NCERT Class 7 Science Thirteenth Chapter Motion and Time Exercise Solutions
Motion and Time
Define Speed.
Ans. The distance covered by an object in a unit time as the speed of the object.
Speed = Total distance covered / Total time taken.
What is called non-uniform motion.
Ans. The speed of an object moving along a straight line keeps changing, its motion is said to be non-uniform.
What is called uniform motion.
Ans. An object moving along a straight line with a constant speed is said to be in uniform motion.
Give a example of periodic motion.
Ans. simple pendulum.
Write a short note on simple pendulum
Ans. A simple pendulum consists of a small metallic ball or a piece of stone suspended from a rigid stand by a thread. The metallic ball is called the bob of the pendulum.
What is called time period.
Ans. The time taken by the pendulum to complete one oscillation is called its time period.
What is the basic unit of time?
Ans. Second.
Name some measuring device which are used in ancient times.
Ans. Sundials, water clocks and sand clocks.
Distance covered = Speed × Time
What is called speedometer.
Ans. One of the meters has km/h written at one corner. This is called a speedometer.
What is called odometer.
Ans. There is also another meter that measures the distance moved by the vehicle. This meter is known as an odometer.
Exercise Questions with Answers:
(1) Classify the following as motion along a straight line, circular or oscillatory motion:
(i) Motion of your hands while running.
Ans. (i) Oscillatory motion.
(ii) Motion of a horse pulling a cart on a straight road.
Ans. (ii) Linear motion
(iii) Motion of a child in a merry-go-round.
Ans. (iii) Circular motion
(iv) Motion of a child on a see-saw.
Ans. (iv) Oscillatory motion
(v) Motion of the hammer of an electric bell.
Ans. (v) Oscillatory motion
(vi) Motion of a train on a straight bridge.
Ans. (vi) Linear motion
(2) 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 constant.
(v) The speed of a train is expressed in m/h.
Ans. (ii), (iv), (v).
(3) A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Ans. 20 oscillations completed at 32 s
Therefore, 1 oscillations completed at 32 / 20 = 1.6 s.
The time period of the pendulum is 1.6 s.
(4) The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.’
Ans. Here Distance = 240 km, Time 4 hr.
We know, speed = Distance covered / Time period
= 240 km / 4 hr. = 6 km/hr.
The speed of the train = 6 km/hr.
(5) 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. Distance moved by a car = 57336.0 km – 57321.0 km = 15.0 km.
Total Time needed = 08:50 AM – 08:30 AM = 20 Min.
We know, Speed = Distance covered / Time period
= 15.0 km / 20 Min = 3/4 km/min.
Speed of the car in km/min during this time = 3/4 km/min.
Time = 20 / 60 = 1/3 hr.
Speed = 15.0 km x 3 hr = 45 km/hr.
(6) 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. Here Speed = 2 m/s
Time taken = 15 minutes = 15 x 60 = 900 sec.
Distance covered = Speed x Time = 2 m/sec x 900 sec = 1800 m.
(7) 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.
(8) Which of the following relations is correct?
(i) Speed = Distance × Time
(ii) Speed = Distance/Time
(iii) Speed = Time/Distance
(iv) Speed = 1/Distance x Time
Ans. (ii) Speed = Distance/Time
(9) The basic unit of speed is:
(i) km/min (ii) m/min (iii) km/h (iv) m/s
Ans. (iv) m/s
(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:
(i) 100 km
(ii) 25 km
(iii) 15 km
(iv) 10 km
Ans.(ii) 25 km
(Solution: First 15 minutes,
Speed = 40 km/hr
Time = 15 minutes / 60 = 1/4 hr.
Distance covered at 1st 15 minutes = 40 / 4 = 10 km.
Distance covered at 2nd 15 minutes = 60 / 4 = 15 km.
Therefore, Total 10km + 15km = 25 km.)
(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 metres is shown by 1 cm in these photographs, calculate the speed of the blue car.
Ans. 10 m/s
(12) Fig. 13.15 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?
Ans: ‘A’ is moving faster.
(13) Which of the following distance-time graphs shows a truck moving with speed which is not constant?
Ans: (iii)