Solutions of NCERT Exemplar, Ch 13, Class 7

 1. The answer is (d) Blinking of eyes.

Explanation:

Blinking of the eye is not a periodic phenomenon. Eyes do not blink at a fixed interval of time. Hence Blinking of eyes cannot be used for measurement of time.

2. The answer is (c) Time interval of 5 minutes can be measured by both A and B.

Explanation:

Clock A doesn’t have a seconds hand. Hence seconds cannot be measured by clock A. 5 minutes can be measured by using both the clocks.

3. The answer is (a) both A and B.

Explanation:

Since speed for A and B is constant, graph for A and B is a straight line as shown in the figure.

4. Answer is (b) 10 m/s

Explanation:

Speed = Distance / Time

Distance = 54km = 54 x1000 = 54000m

Time = 90 minutes = 90×60= 5400s

Speed = 54000/5400 = 10 m/s

5. Answer is (c) S = 1/SxD

Explanation:

Option c) is the correction equation because Speed = Distance/Time

6. The answer is (a) A to B and back to A.

7. Unit of speed is meter/ second hence the answer is (a) 5 m/s .