each trial, and repeat the combination closest to 24 minutes.
Answer
Record repeated trials; change one feature at a time.
Question Project 2
How can students measure pulse rate, use pulse beats to estimate time, and develop a story about it?
Steps only
Measure beats per minute, use the rate to estimate a short time, and write a story about checking the estimate.
Answer
At 75 beats per minute, 25 beats are about 20 seconds; pulse timing is approximate.
Question Project 3
Why might repeated time-period readings for a pendulum differ slightly, and how can the experiment be improved?
Steps only
Use the same length and release angle, time many swings, repeat, and average.
Answer
Consistent setup and repeated measurements reduce timing differences.
Question Project 4
Measure a swing’s time for 10 oscillations with children of different weights, then compare swings of different lengths. Does weight affect the time period? How does length affect it? Is a swing a pendulum?
Steps only
Time 10 swings, divide by 10, then compare average periods for different riders and lengths.
Answer
A swing acts like a pendulum. Its length affects the period; rider weight has little effect.
Question Project 5
Find the winners’ times for the 100 m, 200 m, and 400 m races for men and women at the last two Olympic Games. Calculate and compare their speeds. Which event has the greatest speed?
Steps only
Speed = distance ÷ time. For example, Paris 2024 men’s 200 m: 200 ÷ 19.46 ≈ 10.28 m/s.
Answer
In this sample, Paris 2024 men’s 200 m has the greatest average speed: about 10.28 m/s.
Activities and In-text Questions
Textbook pages 107–117
Question Activity 8.1
How can you use the marked bottle to measure time after the water has dripped into its lower half?
Steps only
How can you use the marked bottle to measure time after the water has dripped into its lower half?
Answer
Mark the water level in the lower half after each minute as the water drips through the small cap hole.
Question Activity 8.2
How do you calculate a pendulum’s time period, and what should repeated readings show?
Steps only
How do you calculate a pendulum’s time period, and what should repeated readings show?
Answer
Count 10 complete oscillations and measure their total time.
Question Think like a scientist: pendulum variables
How can you test whether a pendulum’s length or bob mass affects its time period?
Steps only
How can you test whether a pendulum’s length or bob mass affects its time period?
Answer
Change one factor at a time.
Question Activity 8.3
What is the smallest time interval shown by the wall clock in the figure?
Steps only
What is the smallest time interval shown by the wall clock in the figure?
Answer
The smallest interval shown is 1 second.
Question Activity 8.4
How can you compare train speeds between two stations using a railway timetable?
Steps only
How can you compare train speeds between two stations using a railway timetable?
Answer
For each train, note the distance between the stations and subtract its departure time from its arrival time.
Question Table 8.3: Comparing trains
Which train moves with uniform linear motion between 10:00 and 11:00, based on the distances in Table 8.3?
Steps only
Which train moves with uniform linear motion between 10:00 and 11:00, based on the distances in Table 8.3?
Answer
Train X moves uniformly because it covers 20 km in each 10-minute interval.