**Motion and Time** is** Chapter 13** taken from the **JKBOSE** textbook of Science for students of **Class 7th**. In the previous post, I provided you with **Questions and Answers for Chapter 12 Reproduction in Plants.** This post is about **Motion and Time** **Class 7 Question Answers. **Before we proceed to Question Answers, you need to have basic information about the chapter. So here is a brief overview of the chapter:

**Motion and Time Class 7 Question Answers**

**Overview of Chapter**

**SLOW OR FAST.****SPEED.****MEASUREMENT OF TIME.****Units of time and speed.**

**MEASURING SPEED.****DISTANCE-TIME GRAPH.**

The movement of an object is called motion. The moment in which things occur is called time. When an object changes its position with time, it is said to be in motion.

**SLOW OR FAST:** An object which takes a long time to cover a certain distance is said to be slow while another object which takes a shorter time to cover the same distance is said to be faster.

**SPEED:** The distance travelled by an object per unit of time is known as the speed of the object. A slow-moving object is said to have a low speed and a fast-moving object is said to have a fast speed.

**Speed = Distance Travelled/Time taken**

- Units of Speed: The standard unit of speed is ‘metres per second. The larger speed values are expressed in units of ‘kilometres per hour. The small values of speed are expressed in units of centimetres per second.

**MEASUREMENT OF TIME:** A moment or duration in which things occur is known as time. We measure time with the help of clocks and watches. In ancient times, people used some natural events which repeated regularly after fixed time intervals, to measure time. Some of the important time-measuring devices used in ancient times were: Sundial, sand clock and water clock.

- Units of Time: The standard unit of time is ‘second’. The larger unit of time is a minute and an hour.

**MEASURING SPEED:** We need two things to measure the speed of a moving object one is the distance travelled by the object and the time taken to cover the said distance. A speedometer is an instrument which indicates the speed of the vehicle when it is moving.

**DISTANCE-TIME GRAPH:** A distance-time graph shows how the distance travelled by a moving object changes with time.

- The distance-time graph of an object moving with a uniform speed is a straight line.
- The distance-time graph for an object moving with non-uniform speed is a curved line.
- The distance-time graph for an object which stops moving is a straight line parallel to the time axis (x-axis)

**Motion and Time ****Class 7 Questions **

**Exercises:**

**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.**

Ans. i) Oscillatory motion.

(ii) Linear motion/ Straight line motion.

(iii) Circular motion.

(iv) Oscillatory motion.

(v) Oscillatory motion.

(vi) Linear motion/Straight line motion.

**Which of the following is 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.**

Ans. The statements (ii), (iv) and (v) are not correct.

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

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

**The odometer of a car reads 57321.0 km when the clock shows the time 08:30 AM. What is the distance moved by 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.**

**Geeta 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.**

**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.**

Ans.

**Which of the following relations is correct?**

**The basic unit of speed is:**

**(i) km/min **

**(ii) m/min**

**(iii) km/h **

**(iv) m/s**

Ans. (iv) m/s

**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**

**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.**

Ans. Since the scale given is 1 cm = 100 cm. So, the blue car has moved only 1 cm in the picture. Therefore, the distance covered by the blue car is 100 m. Time taken by car to cover this distance is 10 seconds.

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

Ans. From the graph, we can see that slope of A is greater than the slope of B. So, the distance covered by A at in same time is greater with respect to B. It means A is moving faster than B.

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

Ans. Graph (iii) will show the movement of the truck with a speed which is not constant

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