# NCERT Science Class-VII Chapter-13: Motion and Time Solutions

**Exercises**

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

(i) Motion of your hands while running.

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

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

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

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

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

**Answer**

(i) Oscillatory motion

(ii) Straight line motion

(iii) Circular motion

(iv) Oscillatory motion

(v) Oscillatory motion

(vi) Straight line motion

**2. Which of the following are not correct?**

(i) The basic unit of time is second.

(ii) Every object moves with a constant speed.

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

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

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

**Answer.**

(i) Correct

(ii) Not correct

(iii) Correct

(iv) Not correct

(v) Not correct

**3. 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

**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. **

Distance between two stations = 240 km

Time taken = 4 hrs

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. **

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

**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**

Speed of bicycle = 2 m/s

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

⇒ Distance = Speed × Time = 2 × 900 metres = 1800 metres = 1.8 km

**7. Show the shape of the distance-time graph for the motion in the following cases:
(i) A car moving with a constant speed.
(ii) A car parked on a side road.**

**Answer**

(i) The graph will be a straight line passing through the origin.

(ii) The graph is a straight line parallel to x-axis or time axis.

**8. Which of the following relations is correct?
** (i) Speed = Distance × Time (ii) Speed = Distance/Time

(iii) Speed = Time/Distance (iv) Speed = 1/Distance × Time

**Answer**

(ii) Speed = Distance/Time

**9. The basic unit of speed is:
** (i) km/min (ii) m/min

(iii) km/h (iv) m/s

**Answer**

(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: **

(i) 100 km (ii) 25 km

(iii) 15 km (iv) 10 km

**Answer**

(ii) 25 km

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

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

Distance covered,

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

Distance covered,

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

**11. 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 metres is shown by 1 cm in these photographs, calculate the speed of the blue car.**

Answer.

First, measure the distance with the help of scale and then proceed as given below.

Suppose the distance measured by scale is 2cm.

Multiply 2 with 100 to get the actual distance = 2 × 100 = 200 m. (1cm = 100m)

Interval time between the photos taken = 10 s.

Speed of the blue car = 200/10 m/s = 20 m/s

**12. Fig. 13.15 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?**

**Answer. **Vehicle A is moving faster because it has more slope than vehicle B. In distance-time graph, speed is measured by its slope.

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

(iii) (iv)

**Answer**

Option (iii) because the slope of the graph is not a straight line and hence it does not show a uniform motion.