## Chapter 13 - Motion and Time

**Question and AnswersQuestion 1: Classify the following as motion along a straight line, circular or oscillatory motion: (i) Motion of your hands while running. **(Oscillatory Motion)

**(Motion along a straight line)**

(ii) Motion of a horse pulling a cart on a straight road.

(ii) Motion of a horse pulling a cart on a straight road.

**(Circular motion)**

(iii) Motion of a child in a merry-go-round.

(iii) Motion of a child in a merry-go-round.

**(Oscillatory motion)**

(iv) Motion of a child on a see-saw.

(iv) Motion of a child on a see-saw.

**(Oscillatory motion)**

(v) Motion of the hammer of an electric bell.

(v) Motion of the hammer of an electric bell.

**(Motion along a straight line)**

(vi) Motion of a train on a straight bridge.

(vi) Motion of a train on a straight bridge.

**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.

Answer:Incorrect statements:

(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.

Answer:

(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?**

Answer:

Answer:

Number of oscillations = 20

Time taken to complete 20 oscillations = 32 seconds

Time period = Number of oscillations/Time taken

= 32/20

= 1.6 seconds

Therefore, the time period of the simple pendulum is 1.6 seconds.

**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.**

Answer:

Answer:

Distance between two stations = 240 kilometre

Time taken to reach from station 1 to station 2 = 4 hours

Speed = Distance/Time

= 240/4

= 60 km/hr

Therefore, the speed of the train is 60 km/hr.

**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.**

Answer:

Answer:

Initial reading of the odometer = 57321.0 km

Final reading of the odometer = 57336.0 km

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

= 57336.0 - 57321.0 = 15 km

Starting time = 08:30 AM

Ending time = 08:50 AM

Time taken to complete 15 km = Starting time - Ending time

= 08:50 - 08:30 = 20 minutes

(Speed of car in km/min)

Speed = Distance/Time

= 15/20

= 3/4 km/min

(Speed of car in km/hr)

1 hour = 60 minutes

20 minutes = 1/6 x 20

= 1/3 hr

Speed = Distance/Time

= 15/1/3 km/hr

Therefore, the speed of car in km/min is 3/4 km/min and the speed of the car in km/hr is 15/1/3 km/hr.

**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.**

Answer:

Answer:

Time taken by Salma to reach school from home = 15 minutes

Speed of bicycle = 2 m/s

Distance between her school and her home = ?

15 minutes = _____ seconds

= 15 x 60

= 900 seconds

Distance = Speed x Time

= 2 x 900

= 1800 m

1800 m = ________ km

= 1800/1000

= 1.8 km

Therefore, the distance between her house and her home is 1.8 km.

**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.

Answer:

(i) A car moving with a constant speed.

(ii) A car parked on a side road.

Answer:

(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 TimeAnswer:** (ii) Speed = Distance/Time

**9. The basic unit of speed is:**

(i) km/min

(ii) m/min

(iii) km/h

(iv) m/s

Answer:(iv) m/s

(i) km/min

(ii) m/min

(iii) km/h

(iv) m/s

Answer:

**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

Answer:(ii) 25 km

(i) 100 km

(ii) 25 km

(iii) 15 km

(iv) 10 km

Answer:

**(When the speed of the car is 40 km/hr)**

Time taken = 15 min (15/60 = 1/4 hr)

Speed = Distance/Time

Distance covered = speed x time taken

= 40 x 1/4 = 10 km

**(When the speed of the Car is 60 km/ h)**

Speed = Distance/Time

Distance covered = speed x time taken

= 60 x 1/4= 15 km

Total distance covered by the car = 10 + 15 = 25 km

**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.**

**Answer:**

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.

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/Time

= 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?**

**Answer:** 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?**

**Answer: **(iii)

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