Question 9: The basic unit of speed is:

(i) km/min (ii) m/min (iii) km/h (iv) m/s

**Answer:** (iv) m/s

Question 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:** Given, Speed for first 15 minute = 40km/h

Speed for next 15 minute = 60 km/h

We know that,

Distance = Speed × Time

Therefore, Distance covered in first 15 minute i.e. 0.25 hour = 40 km/h × 0.24 hour = 10 km

Distance covered in next 15 minute = 60 km/h × 0.25 hour = 15 km

15 minute = ΒΌ hour = 0.25 hour

Thus, total distance covered = 10 km + 15 km = 25 km

Hence, Answer (ii) is correct

Question 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:** Do the following steps to calculate the speed:

- Measure the distance cover by blue car.
- Multiply the distance measured in cm by 100 to get the distance covered by car in meter.
- Divide the distance in meter by 10 second (time given in question).
- Result will be the speed of car in meter/second.

Suppose the distance of car is 3.5 cm.

Thus, Distance covered by car = 3.5 × 100 = 350 meter.

Time (given in question) = 10 second

Speed = Distance ÷ Time

= 350 m ÷ 10 sec = 35 m per second

Question 12: Figure given here 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 which is evident from higher slope for distance time graph of this vehicle; compared to that of the other vehicle.

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

**Answer:** (iii) The slope of the graph in this option is not a straight line and hence it does not show a uniform motion.

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