RBSE Solutions Class 7 Science Chapter 8 Measurement of Time and Motion

 

Class 7 Science Curiosity Chapter 8 Question Answer (Exercise)

(Let Us Enhance Our Learning)

 

Question 1. Calculate the speed of a car that travels 150 meters in 10 seconds. Express your answer in km/h.
Answer: Distance covered \( = 150 \text{ m} \)
Time taken \( = 10 \text{ s} \)
Speed \( = \frac{\text{Distance}}{\text{Time}} \)
Speed \( = \frac{150 \text{ m}}{10 \text{ s}} \)
Speed \( = 15 \text{ m/s} \)
To convert m/s to km/h, we multiply by \( \frac{18}{5} \). This is because 1 km = 1000 m and 1 hour = 3600 s.
Speed in km/h \( = 15 \times \frac{18}{5} \)
Speed in km/h \( = 3 \times 18 \)
Speed in km/h \( = 54 \text{ km/h} \)
In simple words: First, find the speed in meters per second by dividing distance by time. Then, change this speed to kilometers per hour by multiplying it by a special fraction.

🎯 Exam Tip: Remember the conversion factor: to convert m/s to km/h, multiply by \( \frac{18}{5} \); to convert km/h to m/s, multiply by \( \frac{5}{18} \).

 

Question 2. A runner completes 400 metres in 50 seconds. Another runner completes the same distance in 45 seconds. Who has a greater speed and by how much?
Answer: For the first runner:
Distance \( = 400 \text{ m} \)
Time \( = 50 \text{ s} \)
Speed \( = \frac{\text{Distance}}{\text{Time}} \)
Speed of first runner \( = \frac{400}{50} = 8 \text{ m/s} \)
For the second runner:
Distance \( = 400 \text{ m} \)
Time \( = 45 \text{ s} \)
Speed \( = \frac{\text{Distance}}{\text{Time}} \)
Speed of second runner \( = \frac{400}{45} \approx 8.89 \text{ m/s} \)
To find who has a greater speed and by how much, we compare the speeds. The second runner has a higher speed.
Difference in speed \( = 8.89 - 8.00 = 0.89 \text{ m/s} \)
So, the second runner had a greater speed by 0.89 m/s. Faster times mean faster speeds over the same distance.
In simple words: Calculate the speed for each runner by dividing distance by time. The runner with the higher speed is faster. Subtract the slower speed from the faster speed to see how much faster.

🎯 Exam Tip: When comparing speeds for the same distance, the person who takes less time is always faster. Always show both speed calculations clearly.

 

Question 3. A train travels at a speed of 25 m/s and covers a distance of 360 km. How much time does it take?
Answer: Given speed of the train \( = 25 \text{ m/s} \)
Distance to cover \( = 360 \text{ km} \)
First, convert the speed from m/s to km/h so the units match. Remember, 1 m/s \( = 3.6 \text{ km/h} \).
Speed in km/h \( = 25 \times 3.6 \)
Speed in km/h \( = 90 \text{ km/h} \)
Now, we can find the time taken using the formula: Time \( = \frac{\text{Distance}}{\text{Speed}} \)
Time \( = \frac{360 \text{ km}}{90 \text{ km/h}} \)
Time \( = 4 \text{ hours} \)
The train will take 4 hours to cover the distance. This calculation helps us understand how long journeys take.
In simple words: Change the train's speed from meters per second to kilometers per hour. Then, divide the total distance by this new speed to find out how many hours the journey will take.

🎯 Exam Tip: Always make sure your units are consistent (e.g., both distance in km and speed in km/h) before performing calculations. If units are mixed, convert them first.

 

Question 4. A train travels 180 km in 3 h. Find its speed in:
(i) km/h
(ii) m/s
(iii) What distance will it travel in 4 h if it maintains the same speed throughout the journey?
Answer:
Given: Distance \( = 180 \text{ km} \), Time \( = 3 \text{ h} \)
(i) Speed in km/h:
Speed \( = \frac{\text{Distance}}{\text{Time}} \)
Speed \( = \frac{180 \text{ km}}{3 \text{ h}} \)
Speed \( = 60 \text{ km/h} \)
(ii) Speed in m/s:
To convert km/h to m/s, we multiply by \( \frac{5}{18} \). This ratio comes from \( \frac{1000 \text{ m}}{3600 \text{ s}} \).
Speed in m/s \( = 60 \times \frac{5}{18} \)
Speed in m/s \( = \frac{300}{18} \)
Speed in m/s \( = \frac{50}{3} \approx 16.67 \text{ m/s} \)
(iii) Distance travelled in 4 h at the same speed:
Speed \( = 60 \text{ km/h} \)
New time \( = 4 \text{ h} \)
Distance \( = \text{Speed} \times \text{Time} \)
Distance \( = 60 \text{ km/h} \times 4 \text{ h} \)
Distance \( = 240 \text{ km} \)
This shows how we can use a constant speed to predict future distances. The train will travel 240 km in 4 hours.
In simple words: First, calculate the speed in kilometers per hour. Then, change this speed to meters per second using the conversion factor. Finally, multiply the speed in km/h by the new time (4 hours) to find the distance covered in that time.

🎯 Exam Tip: Make sure to show all unit conversions clearly. For example, explicitly write \( 1 \text{ km} = 1000 \text{ m} \) and \( 1 \text{ hour} = 3600 \text{ s} \) when converting between km/h and m/s.

 

Question 6. Distinguish between uniform and non-uniform motion using the example of a car moving on a straight highway with no traffic and a car moving in city traffic.
Answer:
**Uniform motion:** This happens when an object covers the same distance in the same amount of time. If a car moves on a straight highway with no traffic, it can maintain a constant speed, covering equal distances in equal time intervals. This makes its motion uniform.
**Non-uniform motion:** This occurs when an object covers different distances in the same amount of time. For a car moving in city traffic, it has to speed up, slow down, and stop often due to other vehicles and signals. Because its speed is always changing, it covers unequal distances in equal time intervals, which is non-uniform motion. Uniform motion is rare in real-world scenarios due to various external factors.
In simple words: Uniform motion means moving at a steady speed, covering the same distance in the same time, like a car on an empty highway. Non-uniform motion means changing speed, covering different distances in the same time, like a car in busy city traffic.

🎯 Exam Tip: Clearly define both types of motion and provide distinct, easy-to-understand examples for each to earn full marks. Focus on the idea of 'equal distances in equal time intervals'.

 

Question 7. Data for an object covering distances in different intervals of time are given in the following table. If the object is in uniform motion, fill in the gaps in the table.

Answer: The table above has been filled assuming uniform motion. For uniform motion, the object covers equal distances in equal time intervals. Here, the time interval is 10 seconds.
From 0 to 10 s, distance travelled \( = 8 \text{ m} \).
So, in every 10-second interval, the distance travelled must be 8 m.
Distance at 0 s \( = 0 \text{ m} \)
Distance at 10 s \( = 0 + 8 = 8 \text{ m} \)
Distance at 20 s \( = 8 + 8 = 16 \text{ m} \)
Distance at 30 s \( = 16 + 8 = 24 \text{ m} \)
Distance at 40 s \( = 24 + 8 = 32 \text{ m} \)
Distance at 50 s \( = 32 + 8 = 40 \text{ m} \)
Distance at 60 s \( = 40 + 8 = 48 \text{ m} \)
Distance at 70 s \( = 48 + 8 = 56 \text{ m} \)
This pattern confirms the definition of uniform motion, where the speed remains constant throughout. The average speed of the object is \( \frac{60 \text{ m}}{100 \text{ s}} = 0.6 \text{ m/s} \).
In simple words: Since it's uniform motion, the object travels the same distance every 10 seconds. We just keep adding 8 meters for each 10-second step to fill the table.

🎯 Exam Tip: To fill a uniform motion table, find the distance covered in one interval, then continuously add that distance for each subsequent interval. Always double-check your calculations.

 

Question 8. A car covers 60 km in the first hour, 70 km in second hour, and 50 km in third hour. Is the motion uniform? Justify your answer. Find the average speed of car.
Answer: The motion of the car is **not uniform**. This is because the car travels different distances in equal time intervals (one hour each). For uniform motion, the distances covered in each equal time interval must be the same.
To justify this, we see:
Distance in 1st hour \( = 60 \text{ km} \)
Distance in 2nd hour \( = 70 \text{ km} \)
Distance in 3rd hour \( = 50 \text{ km} \)
Since 60 km, 70 km, and 50 km are not equal, the motion is non-uniform. Even though the car is moving, its speed keeps changing.
Now, let's find the average speed of the car:
Total distance covered \( = 60 \text{ km} + 70 \text{ km} + 50 \text{ km} = 180 \text{ km} \)
Total time taken \( = 1 \text{ hour} + 1 \text{ hour} + 1 \text{ hour} = 3 \text{ hours} \)
Average speed \( = \frac{\text{Total distance}}{\text{Total time}} \)
Average speed \( = \frac{180 \text{ km}}{3 \text{ hours}} \)
Average speed \( = 60 \text{ km/hr} \)
In simple words: The car's motion is not uniform because it travels different distances each hour. To find the average speed, add up all the distances and divide by the total time taken.

🎯 Exam Tip: Clearly state whether the motion is uniform or not, and explain why using the definition of uniform motion. Average speed is always total distance divided by total time.

 

Question 9. Which type of motion is more common in daily life-uniform or non-uniform? Provide three examples from your experience to support your answer.
Answer: **Non-uniform motion** is much more common in daily life. It is very hard to maintain a perfectly constant speed and direction in most real-world situations due to various factors like traffic, turns, obstacles, and changing terrain. This means our motion usually isn't uniform.
Here are three examples:
1. A bus or auto-rickshaw moving on the road: It speeds up, slows down, and stops at different times because of traffic, passengers, and signals. Its speed is constantly changing.
2. A person walking to school: They might walk fast at times, slow down to greet someone, or stop at a crosswalk. Their speed is not constant.
3. A child playing on a swing: The swing's speed changes as it goes up and down. It is fastest at the bottom and slowest at the top of its path. These variations in speed make most everyday motions non-uniform.
In simple words: Most things we see moving every day show non-uniform motion, meaning their speed changes. Examples are a bus, a person walking, or a swing because their speeds are always going up and down.

🎯 Exam Tip: Clearly state non-uniform motion as more common and provide diverse, simple, everyday examples that highlight the changing speed or direction.

 

Question 11. A vehicle moves along a straight line and covers a distance of 2 km. In the first 500 m, it moves with a speed of 10 m/s and in the next 500 m it moves with a speed of 5 m/s. With what speed should it move the remaining distance so that the journey is complete in 200s? What is the average speed of the vehicle for the entire journey?
Answer: Total distance \( = 2 \text{ km} = 2000 \text{ m} \)
Total desired time \( = 200 \text{ s} \)

**Part 1: First 500 m**
Distance \( = 500 \text{ m} \)
Speed \( = 10 \text{ m/s} \)
Time \( = \frac{\text{Distance}}{\text{Speed}} \)
\( \implies \) Time for first 500 m \( = \frac{500 \text{ m}}{10 \text{ m/s}} = 50 \text{ s} \)

**Part 2: Next 500 m**
Distance \( = 500 \text{ m} \)
Speed \( = 5 \text{ m/s} \)
Time \( = \frac{\text{Distance}}{\text{Speed}} \)
\( \implies \) Time for next 500 m \( = \frac{500 \text{ m}}{5 \text{ m/s}} = 100 \text{ s} \)

**Part 3: Remaining distance and speed**
Distance covered so far \( = 500 \text{ m} + 500 \text{ m} = 1000 \text{ m} \)
Remaining distance \( = 2000 \text{ m} - 1000 \text{ m} = 1000 \text{ m} \)
Time elapsed so far \( = 50 \text{ s} + 100 \text{ s} = 150 \text{ s} \)
Remaining time to complete journey in 200 s \( = 200 \text{ s} - 150 \text{ s} = 50 \text{ s} \)
Required speed for remaining distance \( = \frac{\text{Remaining distance}}{\text{Remaining time}} \)
\( \implies \) Required speed \( = \frac{1000 \text{ m}}{50 \text{ s}} = 20 \text{ m/s} \)

**Part 4: Average speed for the entire journey**
Total distance \( = 2000 \text{ m} \)
Total time \( = 200 \text{ s} \)
Average speed \( = \frac{\text{Total distance}}{\text{Total time}} \)
Average speed \( = \frac{2000 \text{ m}}{200 \text{ s}} \)
Average speed \( = 10 \text{ m/s} \)
The vehicle needs to travel at 20 m/s for the last part to finish on time. This shows how speeds can be adjusted during a journey.
In simple words: First, figure out how long the vehicle took for the first two parts of its journey. Then, see how much distance and time are left. Use these to find the speed needed for the final part. Finally, divide the total distance by the total time to get the average speed for the whole trip.

🎯 Exam Tip: Break down complex problems into smaller, manageable parts. Clearly label calculations for each section of the journey (e.g., first 500m, next 500m, remaining distance) and keep track of elapsed time and distance. Ensure all units are consistent.

 

Question 1. How was time measured when there were no clocks and watches?
Answer: In ancient times, when there were no clocks or watches, people measured time by observing natural events that repeat regularly. They used things like the rising and setting of the sun to mark day and night. The different phases of the moon (like full moon, half moon) helped them understand longer periods, similar to weeks or months. The changing seasons also helped people track the passage of a year. These natural cycles were the original timekeepers. Humans have always found ways to organize their lives around time.
In simple words: Before clocks, people used nature to tell time. They looked at sunrise, sunset, moon phases, and changing seasons because these events happened again and again.

🎯 Exam Tip: When answering history questions, provide specific examples of the methods used (e.g., sunrise/sunset, moon phases, seasons) to show a comprehensive understanding.

 

Question 2. For races covering the same distance, we can tell who was faster by measuring time. But how can we tell that when comparing races for different distances?
Answer: When comparing races that cover different distances, simply looking at who finished first or who took less time won't work. To find out who was faster, we need to calculate the **speed** of each runner. Speed is found by dividing the distance covered by the time taken (Speed \( = \frac{\text{Distance}}{\text{Time}} \)). By calculating each runner's speed, we get a standardized measure that allows us to compare their performance fairly, even if they ran different lengths. This helps us understand their efficiency. The runner with the higher speed is the faster one.
In simple words: To compare speeds in races with different distances, you must calculate each runner's speed. Divide the distance they ran by the time they took. The one with the highest speed is the fastest.

🎯 Exam Tip: The key concept here is "speed." Clearly state that speed needs to be calculated and define the formula for speed, emphasizing why it standardizes comparison across different distances.

 

Question 3. I once watched a part of marathon on a straight road stretch. I noticed that some people seemed to be running at the same speed during that distance while some would speed up or slow down. How were their motion different?
Answer: In the marathon, the people running at the same speed were demonstrating **uniform linear motion**. This means they were moving in a straight line with a constant speed, covering equal distances in equal time intervals. Their pace was steady.
On the other hand, the people who were speeding up or slowing down were exhibiting **non-uniform linear motion**. This means they were moving in a straight line but their speed was changing, causing them to cover different distances in equal time intervals. Marathon runners often adjust their speed based on their energy levels and the terrain. These two types of motion are distinct in how speed changes over time.
In simple words: Runners going at a steady speed showed uniform motion. Those who sped up or slowed down showed non-uniform motion, meaning their speed changed while they ran.

🎯 Exam Tip: Distinguish between uniform and non-uniform motion by clearly defining both and relating them directly to the observed behavior of the runners (constant speed vs. changing speed).