Page 1 Motion and Time 13 I n Class VI, you learnt about different types of motions. You learnt that a motion could be along a straight line, it could be circular or periodic. Can you recall these three types of motions? Table 13.1 gives some common examples of motions. Identify the type of motion in each case. 13.1 SLOW OR FAST We know that some vehicles move faster than others. Even the same vehicle may move faster or slower at different times. Make a list of ten objects moving along a straight path. Group the motion of these objects as slow and fast. How did you decide which object is moving slow and which one is moving fast? If vehicles are moving on a road in the same direction, we can easily tell which one of them is moving faster than the other. Let us look at the motion of vehicles moving on a road. Activity 13.1 Look at Fig. 13.1. It shows the position of some vehicles moving on a road in the same direction at some instant of time. Now look at Fig. 13.2. It shows the position of the same vehicles after some time. From your observation of the two figures, answer the following questions: Which vehicle is moving the fastest of all? Which one of them is moving the slowest of all? The distance moved by objects in a given interval of time can help us to decide which one is faster or slower. For example, imagine that you have gone to see off your friend at the bus stand. Suppose you start pedalling your bicycle at the same time as the bus begins to Table 13.1 Some examples of different types of motion Example of Type of motion motion Along a straight line/circular/ periodic Soldiers in a march past Bullock cart moving on a straight road Hands of an athlete in a race Pedal of a bicycle in motion Motion of the Earth around the Sun Motion of a swing Motion of a pendulum It is common experience that the motion of some objects is slow while that of some others is fast. 2020-21 Page 2 Motion and Time 13 I n Class VI, you learnt about different types of motions. You learnt that a motion could be along a straight line, it could be circular or periodic. Can you recall these three types of motions? Table 13.1 gives some common examples of motions. Identify the type of motion in each case. 13.1 SLOW OR FAST We know that some vehicles move faster than others. Even the same vehicle may move faster or slower at different times. Make a list of ten objects moving along a straight path. Group the motion of these objects as slow and fast. How did you decide which object is moving slow and which one is moving fast? If vehicles are moving on a road in the same direction, we can easily tell which one of them is moving faster than the other. Let us look at the motion of vehicles moving on a road. Activity 13.1 Look at Fig. 13.1. It shows the position of some vehicles moving on a road in the same direction at some instant of time. Now look at Fig. 13.2. It shows the position of the same vehicles after some time. From your observation of the two figures, answer the following questions: Which vehicle is moving the fastest of all? Which one of them is moving the slowest of all? The distance moved by objects in a given interval of time can help us to decide which one is faster or slower. For example, imagine that you have gone to see off your friend at the bus stand. Suppose you start pedalling your bicycle at the same time as the bus begins to Table 13.1 Some examples of different types of motion Example of Type of motion motion Along a straight line/circular/ periodic Soldiers in a march past Bullock cart moving on a straight road Hands of an athlete in a race Pedal of a bicycle in motion Motion of the Earth around the Sun Motion of a swing Motion of a pendulum It is common experience that the motion of some objects is slow while that of some others is fast. 2020-21 SCIENCE 144 move. The distance covered by you after 5 minutes would be much smaller than that covered by the bus. Would you say that the bus is moving faster than the bicycle? We often say that the faster vehicle has a higher speed. In a 100-metre race it is easy to decide whose speed is the highest. One who takes shortest time to cover the distance of 100 metres has the highest speed. 13.2 SPEED You are probably familiar with the word speed. In the examples given above, a higher speed seems to indicate that a given distance has been covered in a shorter time, or a larger distance covered in a given time. The most convenient way to find out which of the two or more objects is moving faster is to compare the distances moved by them in a unit time. Thus, if we know the distance covered by two buses in one hour, we can tell which one is faster. We call the distance covered by an object in a unit time as the speed of the object. When we say that a car is moving with a speed of 50 kilometres per hour, it implies that it will cover a distance of Fig. 13.2 Position of vehicles shown in Fig. 13.1 after some time Fig. 13.1 Vehicles moving in the same direction on a road 50 kilometres in one hour. However, a car seldom moves with a constant speed for one hour. In fact, it starts moving slowly and then picks up speed. So, when we say that the car has a speed of 50 kilometres per hour, we usually consider only the total distance covered by it in one hour. We do not bother whether the car has been moving with 2020-21 Page 3 Motion and Time 13 I n Class VI, you learnt about different types of motions. You learnt that a motion could be along a straight line, it could be circular or periodic. Can you recall these three types of motions? Table 13.1 gives some common examples of motions. Identify the type of motion in each case. 13.1 SLOW OR FAST We know that some vehicles move faster than others. Even the same vehicle may move faster or slower at different times. Make a list of ten objects moving along a straight path. Group the motion of these objects as slow and fast. How did you decide which object is moving slow and which one is moving fast? If vehicles are moving on a road in the same direction, we can easily tell which one of them is moving faster than the other. Let us look at the motion of vehicles moving on a road. Activity 13.1 Look at Fig. 13.1. It shows the position of some vehicles moving on a road in the same direction at some instant of time. Now look at Fig. 13.2. It shows the position of the same vehicles after some time. From your observation of the two figures, answer the following questions: Which vehicle is moving the fastest of all? Which one of them is moving the slowest of all? The distance moved by objects in a given interval of time can help us to decide which one is faster or slower. For example, imagine that you have gone to see off your friend at the bus stand. Suppose you start pedalling your bicycle at the same time as the bus begins to Table 13.1 Some examples of different types of motion Example of Type of motion motion Along a straight line/circular/ periodic Soldiers in a march past Bullock cart moving on a straight road Hands of an athlete in a race Pedal of a bicycle in motion Motion of the Earth around the Sun Motion of a swing Motion of a pendulum It is common experience that the motion of some objects is slow while that of some others is fast. 2020-21 SCIENCE 144 move. The distance covered by you after 5 minutes would be much smaller than that covered by the bus. Would you say that the bus is moving faster than the bicycle? We often say that the faster vehicle has a higher speed. In a 100-metre race it is easy to decide whose speed is the highest. One who takes shortest time to cover the distance of 100 metres has the highest speed. 13.2 SPEED You are probably familiar with the word speed. In the examples given above, a higher speed seems to indicate that a given distance has been covered in a shorter time, or a larger distance covered in a given time. The most convenient way to find out which of the two or more objects is moving faster is to compare the distances moved by them in a unit time. Thus, if we know the distance covered by two buses in one hour, we can tell which one is faster. We call the distance covered by an object in a unit time as the speed of the object. When we say that a car is moving with a speed of 50 kilometres per hour, it implies that it will cover a distance of Fig. 13.2 Position of vehicles shown in Fig. 13.1 after some time Fig. 13.1 Vehicles moving in the same direction on a road 50 kilometres in one hour. However, a car seldom moves with a constant speed for one hour. In fact, it starts moving slowly and then picks up speed. So, when we say that the car has a speed of 50 kilometres per hour, we usually consider only the total distance covered by it in one hour. We do not bother whether the car has been moving with 2020-21 MOTION AND TIME 145 We can determine the speed of a given object once we can measure the time taken by it to cover a certain distance. In Class VI you learnt how to measure distances. But, how do we measure time? Let us find out. 13.3 MEASUREMENT OF TIME If you did not have a clock, how would you decide what time of the day it is? Have you ever wondered how our elders could tell the approximate time of the day by just looking at shadows? How do we measure time interval of a month? A year? Our ancestors noticed that many events in nature repeat themselves after definite intervals of time. For example, they found that the sun rises everyday in the morning. The time between one sunrise and the next was called a day. Similarly, a month was measured from one new moon to the next. A year was fixed as the time taken by the earth to complete one revolution of the sun. Often we need to measure intervals of time which are much shorter than a day. Clocks or watches are perhaps the most common time measuring devices. Have you ever wondered how clocks and watches measure time? The working of clocks is rather complex. But all of them make use of some periodic motion. One of the most well-known periodic motions is that of a simple pendulum. In everyday life we seldom find objects moving with a constant speed over long distances or for long durations of time. If the speed of an object moving along a straight line keeps changing, its motion is said to be non-uniform. On the other hand, an object moving along a straight line with a constant speed is said to be in uniform motion. In this case, the average speed is the same as the actual speed. Fig. 13.3 Some common clocks (b) Table clock (c) Digital clock (a) Wall clock a constant speed or not during that hour. The speed calculated here is actually the average speed of the car. In this book we shall use the term speed for average speed. So, for us the speed is the total distance covered divided by the total time taken. Thus, Total distance covered Speed = Total time taken 2020-21 Page 4 Motion and Time 13 I n Class VI, you learnt about different types of motions. You learnt that a motion could be along a straight line, it could be circular or periodic. Can you recall these three types of motions? Table 13.1 gives some common examples of motions. Identify the type of motion in each case. 13.1 SLOW OR FAST We know that some vehicles move faster than others. Even the same vehicle may move faster or slower at different times. Make a list of ten objects moving along a straight path. Group the motion of these objects as slow and fast. How did you decide which object is moving slow and which one is moving fast? If vehicles are moving on a road in the same direction, we can easily tell which one of them is moving faster than the other. Let us look at the motion of vehicles moving on a road. Activity 13.1 Look at Fig. 13.1. It shows the position of some vehicles moving on a road in the same direction at some instant of time. Now look at Fig. 13.2. It shows the position of the same vehicles after some time. From your observation of the two figures, answer the following questions: Which vehicle is moving the fastest of all? Which one of them is moving the slowest of all? The distance moved by objects in a given interval of time can help us to decide which one is faster or slower. For example, imagine that you have gone to see off your friend at the bus stand. Suppose you start pedalling your bicycle at the same time as the bus begins to Table 13.1 Some examples of different types of motion Example of Type of motion motion Along a straight line/circular/ periodic Soldiers in a march past Bullock cart moving on a straight road Hands of an athlete in a race Pedal of a bicycle in motion Motion of the Earth around the Sun Motion of a swing Motion of a pendulum It is common experience that the motion of some objects is slow while that of some others is fast. 2020-21 SCIENCE 144 move. The distance covered by you after 5 minutes would be much smaller than that covered by the bus. Would you say that the bus is moving faster than the bicycle? We often say that the faster vehicle has a higher speed. In a 100-metre race it is easy to decide whose speed is the highest. One who takes shortest time to cover the distance of 100 metres has the highest speed. 13.2 SPEED You are probably familiar with the word speed. In the examples given above, a higher speed seems to indicate that a given distance has been covered in a shorter time, or a larger distance covered in a given time. The most convenient way to find out which of the two or more objects is moving faster is to compare the distances moved by them in a unit time. Thus, if we know the distance covered by two buses in one hour, we can tell which one is faster. We call the distance covered by an object in a unit time as the speed of the object. When we say that a car is moving with a speed of 50 kilometres per hour, it implies that it will cover a distance of Fig. 13.2 Position of vehicles shown in Fig. 13.1 after some time Fig. 13.1 Vehicles moving in the same direction on a road 50 kilometres in one hour. However, a car seldom moves with a constant speed for one hour. In fact, it starts moving slowly and then picks up speed. So, when we say that the car has a speed of 50 kilometres per hour, we usually consider only the total distance covered by it in one hour. We do not bother whether the car has been moving with 2020-21 MOTION AND TIME 145 We can determine the speed of a given object once we can measure the time taken by it to cover a certain distance. In Class VI you learnt how to measure distances. But, how do we measure time? Let us find out. 13.3 MEASUREMENT OF TIME If you did not have a clock, how would you decide what time of the day it is? Have you ever wondered how our elders could tell the approximate time of the day by just looking at shadows? How do we measure time interval of a month? A year? Our ancestors noticed that many events in nature repeat themselves after definite intervals of time. For example, they found that the sun rises everyday in the morning. The time between one sunrise and the next was called a day. Similarly, a month was measured from one new moon to the next. A year was fixed as the time taken by the earth to complete one revolution of the sun. Often we need to measure intervals of time which are much shorter than a day. Clocks or watches are perhaps the most common time measuring devices. Have you ever wondered how clocks and watches measure time? The working of clocks is rather complex. But all of them make use of some periodic motion. One of the most well-known periodic motions is that of a simple pendulum. In everyday life we seldom find objects moving with a constant speed over long distances or for long durations of time. If the speed of an object moving along a straight line keeps changing, its motion is said to be non-uniform. On the other hand, an object moving along a straight line with a constant speed is said to be in uniform motion. In this case, the average speed is the same as the actual speed. Fig. 13.3 Some common clocks (b) Table clock (c) Digital clock (a) Wall clock a constant speed or not during that hour. The speed calculated here is actually the average speed of the car. In this book we shall use the term speed for average speed. So, for us the speed is the total distance covered divided by the total time taken. Thus, Total distance covered Speed = Total time taken 2020-21 SCIENCE 146 A simple pendulum consists of a small metallic ball or a piece of stone suspended from a rigid stand by a thread [Fig. 13.4 (a)]. The metallic ball is called the bob of the pendulum. Fig. 13.4 (a) shows the pendulum at rest in its mean position. When the bob of the pendulum is released after taking it slightly to one side, it begins to move to and fro [Fig. 13.4 (b)]. The to and fro motion of a simple pendulum is an example of a periodic or an oscillatory motion. The pendulum is said to have completed one oscillation when its bob, starting from its mean position O, moves To set the pendulum in motion, gently hold the bob and move it slightly to one side. Make sure that the string attached to the bob is taut while you displace it. Now release the bob from its displaced position. Remember that the bob is not to be pushed when it is released. Note the time on the clock when the bob is at its mean position. Instead of the mean position you may note the time when the bob is at one of its extreme positions. Measure the time the pendulum takes to complete 20 oscillations. Record your observations Fig. 13.4 (b) Different positions of the bob of an oscillating simple pendulum Fig. 13.4 (a) A simple pendulum to A, to B and back to O. The pendulum also completes one oscillation when its bob moves from one extreme position A to the other extreme position B and comes back to A. The time taken by the pendulum to complete one oscillation is called its time period. Activity 13.2 Set up a simple pendulum as shown in Fig. 13.4 (a) with a thread or string of length nearly one metre. Switch off any fans nearby. Let the bob of the pendulum come to rest at its mean position. Mark the mean position of the bob on the floor below it or on the wall behind it. To measure the time period of the pendulum we will need a stopwatch. However, if a stopwatch is not available, a table clock or a wristwatch can be used. A O B 2020-21 Page 5 Motion and Time 13 I n Class VI, you learnt about different types of motions. You learnt that a motion could be along a straight line, it could be circular or periodic. Can you recall these three types of motions? Table 13.1 gives some common examples of motions. Identify the type of motion in each case. 13.1 SLOW OR FAST We know that some vehicles move faster than others. Even the same vehicle may move faster or slower at different times. Make a list of ten objects moving along a straight path. Group the motion of these objects as slow and fast. How did you decide which object is moving slow and which one is moving fast? If vehicles are moving on a road in the same direction, we can easily tell which one of them is moving faster than the other. Let us look at the motion of vehicles moving on a road. Activity 13.1 Look at Fig. 13.1. It shows the position of some vehicles moving on a road in the same direction at some instant of time. Now look at Fig. 13.2. It shows the position of the same vehicles after some time. From your observation of the two figures, answer the following questions: Which vehicle is moving the fastest of all? Which one of them is moving the slowest of all? The distance moved by objects in a given interval of time can help us to decide which one is faster or slower. For example, imagine that you have gone to see off your friend at the bus stand. Suppose you start pedalling your bicycle at the same time as the bus begins to Table 13.1 Some examples of different types of motion Example of Type of motion motion Along a straight line/circular/ periodic Soldiers in a march past Bullock cart moving on a straight road Hands of an athlete in a race Pedal of a bicycle in motion Motion of the Earth around the Sun Motion of a swing Motion of a pendulum It is common experience that the motion of some objects is slow while that of some others is fast. 2020-21 SCIENCE 144 move. The distance covered by you after 5 minutes would be much smaller than that covered by the bus. Would you say that the bus is moving faster than the bicycle? We often say that the faster vehicle has a higher speed. In a 100-metre race it is easy to decide whose speed is the highest. One who takes shortest time to cover the distance of 100 metres has the highest speed. 13.2 SPEED You are probably familiar with the word speed. In the examples given above, a higher speed seems to indicate that a given distance has been covered in a shorter time, or a larger distance covered in a given time. The most convenient way to find out which of the two or more objects is moving faster is to compare the distances moved by them in a unit time. Thus, if we know the distance covered by two buses in one hour, we can tell which one is faster. We call the distance covered by an object in a unit time as the speed of the object. When we say that a car is moving with a speed of 50 kilometres per hour, it implies that it will cover a distance of Fig. 13.2 Position of vehicles shown in Fig. 13.1 after some time Fig. 13.1 Vehicles moving in the same direction on a road 50 kilometres in one hour. However, a car seldom moves with a constant speed for one hour. In fact, it starts moving slowly and then picks up speed. So, when we say that the car has a speed of 50 kilometres per hour, we usually consider only the total distance covered by it in one hour. We do not bother whether the car has been moving with 2020-21 MOTION AND TIME 145 We can determine the speed of a given object once we can measure the time taken by it to cover a certain distance. In Class VI you learnt how to measure distances. But, how do we measure time? Let us find out. 13.3 MEASUREMENT OF TIME If you did not have a clock, how would you decide what time of the day it is? Have you ever wondered how our elders could tell the approximate time of the day by just looking at shadows? How do we measure time interval of a month? A year? Our ancestors noticed that many events in nature repeat themselves after definite intervals of time. For example, they found that the sun rises everyday in the morning. The time between one sunrise and the next was called a day. Similarly, a month was measured from one new moon to the next. A year was fixed as the time taken by the earth to complete one revolution of the sun. Often we need to measure intervals of time which are much shorter than a day. Clocks or watches are perhaps the most common time measuring devices. Have you ever wondered how clocks and watches measure time? The working of clocks is rather complex. But all of them make use of some periodic motion. One of the most well-known periodic motions is that of a simple pendulum. In everyday life we seldom find objects moving with a constant speed over long distances or for long durations of time. If the speed of an object moving along a straight line keeps changing, its motion is said to be non-uniform. On the other hand, an object moving along a straight line with a constant speed is said to be in uniform motion. In this case, the average speed is the same as the actual speed. Fig. 13.3 Some common clocks (b) Table clock (c) Digital clock (a) Wall clock a constant speed or not during that hour. The speed calculated here is actually the average speed of the car. In this book we shall use the term speed for average speed. So, for us the speed is the total distance covered divided by the total time taken. Thus, Total distance covered Speed = Total time taken 2020-21 SCIENCE 146 A simple pendulum consists of a small metallic ball or a piece of stone suspended from a rigid stand by a thread [Fig. 13.4 (a)]. The metallic ball is called the bob of the pendulum. Fig. 13.4 (a) shows the pendulum at rest in its mean position. When the bob of the pendulum is released after taking it slightly to one side, it begins to move to and fro [Fig. 13.4 (b)]. The to and fro motion of a simple pendulum is an example of a periodic or an oscillatory motion. The pendulum is said to have completed one oscillation when its bob, starting from its mean position O, moves To set the pendulum in motion, gently hold the bob and move it slightly to one side. Make sure that the string attached to the bob is taut while you displace it. Now release the bob from its displaced position. Remember that the bob is not to be pushed when it is released. Note the time on the clock when the bob is at its mean position. Instead of the mean position you may note the time when the bob is at one of its extreme positions. Measure the time the pendulum takes to complete 20 oscillations. Record your observations Fig. 13.4 (b) Different positions of the bob of an oscillating simple pendulum Fig. 13.4 (a) A simple pendulum to A, to B and back to O. The pendulum also completes one oscillation when its bob moves from one extreme position A to the other extreme position B and comes back to A. The time taken by the pendulum to complete one oscillation is called its time period. Activity 13.2 Set up a simple pendulum as shown in Fig. 13.4 (a) with a thread or string of length nearly one metre. Switch off any fans nearby. Let the bob of the pendulum come to rest at its mean position. Mark the mean position of the bob on the floor below it or on the wall behind it. To measure the time period of the pendulum we will need a stopwatch. However, if a stopwatch is not available, a table clock or a wristwatch can be used. A O B 2020-21 MOTION AND TIME 147 Table 13.2 Time period of a simple pendulum Length of the string = 100 cm S.No. Time taken for 20 Time period oscillations (s) (s) 1. 42 2.1 2. 3. cells. These clocks are called quartz clocks. The time measured by quartz clocks is much more accurate than that by the clocks available earlier. Units of time and speed The basic unit of time is a second. Its symbol is s. Larger units of time are minutes (min) and hours (h). You already know how these units are related to one another. What would be the basic unit of speed? Since the speed is distance/time, the basic unit of speed is m/s. Of course, it could also be expressed in other units such as m/min or km/h. You must remember that the symbols of all units are written in singular. For example, we write 50 km and not 50 kms, or 8 cm and not 8 cms. Boojho is wondering how many seconds there are in a day and how many hours in a year. Can you help him? in Table 13.2. The first observation shown is just a sample. Your observations could be different from this. Repeat this activity a few times and record your observations. By dividing the time taken for 20 oscillations by 20, get the time taken for one oscillation, or the time period of the pendulum. Is the time period of your pendulum nearly the same in all cases? Note that a slight change in the initial displacement does not affect the time period of your pendulum. Nowadays most clocks or watches have an electric circuit with one or more There is an interesting story about the discovery that the time period of a given pendulum is constant. You might have heard the name of famous scientist Galileo Galilie (A.D. 1564 –1642). It is said that once Galileo was sitting in a church. He noticed that a lamp suspended from the ceiling with a chain was moving slowly from one side to the other. He was surprised to find that his pulse beat the same number of times during the interval in which the lamp completed one oscillation. Galileo experimented with various pendulums to verify his observation. He found that a pendulum of a given length takes always the same time to complete one oscillation. This observation led to the development of pendulum clocks. Winding clocks and wristwatches were refinements of the pendulum clocks. 2020-21Read More
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