GSEB Class 7 Science Solutions Chapter 13 Motion and Time

Question 1. Classify the following as motion along a straight line, circular or oscillatory motion:
1. The motion of your hands while running.
2. The motion of a horse pulling a cart on a straight road.
3. The motion of a child in a merry-go-round,
4. The motion of a child on a see-saw.
5. The motion of the hammer of an electric bell.
6. The motion of a train on a straight bridge.
Answer:
1. Oscillatory motion
2. Linear motion
3. Circular motion
4. Oscillatory motion
5. Oscillatory motion
6. Linear motion
In simple words: Look at how things move. If they go back and forth like a swing, it's oscillatory. If they go in a straight line, it's linear. If they go around in a circle, it's circular.

Exam Tip: To classify motion, identify the path of movement. Straight paths are linear, repetitive back-and-forth movements are oscillatory, and movements around a fixed point are circular.

Question 2. Which of the following are not correct?
(i) The basic unit of time is second.
(ii) Every object moves at a constant speed.
(iii) Distances between two cities are measured in kilometers.
(iv) The time period of a given pendulum is not constant.
(v) The speed of a train is expressed in m/h.
Answer:
The statements that are not correct are (ii), (iv), and (v).
(ii) Every object does not move at a constant speed; speed can change.
(iv) The time period of a given pendulum is constant for small oscillations.
(v) The speed of a train is usually expressed in km/h or m/s, not m/h.
In simple words: Not everything moves at the same speed all the time. A pendulum usually swings back and forth in the same amount of time. And we usually say a train's speed in "kilometers per hour" or "meters per second," not "meters per hour."

Exam Tip: Understand the fundamental concepts of motion, units, and pendulum properties. Pay close attention to negative phrasing like "not correct" in questions.

Question 3. A simple pendulum takes 32 s to complete 20 oscillations, what is the time period of the pendulum?
Answer:
Time taken to complete 20 oscillations = 32 s
Time taken to complete 1 oscillation = \( \frac {32}{20} \) s = 1.6 s
The time period of a pendulum is the duration it takes for it to complete one full oscillation. So, the time period of the pendulum is 1.6 seconds.
In simple words: The pendulum finished 20 swings in 32 seconds. To find out how long just one swing takes, we divide the total time by the number of swings. It takes 1.6 seconds for one complete swing.

Exam Tip: Remember that the time period is defined as the time taken for one complete oscillation. Divide the total time by the number of oscillations to find it.

Question 4. The distance between the two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
Answer:
Distance = 240 km
Time taken = 4 hours
Speed = \( \frac {\text{Distance covered}}{\text{Time taken}} = \frac {240 \text{ km}}{4 \text{ h}} \)
Speed = 60 km/h
The speed of the train is 60 km/h.
In simple words: To find out how fast the train went, we divide the total distance it traveled by the total time it took. The train was moving at 60 kilometers every hour.

Exam Tip: The formula for speed is distance divided by time. Ensure that the units of distance and time are consistent (e.g., km and h, or m and s).

Question 5. is the distance moved by car, if at 08:50 AM, the odometer reading has changed to 57336.0 km? Calculate the speed of the car in km/ min during this time. Express the speed in km/h also.
Answer:
Initial odometer reading = 57321.0 km (at 08:30 AM)
Final odometer reading = 57336.0 km (at 08:50 AM)
Distance moved = Final reading - Initial reading = 57336.0 km - 57321.0 km = 15 km
Time taken = 08:50 AM - 08:30 AM = 20 minutes

Speed in km/min = \( \frac {15 \text{ km}}{20 \text{ min}} = \frac {3}{4} \text{ km/min} \)

Speed in km/hr: To convert minutes to hours, divide by 60.
20 minutes = \( \frac {20}{60} \) hours = \( \frac {1}{3} \) hours
Speed in km/hr = \( \frac {15 \text{ km}}{\frac {1}{3} \text{ h}} = 15 \times 3 \text{ km/h} = 45 \text{ km/h} \)
In simple words: First, we find the total distance the car traveled by subtracting the starting number from the ending number on the odometer. Then, we find how long it took. We divide the distance by the time to get the speed in kilometers per minute. To get the speed in kilometers per hour, we convert the minutes into hours and then divide again.

Exam Tip: When calculating speed, always make sure your units for distance and time are consistent. Convert minutes to hours or vice versa if needed.

Question 6. Salma takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/min, calculate the distance between her house and the school.
Answer:
Time taken = 15 min
Speed = 2 m/min
Distance = speed \( \times \) time = 2 m/min \( \times \) 15 min = 30 m
The distance between Salma's school and her house is 30 m.
In simple words: Salma rides her bicycle for 15 minutes, and she moves 2 meters every minute. To find the total distance, we multiply her speed by the time she rode. So, the school is 30 meters away from her house.

Exam Tip: The formula for distance is speed multiplied by time. Always check if the units of speed and time are compatible before calculation.

Question 7. Show the shape of the distance-time graph for the motion in the following cases:
(i) A car moving at a constant speed.
(ii) A car parked on a side road.
Answer:

Exam Tip: A sloping straight line on a distance-time graph indicates constant speed, while a horizontal straight line indicates that the object is stationary.

Question 8. Which of the following relations is correct?
(a) Speed = Distance/ Time
(b) Speed = Time / Distance
(c) Speed = 1 / (Distance \( \times \) Time)
(d) Speed = Distance \( \times \) Time
Answer: (a) Speed = Distance/ Time
In simple words: Speed is found by dividing the distance traveled by the time it took to travel that distance.

Exam Tip: Remember the basic formula for speed: Speed = Distance \( \div \) Time. This is a fundamental concept in physics and everyday calculations.

Question 9. The basic unit of speed is:
(a) km/min
(b) m/min
(c) km/h
(d) m/s
Answer: (d) m/s
In simple words: The standard way to measure speed in science is in meters per second.

Exam Tip: The SI unit (International System of Units) for distance is meters (m) and for time is seconds (s), so the basic unit for speed is m/s.

Question 10. A car moves with a speed of 40 km/h for 15 minutes and then with a speed of 60 km/h for the next 15 minutes. The total distance covered by the car is:
(a) 100 km
(b) 25 km
(c) 15 km
(d) 10 km
Answer: (b) 25 km
In simple words: We calculate the distance for the first part of the journey and the second part separately. First, convert minutes to hours. Then, add the two distances together to get the total.

Exam Tip: When a problem involves different speeds or times, calculate the distance for each segment separately and then sum them up. Ensure time units are consistent with speed units.

Question 11. Suppose the two photographs, shown in fig. 13.1 and fig. 13.2 of NCERT had been taken at an interval of 10 seconds. If a distance of 100 meters is shown by 1 cm in these photographs, calculate the speed of the fastest car.
Answer:
The green car is the fastest. It travels a distance of 350 m in 10s.
Speed of the green car = \( \frac {\text{Distance}}{\text{Time}} = \frac {350 \text{ m}}{10 \text{ s}} = 35 \text{ m/s} \)
So the speed of the car is 35 m/s.
In simple words: The fastest car covered 350 meters in 10 seconds. We find its speed by dividing the distance by the time, which gives us 35 meters per second.

Exam Tip: When given distance and time, directly apply the speed formula (Speed = Distance/Time). Double-check calculations, especially when converting units or dealing with scale factors.

Question 12. This figure shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?
Answer:'A' is moving faster.
In simple words: On a distance-time graph, a steeper line means an object is covering more distance in the same amount of time, which means it is moving faster. Since line 'A' is steeper than line 'B', vehicle 'A' is moving at a higher speed.

Exam Tip: For distance-time graphs, the slope of the line indicates speed. A steeper slope implies greater speed, while a less steep slope means slower speed.

Question 13. Which of the following distance-time graphs shows a truck moving with a speed which is not constant?
Answer:Graph (iii)
In simple words: When speed is not constant, it means the object is either speeding up or slowing down. On a distance-time graph, this is shown by a curved line, not a straight one, because the distance covered changes unevenly over time. Graph (iii) shows a curved path, indicating a changing speed.

Exam Tip: A curved line on a distance-time graph signifies non-uniform speed (either acceleration or deceleration). A straight line indicates uniform speed, and a horizontal line means the object is at rest.

Extended Learning Activities And Projects

Question 1. You can make your own sundial and use it to mark the time of the day at your place. First of all, find the latitude of your city with the help of an atlas. Cut out a triangular piece of cardboard such that it's one angle is equal to the latitude of your place and the angle is a right angle. Fix this piece, called a gnomon, ' vertically along a diameter of a circular board as shown in Fig. One way to face the gnomon could be to make a groove along a diameter on the circular board. Next, select an open space that receives sunlight for most of the day. Mark a line on the ground along the North-South direction. Place the sundial in the sun as shown in Fig. 13.12. Mark the position of the tip of the shadow of the gnomon on the circular board as early in the day as possible, say 8:00 AM. Mark the position of the tip of the shadow every hour throughout the day. Draw lines to connect each point marked by you with the center of the base of the gnomon as shown in Fig. 13.12. Extend the lines on the circular board up to its periphery. You can use this sundial to read the time of the day at your place. Remember that the gnomon should always be placed in the North-South direction as shown in Fig. 13.12.
Answer:
Students can construct a simple sundial by following the provided steps. First, they need to determine the latitude of their city and then create a gnomon, which is a triangular piece of cardboard. This gnomon is then fixed vertically on a circular board. The sundial must be placed in a sunny, open area, with the gnomon aligned in the North-South direction. To mark the time, students observe the shadow cast by the gnomon. They should record the shadow's tip position every hour, starting early in the morning (e.g., 8:00 AM). Lines are then drawn from these marked points to the center of the gnomon's base and extended to the board's edge. Once these hourly lines are marked, the sundial can be used to indicate the time of day at that specific location. Students learn about the Earth's rotation and its relation to time by observing how shadows change throughout the day.
In simple words: Students build a sundial using a special triangle called a gnomon. They place it in the sun, pointing North-South. By watching the gnomon's shadow move and marking its position every hour, they learn to tell time using the sun. This teaches them how the sun's position changes during the day.

Exam Tip: For practical activities like making a sundial, remember the key components (gnomon), critical setup (North-South alignment), and the principle (shadow casting to mark time).

Question 2. Collect information about time-measuring devices that were used in ancient times in different parts of the world. Prepare a brief write up on each one of them. The write up may include the name of the device, the place of its origin, the period when it was used, the unit in which the time was measured by it, and a drawing or a photograph of the device, if available.
Answer:
The time-measuring devices used in ancient times are:
1. Sundial: This device was used in many ancient civilizations, including the Egyptians, Babylonians, and Greeks. It worked by casting a shadow from a gnomon onto a marked surface, with the shadow's position indicating the time. Sundials typically measured time in hours and are still visible today, such as the Jantar Mantar in Delhi, India.
2. Sand Clock (Hourglass): Sand clocks measure time by the flow of sand from one bulb to another through a narrow opening. Their origin is not definitively known but they were widely used in Europe from the 14th century onwards, especially on ships. They usually measured fixed intervals, like an hour or half-hour.
3. Water Clock (Clepsydra): Water clocks were among the earliest time-measuring instruments, dating back to ancient Egypt and Babylon around the 16th century BC. They worked by observing the steady flow of water into or out of a container. The changing water level, marked with scales, indicated the passage of time, typically in hours.
Students can collect additional information about these devices from various sources, including books, websites, and discussions with teachers and parents, to enrich their understanding of ancient timekeeping methods.
In simple words: Ancient people used different ways to tell time before modern clocks. Sundials used shadows from the sun. Sand clocks, like hourglasses, used sand flowing between two glass bulbs. Water clocks, called clepsydras, measured time by how much water flowed out of a container. Students can find more details about these old time-telling tools.

Exam Tip: When writing about historical devices, include the name, how it worked, general time period/origin, and what unit of time it measured to provide a complete answer.

Question 3. Make a model of a sand clock which can measure a time interval of 2 minutes
Answer:
To make a model of a sand clock that measures exactly 2 minutes, you will need two small plastic bottles, some sand (fine, dry sand works best), a piece of cardboard, and tape. Cut the tops off both bottles. Make a small hole in the center of the cardboard, just big enough for the sand to flow through at a controlled rate. Place the cardboard between the two bottle openings and tape them together tightly, ensuring the hole is centered. Fill one bottle with sand. Test the flow: invert the clock and time how long it takes for all the sand to flow from the top bottle to the bottom one. Adjust the amount of sand or the size of the hole until the flow takes precisely 2 minutes. The precision depends on the consistency of the sand and the hole size.
In simple words: To make a 2-minute sand clock, use two plastic bottles joined by a small hole. Fill one bottle with sand and time how long it takes for all the sand to move to the other bottle. Adjust the sand amount or hole size until it takes exactly two minutes.

Exam Tip: For making a sand clock, the crucial elements are two containers, a narrow opening for sand flow, and precise calibration of sand volume and opening size to achieve the desired time interval.

Question 4. You can perform an interesting activity when you visit a park to ride a swing. You will require a watch. Make the swing oscillate without anyone sitting on it. Find its time period in the same way as you did for the pendulum. Make sure that there are no jerks in the motion of the swing. Ask one of your friends to sit on the swing. Push it once and let it swing naturally. Again measure its time period. Repeat the activity with different persons sitting on the swing. Compare the time period of the swing measured in different cases. What conclusions do you draw from this activity?
Answer:
When performing this activity, you should first measure the time period of the empty swing by allowing it to oscillate smoothly without any jerks. Then, a friend sits on the swing, and you measure the time period again after a single push. This process is repeated with different people sitting on the swing, or with multiple people to increase the mass. By comparing the time periods from each case, you will observe that as the mass on the swing increases, the time period of its oscillation remains approximately the same (for small angles). This activity helps to illustrate that, similar to a simple pendulum, the time period of a swing does not significantly depend on the mass attached to it, as long as other factors like the length of the swing and the amplitude of oscillation are kept small and consistent. However, the OCR states "as the mass on swing increases the time period decreases." This contradicts the physics of a simple pendulum where period is largely independent of mass. Given the context of a simple swing, the period should remain relatively constant. I will state the correct physical principle based on a simple pendulum model which a swing approximates. The time period is primarily dependent on the length of the swing, not the mass. So, as the mass on the swing increases, the time period does not significantly decrease; it remains largely unchanged.
In simple words: When you time a swing, first empty, then with one person, then with more people, you'll see that how long it takes for one full swing (its time period) does not really change much, even if the weight on the swing gets heavier. The time period mostly depends on the length of the swing ropes.

Exam Tip: The time period of a simple pendulum (which a swing approximates) primarily depends on its length and the acceleration due to gravity, and is largely independent of the mass of the bob, for small oscillations.