Science STD 7 Chapter 9: Motion and Time - Exercises

Science STD 7 Chapter 9: Motion and Time - Exercises

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

  1. (i) Motion of your hands while running.

    Oscillatory motion

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

    Motion along a straight line

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

    Circular motion

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

    Oscillatory motion

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

    Oscillatory motion

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

    Motion along a straight line

2. Which of the following are not correct?

  1. (i) The basic unit of time is second.
  2. (ii) Every object moves with a constant speed.
  3. (iii) Distances between two cities are measured in kilometres.
  4. (iv) The time period of a given pendulum is constant.
  5. (v) The speed of a train is expressed in m/h.

Statements (ii) and (v) are not correct.

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

Time period = Total time taken / Number of oscillations
Time period = 32 s / 20 = 1.6 s

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

Speed = Distance / Time
Speed = 240 km / 4 h = 60 km/h

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.

Distance moved by the car = Final odometer reading - Initial odometer reading
Distance = 57336.0 km - 57321.0 km = 15.0 km

Time taken = 08:50 AM - 08:30 AM = 20 minutes

Speed in km/min = Distance / Time = 15 km / 20 min = 0.75 km/min

To express speed in km/h: Speed = (15 km / 20 min) * (60 min / 1 h) = 45 km/h

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.

First, convert time to seconds: Time = 15 minutes * 60 s/min = 900 s

Distance = Speed * Time
Distance = 2 m/s * 900 s = 1800 m

Distance in km = 1800 m / 1000 m/km = 1.8 km

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

  1. (i) A car moving with a constant speed.

    A straight line sloping upwards from the origin.

    [Image of a distance-time graph for constant speed]

  2. (ii) A car parked on a side road.

    A horizontal line parallel to the time axis.

    [Image of a distance-time graph for a stationary object]

8. Which of the following relations is correct?

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

(ii) Speed = Distance / Time

9. The basic unit of speed is:

  1. km/min
  2. m/min
  3. km/h
  4. m/s

(iv) m/s

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:

Distance covered in the first 15 minutes = Speed × Time = 40 km/h × (15/60) h = 10 km

Distance covered in the next 15 minutes = Speed × Time = 60 km/h × (15/60) h = 15 km

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

  1. 100 km
  2. 25 km
  3. 15 km
  4. 10 km

11. Suppose the two photographs, shown in Fig. 9.1 and Fig. 9.2, 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.

Based on the provided figures (Fig. 9.1 and Fig. 9.2), the car that moves the furthest in the given time is the fastest. By measuring the distance the cars have moved in the figures and applying the scale (1 cm = 100 m) and the time interval (10 s), we can find the speed of the fastest car.

Let's assume the fastest car moved about 2 cm in the time interval.
Actual distance = 2 cm × 100 m/cm = 200 m
Speed = Distance / Time = 200 m / 10 s = 20 m/s

12. Fig. 9.15 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?

Vehicle A is moving faster. In a distance-time graph, a steeper slope indicates a higher speed. Since the line for Vehicle A is steeper than the line for Vehicle B, Vehicle A is moving at a higher speed.

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

The graph which is not a straight line shows a truck moving with a speed which is not constant. This is because a straight line in a distance-time graph represents uniform or constant speed, whereas a curved line represents non-uniform or changing speed.

[Image of a distance-time graph showing non-uniform motion]

Suggested Activities and Projects

1. You can make your own sundial and use it to mark the time of the day at your place. First of all find the latitude of your city with the help of an atlas. Cut out a triangular piece of a cardboard such that its one angle is equal to the latitude of your place and the angle opposite to it is a right angle. Fix this piece, called gnomon, vertically along a diameter of a circular board a shown in Fig. 9.16. One way to fix the gnomon could be to make a groove along a diameter on the circular board. Next, select an open space, which receives sunlight for most of the day. Mark a line on the ground along the North-South direction. Place the sundial in the sun as shown in Fig. 9.16. Mark the position of the tip of the shadow of the gnomon on the circular board as early in the day as possible, say 8:00 AM. Mark the position of the tip of the shadow every hour throughout the day. Draw lines to connect each point marked by you with the centre of the base of the gnomon as shown in Fig. 9.16. Extend the lines on the circular board up to its periphery. You can use this sundial to read the time of the day at your place. Remember that the gnomon should always be placed in the North-South direction as shown in Fig. 9.16.

Outline: This project is a practical activity to build and use a sundial. It helps you understand how the position of the sun and the shadows it casts can be used to tell the time. By building the sundial and marking the positions of the shadows at different times, you can create your own time-measuring device, demonstrating the principles of ancient timekeeping.

2. Collect information about time-measuring devices that were used in the ancient times in different parts of the world. Prepare a brief write up on each one of them. The write up may include the name of the device, the place of its origin, the period when it was used, the unit in which the time was measured by it and a drawing or a photograph of the device, if available.

Outline: This activity involves researching historical time-measuring devices. You would collect information on devices like sundials, water clocks (clepsydra), and sand clocks (hourglasses). The write-up would detail their origin, usage period, and how they worked, highlighting the ingenuity of our ancestors in measuring time based on periodic events in nature.

3. Make a model of a sand clock which can measure a time interval of 2 minutes (Fig. 9.17).

Outline: This is a hands-on project to create a functional sand clock. You would use two bottles and a small tube to allow sand to flow from one bottle to the other. By experimenting with the size of the tube and the amount of sand, you would calibrate the clock to measure a 2-minute interval. This project helps in understanding the concept of a constant flow to measure time accurately.

4. You can perform an interesting activity when you visit a park to ride a swing. You will require a watch. Make the swing oscillate without anyone sitting on it. Find its time period in the same way as you did for the pendulum. Make sure that there are no jerks in the motion of the swing. Ask one of your friends to sit on the swing. Push it once and let it swing naturally. Again measure its time period. Repeat the activity with different persons sitting on the swing. Compare the time period of the swing measured in different cases. What conclusions do you draw from this activity?

Outline: This activity is a fun way to explore oscillatory motion and time period. You would measure the time period of a swing with and without a person sitting on it. You would conclude that the time period of a pendulum (or a swing) does not depend on the mass of the bob (the person). The time period is primarily dependent on the length of the pendulum (the ropes of the swing), which remains constant.