Tuesday , July 5 2022
Time and Motion

NCERT 7th Class (CBSE) Science: Time and Motion

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

  1. Motion of your hands while running – Oscillatory motion
  2. Motion of a horse pulling a cart on a straight road – Straight line motion
  3. Motion of a child in a merry-go-round – Circular motion
  4. Motion of a child on a see-saw – Oscillatory motion
  5. Motion of the hammer of an electric bell – Oscillatory motion
  6. Motion of a train on a straight bridge – Straight line motion

Question: Which of the following are not correct?

Answer: 1) The basic unit of time is second – Correct

2) Every object moves with a constant speed – Not correct, Every object may or may not moves with a constant speed

3) Distances between two cities are measured in kilometers – Correct

4) The time period of a given pendulum is not constant – Not correct, Time period of a given pendulum is always constant because it depends on the length of the pendulum.

5) The speed of a train is expressed in m/h – Not correct, The speed of a train is measured in km/h or in m/s.

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

Answer: Numbers of oscillations = 20

Time taken = 32 sec

Time period of the pendulum = Time taken / Numbers of oscillations

= 32 / 20 = 1.6 s.

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

Answer: Distance between two stations = 240 km

Time taken = 4 hrs

Speed of train = Distance / Time taken

= 240 / 4 = 60 km / hr

Question: 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: Initial reading of odometer = 57321.0 km

Final reading of odometer = 57336.0 km

Initial Time = 8:30 AM

Final Time = 8:50 AM

Total distance covered = Final reading of odometer – Initial reading of odometer
=57336.0 – 57321.0 = 15.0 km

Total time taken = Final Time – Initial Time

= 8:50 AM – 8:30 AM = 20 minutes

20 minutes = 20/60 hrs = 1/3 hrs

Speed of the car = Total distance covered/Total time taken = 15.0/20 = 0.75 km/min

Speed of the car in km/hr =15/(1/3) = 45 km/hr

Question: 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: Speed of bicycle = 2 m/s

Time taken = 15 mins = 15 * 60 secs = 900 secs

As per question,

Speed = Distance/Time

= Distance = Speed * Time = 2 * 900 m = 1800 m = 1.8 km

Question: 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: Firstly, the car is moving with 40 km/h for 15 minutes.

15 minutes = 15/60 hrs = 1/4 hrs

Distance covered = 40 * 1/4 = 10 km

Secondly, the car is moving with 60 km/h for 15 minutes.

Distance covered = 60 * 1/4 = 15 km

Total distance covered = 10 km + 15 km = 25 km

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