# Chapter 13 Motion and Time

• When object is moving, then we say that it is in motion.
• Motion is of different types- Rectilinear motion, oscillatory motion, circulatory motion and vibratory motion etc.
• Speed is the distance covered by the object in a unit time.
• When an object is moving along a straight line with constant speed is known as uniform motion.
• When an object is moving along a straight line with change in speed is known as non- uniform motion.
• A motion which occur in the same internal of time is called periodic motion. Example- pendulum.
• A metallic round shaped of the pendulum is called as bob.
• Unit of time is second.
• Unit of speed is ms-1 or m/s.
• Measurement of speed is record on speedometer.
• Odometer is a device which measure the distance covered by the vehicle.
• A graphical display of data using bars of different heights is called as graph.
• Visual representation of the given data called as graph.
• Time taken by the pendulum to complete one oscillation is called its time period.
• Graph shows a straight line, which it make between distance and time.
• Shortest distance covered by the body or object is known as displacement.

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### Exercise 1 ( Page No. : 157 )

•  Q1 Classify the following as motion along a straight line, circular or oscillatory motion: (i) Motion of your hands while running. (ii) Motion of a horse pulling a cart on a straight road. (iii) Motion of a child in a merry-go-round. (iv) Motion of a child on a see-saw. (v) Motion of the hammer of an electric bell. (vi) Motion of a train on a straight bridge. Ans: (i) Oscillatory motion                                                                                                                                                                                                                  Motion of our hands while running is oscillatory because while running our hands does to and froth motion in the same interval of time, is called oscillatory motion and is also called Periodic motion. The swinging of a pendulum is an example of periodic motion. (ii) Straight line motion                                                                                                                                                                                                                      If an object changes its position with respect to its surroundings with time, then it is called in motion. It is the straight line motion because in this the object changes its position along the straight line with respect to time. For example: car is running on the straight road.   (iii) Circular motion                                                                                                                                                                                                                          When an object moves along a circular path without changing its position, it is called circular motion. For example: the earth moves around the sun in a circular path in its particular orbit, so the motion of the earth around the sun is circular motion; ceiling fan, wheel of the bicycle. (iv) Oscillatory motion                                                                                                                                                                                                                   The motion which repeats itself after regular intervals of time, is called oscillatory motion and also called Periodic motion. The swinging of a pendulum is an example of periodic motion; swinging of the swing, vibration of the mobile phone etc. (v) Oscillatory motion                                                                                                                                                                                                                    The motion which repeats itself after regular intervals of time, is called oscillatory motion. When the hammer hits the electric bell then it vibrates repetitively, so this repetitively motion is oscillatory motion. (vi) Straight line motion                                                                                                                                                                                                                      If an object changes its position with respect to its surroundings with time, then it is called in motion. In this, the train is moving on the straight bridge, it shows the change in position of the train along the straight line that shows straight line motion. Q2 Which of the following are not correct? (i) The basic unit of time is second. (ii) Every object moves with a constant speed. (iii) Distances between two cities are measured in kilometres. (iv) The time period of a given pendulum is not constant. (v) The speed of a train is expressed in m/h. Ans: (i) Correct                                                                                                                                                                                                                                        The SI (Systeme international) unit of time is second according to the metric scale. (ii) Not correct                                                                                                                                                                                                                             Every object has its own speed and variable from others. For example; in comparison to: a running horse and a moving tortoise. (iii) Correct                                                                                                                                                                                                                                      The distance between two cities is very large so we cannot measure it in meters; so kilometres is a bigger unit thus it is used to measure the distance between the two cities. (iv) Not correct                                                                                                                                                                                                                                The time period of the pendulum is constant, because it shows to and froth motion which is an example of periodic motion (occurs in the same interval of time). (v) Not correct                                                                                                                                                                                                                                 We cannot express the speed of train in m/h because meter is a too short unit for measuring the distance covered by the train per hour. So, we use km/h to measure the distance covered by train. Q3 A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum? Ans: The time taken to complete one oscillation is known as time period of the pendulum. Time period = Total time taken                      Number of oscillations Given - 20 oscillation taking 32 s to complete Therefore, 1 oscillation will take = 32 sec = 1.6 second.                                                        20 Therefore, 1.6 is the time period of the pendulum. Q4 The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train. Ans: Given; Speed = Distance travelled                            Time = 240 km/ 4 h          (Given) = 60 km/h Therefore, the speed of the train is 60km/hr. Q5 The odometer of a car reads 57321.0 km when the clock shows the time 08:30 AM. What is the distance moved by the 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. Ans: Initial reading of the odometer of the car = 57321.0 km Final reading of the odometer of the car = 57336.0 km The car starts at 8:30 AM and stops at 8:50 AM Distance covered by car = (57336 - 57321) km = 15 km Time taken between 8:30 AM to 8:50 AM = 20 minutes =20/60 hour = 1/3 hour So speed in km/min Speed = Distance travelled                            Time =15 km / 20 min 0.75 km/ min Speed in km/h Speed = Distance travelled                           Time = 15 km/ 1/3 h = 15 x 3 km/ 1 h = 45 km/h Q6 Salma takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/s, calculate the distance between her house and the school. Ans: Speed = 2 m/s Time taken to reach school = 15 minutes = 15 x 60 seconds                                                                   = 900 seconds Speed = Distance travelled                          Time Distance= speed x time                = 2 x 900                = 1800 m Also, 1 km = 1000 m Therefore, 1800 x 1/1000 = 1.8 km. Hence, the distance between her house and the school is 1.8 km. Q7 Show the shape of the distance-time graph for the motion in the following cases: (i) A car moving with a constant speed. (ii) A car parked on a side road. Ans: (i) When a car is moving with the constant speed it means the car covers the equal distance in equal interval of time then the graph shows straight line.       (ii) A car parked on the road side, it means that there is no change in the position of the car with respect to time. Then the distance time graph represents the parallel line along with the time. Q8 Which of the following relations is correct? (i) Speed = Distance × Time      (ii) Speed = Distance                                                                         Time (iii) Speed = Time                       (iv) Speed =            1                              Distance                                      Distance × Time Ans: (ii) The speed of an object is given by the relation: Speed = Distance travelled                         Time The speed is defined as the distance covered by an object per unit time. The SI unit of time is m/s. Q9 9. The basic unit of speed is: (i) km/min       (ii) m/min (iii) km/h         (iv) m/s Ans: (iv) m/s The basic unit of distance is meter (m) and of time is second(s). Speed = Distance travelled                            Time Therefore, the basic unit of speed is m/s. Q10 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: (i) 100 km          (ii) 25 km (iii) 15 km          (iv) 10 km Ans: (ii) 25 km Case 1. Speed = 40 km/h Time = 15 min = (15/60) hour Distance (d1) = speed x time = 15/60= 10 km Case 2. Speed = 60 km/h Time = 15 min = (15/60)hour Distance(d2) = speed x time = 60*(15/60) = 15 km Total distance (d) = (d1 + d2) = 10 km + 15 km = 25 km Therefore, the total distance covered by the car = 25 km. Q11 Suppose the two photographs, shown in Fig. 13.1 and Fig. 13.2, had been taken at an interval of 10 seconds. If a distance of 100 metres is shown by 1 cm in these photographs, calculate the speed of the blue car. Ans: With the help of the scale we will first measure the distance: Suppose the distance measured is 2 cm So, the distance covered = 2*100 = 2 m (because 1 m = 100 cm) Time taken = 10 seconds Speed = (distance/ time) = (200 m/10 s) = 20 m/s Therefore, the speed of the blue car = 20 m/s. Q12 Fig. 13.15 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?                                                                              Fig. 13.15 Distance-time graph for the motion of two cars Ans: This graph shows that vehicle A is moving faster than B. Speed     =    Distance travelled                                                                                                                                                                                                                        Time Vehicle A is faster that can be explained by drawing a perpendicular line along to the time. This perpendicular line cuts the B line at point b and A line is cut by the point a. Now compare point a and point b. Point a shows that more distance covered in time t. Point b shows less distance covered with respect to time. Now it is evident that, from the graph in the given time t, distance covered by A is more than B. Q13 Which of the following distance-time graphs shows a truck moving with speed which is not constant? Ans: Graph 1st is not correct because this shows the constant speed of the truck which is represented by the straight line. Graph 2nd is not correct because the distance/ time graph shows a line that is parallel along the time axis. This shows that truck is not moving. Graph 4th is not correct because the distance/ time graph indicates that the truck is moving with a constant speed with the same interval of time. Graph 3rd is correct.     Because in the distance/ time graph, a curved line indicates that the truck is moving with a variable speed with respect to time.