Class 7 Science

Motion

NCERT Solution

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

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

Answer:

  1. Periodic motion or Oscillatory motion
  2. Along a straight line (Linear motion)
  3. Circular motion
  4. Periodic motion or oscillatory motion
  5. Periodic motion or oscillatory motion
  6. Linear motion

Question 2: Which of the following are not correct?

  1. The basic unit of time is second.
  2. Every object moves with a constant speed.
  3. Distances between two cities are measured in kilometres.
  4. The time period of a given pendulum is not constant.
  5. 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 × Time

= 2 m/s × 900 s = 1800 m = 1.8 km

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

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

Answer:

distance time graph

Question 8: Which of the following relations is correct?

  1. Speed = Distance × Time
  2. Speed = Distance ÷ Time
  3. Time = Distance ÷ Speed
  4. Speed = 1 ÷ (Distance.Time)

Answer: (b) Speed = Distance/Time

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