Motion and Time | Science | Chapter 13 | Class 7th | Questions and Answers |

JANDKNCERT | Science | Class 7th |

Motion and Time | Science | Chapter 13 | Class 7th | Questions and Answers |

 

Chapter 13: Motion and Time

EXERCISE

Q1. Classify the following as motion along a straight line, circular or oscillatory motion:

(i) The motion of your hands while running.

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

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

(iv) The motion of a child on a see-saw.

(v) The motion of the hammer of an electric bell.

(vi) The motion of a train on a straight bridge.

Ans.

(i) The motion of your hands while running. Oscillatory motion

(ii) The motion of a horse pulling a cart on a straight road. Motion along a straight line

(iii) The motion of a child in a merry-go-round. Circular motion

(iv) The motion of a child on a see-saw. Oscillatory motion

(v) The motion of the hammer of an electric bell. Oscillatory motion

(vi) The motion of a train on a straight bridge. Motion along a straight line

 

 

Q2. Which of the following are not correct?

(i) The basic unit of time is second.

(ii) Every object moves at a constant speed.

(iii) Distances between two cities are measured in kilometers.

(iv) The time period of a given pendulum is not constant.

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

Ans.

(i) The basic unit of time is second. Correct

(ii) Every object moves at a constant speed. Incorrect

(iii) Distances between two cities are measured in kilometers. Correct

(iv) The time period of a given pendulum is not constant. Incorrect

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

 

 

Q3. A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?

Ans.

No. of oscillations = 20

Time taken to complete 20 oscillations = 32 s

Time Period = Total time taken / No. of oscillations

Therefore,

Time period = 32 s / 20

= 1.6 s

 

Q4. The distance between the two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.

Ans.

Distance between two stations = 240 km

Time taken to cover the distance = 4 hours (240 minutes)

Speed = Distance / Time

Therefore,

Speed of the train = 240 km / 4 h

= 60 km/h

 

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 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 = 57336.0 km

Distance covered by the car = Final reading – Initial reading

= 57336.0 km – 57321.0 km

= 15 km

According to the question;

The car starts at 8:30am and stops at 8:50am

Therefore,

Time taken by the car to cover the distance is (8:50am – 8:30am) = 20 min

Distance covered by the car = 15 km

Time taken by the car to cover the distance = 20 min

Speed = Distance / Time taken

= 15 km / 20 min

= 0.75 km/min

Again 60 min = 1 h

20 min = 20 / 60 h

= 1/3h

Therefore,

Time taken by the car to cover the distance = 1/3h

Hence,

Speed of the car in km/h = 15km / 1/3h

=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.

Time taken by Salma to reach her school by bicycle = 15 min (900s)

Speed of the bicycle = 2 m/s

We know that,

Speed = Distance / Time

Therefore,

Distance = Speed x Time

Distance = 2 m/s x 900s

= 1800m or 1.8km (Because 1m = 1 / 1000km)

 

Q7. Show the shape of the distance-time graph for the motion in the following cases:

(i) A car moving at a constant speed.

(ii) A car parked on a side road.

Ans.

(i) A car moving at a constant speed.

 

 

 

  

(ii) A car parked on a side road.

 

 

 

 

Q8. Which of the following relations is correct?

(i) Speed = Distance × Time

(ii) Speed = Distance / Time

(iii) Speed = Time / Distance

(iv) Speed = 1 / Distance × Time

Ans. (ii) Speed = Distance / Time

 

Q9. The basic unit of speed is:

(i) km/min

(ii) m/min

(iii) km/h

(iv) m/s

Ans. (iv) 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

Solution:

Initial Speed = 40 km/h

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

Distance = Speed x Time

Distance = 40 km/h x 0.25h

= 10 km

Final Speed = 60 km/h

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

Distance = Speed x Time

Distance = 60 km/h x 0.25h

= 15 km

Total Distance = 10 km + 15 km

= 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 meters is shown by 1 cm in these photographs, calculate the speed of the blue 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.

Given that,

1 cm is equivalent to 100 m.

Therefore,

1.2 cm = 120 m.

Distance traveled by car = 120 m

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

Speed = Distance / Time

Therefore,

Speed = 120m / 10s

= 12 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?

 

 

 

 

 

Ans. Vehicle A is moving faster than vehicle B.

 

Q13. Which of the following distance-time graphs shows a truck moving with speed which is not constant?

 

 

 

 

 

  

Ans. Truck (iii) is moving with variable speed.