Class:-7, Science, Chapter:- 13 Motion & Time Notes, NCERT Solutions & Frequently Asked Questions

Science

Class:-7, Chapter:- 13

Motion & Time 

Notes, NCERT Solutions & Frequently Asked Questions

-- Notes --

⭐ Distance:- Distance is the total path covered by the object in the given interval of time.

• Distance=Speed×Time.

• Unit of distance is generally measured in units of length like metres, kilometres etc.

⭐ Motion:- An object is said to be in motion if it changes its position with respect to time. Eg: A car moving on a road.

• An object is said to be at rest if the object does not change its position with time. Eg: A person standing on the ground.

⭐ Types of motion:- There are 3 types of motions:-

a) Rectilinear/Translatory/Straight Line motion:- When a body is moving in a straight line, without changing its direction, then the body is said to possess translatory/rectilinear motion.

• Eg.- A car moving on a straight road.

b) Circular motion:- When a body moves in the shape of a circle about a fixed point and a fixed radius, then the body is said to be in circular motion.

• Eg.- Motion of planets around the sun.

c) Periodic/Oscillatory motion:- When the motion of a body repeats after fixed intervals of time, then the body is said to in the periodic/oscillatory motion.

• Eg.-To and fro motion of a simple pendulum.

⭐ Oscillations of a simple pendulum:- When the bob of a simple pendulum moves from its mean position B to A and back to B again, then from B to C and back to B again, the pendulum is said to complete 1 oscillation.

• In the case of the simple pendulum, the time period is the total time taken by the pendulum to complete one oscillation.

⭐ Speed:- Speed is the total distance travelled by the object in a given interval of time.

• Speed = Distance travelled/time taken

• Unit of speed is generally measured in metre per second(m/s), kilometre per hour(km/h).

⭐ Uniform and Non-Uniform Motion:- 

a) Uniform motion:- When an object moving along a straight line moves with a constant speed, then the object is said to be in uniform motion.

• Eg.- A car moving in a straight line with a constant speed.

b) Non-uniform motion:- When an object moving along a straight line changes its speed with respect to time, then the object is said to be in uniform motion.

• Eg.- The motion of a train.

⭐ Measurement of Time:- 

• Unit of time is generally measured in seconds, minutes, hours.

• The time period is the total time taken by an object to complete one oscillation.

• Unit of time is generally measured in seconds, minutes, hours.

⭐ Speed:- 

• Speed is the total distance travelled in the given time interval.

• Unit of speed is generally measured in metre per second (m/s) and kilometre per hour (Km/h).

⭐ Basics of Standard Units:- 

• The standard unit of distance is in metres.

• The standard unit of time is in seconds (s).

• The standard unit of speed is in metre per second (m/s).

⭐ Speedometer and Odometer:- 

• The speedometer is an instrument on a vehicle’s dashboard which indicates the speed of the vehicle when it is moving. This instrument tells us the speed of a running vehicle at that instant of time in kilometre per hour.

• An instrument which is used for measuring the distance travelled by a vehicle is known as an odometer. 

⭐ Distance-time graph:- Distance-time graph is the graph plotted between the distance (in y-axis) and time (in x-axis).

• The slope of a distance-time graph gives the speed of an object.

-- NCERT Solutions --

Question 1.- 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.

Solution:-

i) Oscillatory

ii) Motion along a straight line

iii) Circular motion

iv) Oscillatory motion

v) Oscillatory motion

vi) Motion along a straight line.

Question 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.

Solution:- Incorrect statements are:

ii) Every object moves with a constant speed.

iv) The time period of a given pendulum is constant.

v) The speed of a train is expressed in m/h.

Question 3.- A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?

Solution:- 

     Number of oscillations = 20

     Total time taken to complete 20 oscillations = 32 s 

Question 4.- The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.

Solution:-

Distance between two stations = 240 kms

Total time take = 4 hrs

Speed = 60 km/h

Question 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.

Solution:-

Initial reading of the odometer = 57321.0

Final reading of the odometer = 57336.0

Distance covered by the car = Final reading of the odometer – Initial reading of the odometer

= 57336.0 – 57321.0 = 15 kms

Starting time of car is 8:30 and it stops at 8:50

Hence, time taken by car = 20 mins

Speed = 45 km/h

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/s, calculate the distance between her house and the school.

Solution:-

Time taken by Salma to reach her school by bicycle = 15 mins = 15 x 60 = 900s

Speed of Salma’s bicycle= 2m/s

Distance covered = speed x time taken

= 2 x 900 = 1800 m

(1000m = 1 km)

Distance = 1.8 kms

Question 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.

Solution:-

i)

ii) 

Question 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

Solution:- ii) Speed = Distance/Time

Question 9.- The basic unit of speed is:-

i) km/min

ii) m/min

iii) km/h

iv) m/s

Solution:- iv) 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:-

i) 100 km 

ii) 25 km 

iii) 15 km 

iv) 10 km

Solution:- ii) 25 km

Calculation:-

When the speed of the car is 40 km/h

Time taken = 15 min = 15/60 = 0.25 h

Distance covered d1 = speed x time taken

                                      = 40 x 0.25 = 10 kms

When the speed of the car is 60 km/ h

Distance covered d2 = speed x time taken

                                       = 60 x 0.25= 15 kms

Hence, the total distance covered by the car = d1 + d2

= 10 + 15

= 25 kms

Question 11.- 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 fastest car.

Solution:-

The distance covered by the blue car (as evident from the photograph) from one horizontal white strip to another, which is measured by scale is 1.2 cm.

It is given that 1 cm is equivalent to 100 m.

Therefore, 1.2 cm is equivalent to 120 m.

Distance travelled by the car = 120 m

Time taken to cover this distance = Time interval between the two photographs = 10 s 

Speed = Distance Covered / Time taken

= 120/10

= 12 m/s

Question 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?

Solution:- Vehicle A is moving faster than vehicle B.

Question 13.- Which of the following distance-time graphs shows a truck moving with speed which is not constant?

Solution:- iii)

-- Frequently asked Questions --

Question 1.- What is ‘Rectilinear’ motion?

Solution:- It is a type of linear motion in which the direction of the velocity remains constant and the path is a straight line.

Question 2.- What is ‘Average speed’?

Solution:- Average speed is calculated by dividing the total distance that something has traveled by the total amount of time it took it to travel that distance.

Question 3.- What are the uses of ‘Uniform motion’?

Solution:- If an object is travelling with uniform motion, it is moving in a straight line at a constant speed.