1. Motion -
ü If an object keeps on changing its position with respect to time, it is said to be moving or in motion.
ü Linear or straight in which the object travels in a straight line.
ü Circular in which the object travels along a circular path.
ü Curvilinear in which the object moves along a curve.
2. Speed Of Moving Body-
ü If one object covers a particular distance in less time and another object covers the same distance in more time then the first object is said to be moving slowly while the second object is said to be moving faster.
ü The distance travelled by an object in unit time is called its Speed.
ü Uniform Speed - When the object travels a fixed distance same time gaps, it is said to have a uniform speed.
ü Non-uniform speed - When an object covers different distances in different time gaps, it is said to have a non-uniform speed.
ü Average speed - The total distance travelled by an object divided by the total time taken by the object is called its average speed.
Figure 01- Speed, time and distance relationship
3. Measuring Time-
ü Morning - The rising of the sun
ü Day and Night - The time between the sunrise and sunset
ü Month - The time between two new moons
ü Year - The time the earth takes to complete its one revolution around the sun
ü Time measuring gadgets or clocks - Clocks use the concept of periodic motion to measure time. It means that it uses motion that repeats itself in equal amounts of time. There are different types of time measuring devices.
1.Sundial – It uses the position of the sun to depict time.
2.Sand Clock (hourglass) – It uses sand to measure time.
3.Water Clock – It uses water to measure time.
4.Pendulum Clock – It uses a pendulum to measure time.
5.Quartz Clocks – They have an electric circuit that works with the help of cells. They provide accurate time.
4. Simple Pendulum-
Figure 03- Simple pendulum
ü A simple pendulum contains a Bob. It is a metallic ball or a stone which is suspended from a rigid stand with the help of a thread.
ü Oscillatory motion - The to and fro motion of the pendulum is called as Oscillatory Motion. The bob of the pendulum does move from the center (mean position) of the pendulum to its extreme positions on the other side.
ü Oscillation - When the bob moves from its center (mean position) to its extreme ends it is said to complete one oscillation.
ü Time Period of a pendulum - The time taken by the pendulum bob to complete one oscillation is called its Time Period.
5. Units to Measure Time And Speed-
Time | Second (s) Minutes (min) Hours (h) |
Speed = Distance/time | Meter/Second (m/s) Meter/minute (m/min) Kilometer/hour (km/h) |
Table 01- Units of measurement
Figure 04- Conversion between km/hr and m/s
ü Speedometer - It is a device which is used in vehicles such as cars and trucks which measures the speed in kilometer per hour.
ü Odometer - It is a device which measures the distance travelled by a vehicle in meters or kilometers.
Figure 05- Measurement of distance and speed of a car
6. Distance-time Graphs-
ü A graph which represents the distance travelled by an object with respect to time is called a distance-time graph.
Shape of Graph | Interpretation |
Straight line | The object has a uniform or constant speed |
Parallel to time-axis | It is a stationary object |
Curve shape | The object has a non-uniform speed |
Table 02- Interpretation of data
Figure 06- Distance-time Graph
ü Speed = distance/time = (final position of object – initial position of object)/time taken by object
ü The speed of the distance-time graph can be calculated by the Slope of a graph. The steeper the slope of the graph, the more is the speed of the object. For example, in the graph given below object A has a steeper slope. This means that object A is moving at a higher speed than object B.
Figure 06- Distance-time graph of two objects
NCERT PRACTICE QUESTIONS
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.
Ans.- i) oscillatory ii) Motion along a straight line iii) Circular motion iv) Oscillatory motion v) Oscillatory motion vi) Motion along a straight line.
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) 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.
3. A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Ans.- Number of oscillations = 20
Total time taken to complete 20 oscillations = 32 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.
Solution:
Ans.- Distance between two stations = 240 kms
Total time take = 4 hrs/240 minutes
= 60 km/h
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.- 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
= 45 km/h
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.- Time taken by Salma to reach her school by bicycle = 15 mins= 15 x 60 = 90 s
Speed of Salma’s bicycle= 2m/s
Distance covered = speed x time taken
= 2 x 900 = 1800 m
1000m = 1 km
= 1.8 kms
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.
Ans.
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
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
Total distance covered by the car = d1 + d2
= 10 + 15
= 25 kms
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.
Ans.- 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
= 120/10
= 12 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.- Vehicle A is moving faster than vehicle B.
13. Which of the following distance-time graphs shows a truck moving with speed which is not constant?
Ans.- iii)
Junior Science Topic - Motion And Time. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 7, CBSE syllabus, chapter 13)
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