**NCERT Exemplar Problems Class 7 Science – Motion and Time**

**Question 1: **Which of the following cannot be used for the measurement of time?

(a) A leaking tap

(b) Simple pendulum

(c) Shadow of an object during the day

(d) Blinking of eyes

**Answer:**

**(d)**Blinking of eyes is not a periodic phenomenon (i.e. it does not repeat after definite intervals of time), so we cannot measure the time using this phenomenon.

**Question 2: **Two clocks A and B are shown in figure. Clock A has an hour and a minute hand whereas clock B has an hour hand, minute hand as well as a second hand. Which of the following statement is correct for these clocks?

(a) A time interval of 30 seconds can be measured by clock A

**(b) A time interval of 30 seconds cannot be measured by clock B**

(c) Time interval of 5 minutes can be measured by both A and B

(d) Time interval of 4 minutes 10 seconds can be measured by clock A

**Answer:**

**(c)**The time interval of 5 min can be measured by both A and B. Since, the time interval of 30 seconds cannot be measured by clock A and can be measured by clock B. Also, the time interval of 4 min 10 seconds cannot be measured by clock A, but can be measured by clock B.

**Question 3: **Two students were asked to plot a distance-time graph for the motion described by Table A and Table B.

The graph given in figure is true for

(a) Both A and B

(b) Only A

(c) Only B

(d) Neither A Nor B

**Answer:**

**(a)**Since, speed is constant for A and B, so graph for A and B will be a straight line as given in figure.

**Question 4: **A bus travels 54 km in 90 min. The speed of the bus is

(a) 6 m/s

(b) 10 m/s

(c) 5.4 m/s

(d) 3.6 m/s

**Answer:**

(b) As, we know that

**Question 5: **If we denote speed by S, distance by D and time by T, the relationship between these quantities is

**Answer:**

(c) The correct relationship is

**Question 6: **Observe the given figure.

The time period of a simple pendulum is the time taken by it to travel from

(a) A to B and back to A

(b) O to A, A to B and B to A

(c) B to A, A to B and B to O

(d) A to B

**Answer:**

**(a)**Time period of a simple pendulum is the total time taken to complete one full cycle, (i.e. from A to 6 and back to A).

**Question 7: **Figure shows an oscillating pendulum. Time taken the bob to move from A to C is t

_{1}and from C to 0 is The time period of this simple pendulum is

(a) (t

_{1}+ t

_{2})

(b) 2 (t

_{1}+ t

_{2})

(c) 3 (t

_{1}+ t

_{2})

(d) 4 (t

_{1}+ t

_{2})

**Answer:**

**(d)**The total time taken by the bob to move from A to O is (t

_{1}+ t

_{2})

which is 1/4 th time of one full cycle of pendulum. Time period of pendulum i.e. time taken by pendulum to complete one oscillation from A to B and back to A will be 4 (t

_{1}+ t

_{2}).

**Question 8: **The correct symbol to represent the speed of an object is

(a) 5 m/s

(b) 5 mp

(c) 5 m/s

^{-1}

(d) 5 s/m

**Answer:**

**(a)**5 m/s is the correct symbol to represent the speed of an object because the unit of speed is m/s (or m/s

^{-1}).

**Question 9: **Boojho walks to his school which is at a distance of 3 km from his home in 30 min. On reaching, he finds that the school is closed and comes back by a bicycle with his friend and reaches home in 20 min. His average speed in km/h is

(a) 8.3 (b) 7.2 (c) 5 (d) 3.6

**Answer:**

**(b)**Given, total distance = 3 + 3= 6km, total time = 30 + 20 = 50min = 50/ 60 h

**Very Short Answer Type Questions**

**Question 10: **A simple pendulum is oscillating between two points A and B as shown in figure. Is the motion of the bob uniform or’non-uniform?

**Answer:**

The motion of bob is non-uniform, as it does not cover equal distance in equal intervals of time.

**Question 11: **Paheli and Boojho have to cover different distances to reach their school but they take the same time to reach the school. What can you say about their speed?

**Answer:**

They do not have equal speed because they cover unequal distances in equal intervals of time. One of them has higher speed, whom has to cover larger distance with respect to other.

**Question 12: **If Boojho covers a certain distance in one hour and Paheli covers the same distance in two hours, who travels in a higher speed?

**Answer:**

Boojho travels with a higher speed, as he has covered the same distance in lesser time than Paheii.

**Short Answer Type Questions**

**Question 13: **Complete the data of the table given below with the help of the distance-time graph given in figure.

**Answer:**

From the graph given above, it is a case of uniform motion, so

**Question 14: **The average age of children of class VII is 12 years and 3 months. Express this age in seconds.

**Answer:**

**Question 15: **A spaceship travels 36000 km in one hour. Express its speed in km/s.

**Answer:**

**Question 16: **Starting from A, Paheli moves along a rectangular path ABCD as shown in figure. She takes 2 min to travel each side. Plot a distance-time graph and explain whether the motion is uniform or non-uniform

**Answer:**

Since, the speed i.e., the distance covered per unit time for the entire distance covered is not the same, therefore, the motion is a non-uniform motion.

**Question 17: **Plot a distance-time graph of the tip of the second hand of a clock by selecting 4 points on X-axis and /-axis, respectively. The circumference of the circle traced by the second hand is 64 cm.

**Answer:**

The circumference of circle traced by a second hand in 60 sec is 64 cm.

**Long Answer Type Questions**

**Question 18: **Given below as figure is the distance-time graph of the motion of an object.

(a) What will be the position of the object at 20 s?

(b) What will be the distance travelled by the object in 12 s?

(c) What is the average speed of the object?

**Answer:**

(a) From the graph, it is clear that the distance at 20 s is 8 m.

(b) Distance travelled by the object in 12 s is 6 m.

**Question 19: **Distance between Bholu’s and Golu’s house is 9 km. Bholu has to attend Golu’s birthday party at 7 0′ clock. He started his journey from his home at 6 O’ clock on his bicycle and covered a distance of 6 km in 40 min. At that point, he met Chintu and he spoke to him for 5 min and reached Golu’s birthday party at 7 O’ dock. With what speed, did he cover the second part of the journey? Calculate his average speed for the entire journey.

**Answer:**

From the question, Bholu covers the remaining 3 km distance in 15 min

**Question 20: **Boojho goes to the football ground to play football. The distance-time graph of his journey from his home to the ground is given as figure.

(a) What does the graph between points B and C indicate about the motion of Boojho?

(b) Is the motion between 0 to 4 min uniform or non-uniform?

(c) What is his speed between 8 and 12 minutes of his journey?

**Answer:**

(a) Since, the graph between B and C is parallel to time-axis, so it indicates that Boojho is at rest i.e. his speed is zero.

(b)Since, the graph is not straight line, so it is a non-uniform motion.