Class 7 Science

Motion

NCERT Solution

Question 1: Classify the following as motion along a straight line, circular or oscillatory motion:

  • Motion of your hands while running.
  • Motion of a horse pulling a cart on a straight road.
  • Motion of a child in a merry-go-round.
  • Motion of a child on a see-saw.
  • Motion of the hammer of an electric bell.
  • Motion of a train on a straight bridge.

Answer:

  • Periodic motion or Oscillatory motion
  • Along a straight line (Linear motion)
  • Circular motion
  • Periodic motion or oscillatory motion
  • Periodic motion or oscillatory motion
  • Linear motion



Question 2: Which of the following are not correct?

  • The basic unit of time is second.
  • Every object moves with a constant speed.
  • Distances between two cities are measured in kilometres.
  • The time period of a given pendulum is not constant.
  • The speed of a train is expressed in m/h.

Answer: (ii), (iv) and (v)

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

Answer: Given, Number of oscillation = 20
Time taken = 32 second
We know that,
Time Period = Time ÷ Number of oscillations
= 32 s ÷ 20 = 1.6 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.

Answer: Given, distance = 240 km
Time taken = 4 hour
We know that,
Speed = Distance ÷ Time
= 240 km ÷ 4hour = 60 km per hour



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: Given, Initial reading of odometer = 57321.0 km
Final reading of odometer = 57336.0 km
Initial time = 08:30 AM
Final time = 08:50 AM

Thus, Distance covered = Final reading of odometer – Initial reading of odometer
= 57336.0 km – 57321.0 km = 15 km
Total time taken = Final time – Initial time = 08:50 AM – 08:30 AM = 20 minute
20 minute = 20 ÷ 60 hour = 1/3 hour

We know that, speed = distance ÷ time
= 15 km ÷ 20 minute = 0.75 km ÷ minute
Or, speed = 15km ÷ (1/3) hour = 45 km/hour
So, speed = 0.75 km/minute or 45km/hour

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: Given, Speed = 2 m/s
Time taken = 15 minute = 900 s
Distance =?
We know that,
Speed = Distance ÷ Time
Or, Distance = Speed X Time
= 2 m/s X 900 s = 1800 m = 1.8 km



Question 7: Show the shape of the distance-time graph for the motion in the following cases:

  • A car moving with a constant speed.
  • A car parked on a side road.

Answer:

distance time graph

Question 8: Which of the following relations is correct?

  • Speed = Distance X Time
  • Speed = Distance ÷ Time
  • Time = Distance ÷ Speed
  • Speed = 1 ÷ (Distance.Time)

Answer: (b) Speed = Distance/Time