Class 7 Science
Chapter 13 – Motion and Time
Very Important Questions Answers Set 3
Q.1. Which of these can only measure time interval and not the time of day?
(a) sand clock
(b) sundial
(c) pendulum clock
(d) quartz clock
Ans: (a) sand clock
It only measures the fixed time interval and not the time of day.
Q.2. Which of these is true?
(a) distance = speed / time
(b) distance = speed × time
(c) distance = time / speed
(d) none of these
Ans: (b) distance = speed × time
The distance covered by a moving object in unit time is known as its speed.
So, distance = speed × time
Q.3. The distance-time graph of a car is plotted with on the X-axis and distance on the Y-axis. The graph is a straight line parallel to the X-axis. Which of the following is true about the car?
(a) It is in uniform motion
(b) It is in non-uniform motion
(c) It is not moving
(d) It is moving very fast.
Ans: (c) It is not moving
In a distance–time graph, if the graph is a straight line parallel to the time axis, then the object, here the car, is at rest (i.e., its distance is constant with time).
Q.4. A car with non-uniform motion travels 100 km in 4 hours. What can be said about the speed of the car?
(a) Its speed is constant at 25 km/h during the journey.
(b) Its average speed is 25 km/h during the journey.
(c) Its speed is 25 km/h at the beginning and end of the journey and varies in between.
(d) Its speed is 25 km/h for most of the journey.
Ans: (b) Its average speed is 25 km/h during the journey.
Q.5. Which of these is not a standard unit?
(a) second
(b) metre
(c) hand span
(d) gram
Ans: (c) hand span
This unit is not reliable because it varies from person to person. So, it cannot be considered as a standard unit.
Q.6. Which of the following instruments measures time most accurately?
(a) sundial
(b) quartz watch
(c) pendulum clock
(d) watch with balance wheel
Ans: (b) quartz watch
These watches have the crystals of a substance called quartz.
These crystals can vibrate very fast and at a very precise rate. These vibrations are used to measure time accurately.
Q.7. Which of these is closest to uniform motion?
(a) car travelling on a busy straight road
(b) car travelling on a busy curved road
(c) car travelling on a straight empty road
(d) car travelling on a curved empty road
Ans: (c) car travelling on a straight empty road
A body is said to be in uniform motion if it travels in a straight line and covers equal distances in equal intervals of time.
Q.8. The SI system is used all over the world in everyday measurements. True or false?
Ans: True.
The SI system or international system of units is the modern form of the metric system, and it is used all over the world in everyday measurements.
Q.9. Time could not be accurately measured before the principle of the pendulum was discovered. True or false?
Ans: True.
Clocks that measure time accurately could be made only after the discovery of the simple pendulum.
Q.10. The time period of a simple pendulum remains constant even if its length is changed. True or false?
Ans: False.
The time period of a simple pendulum changes if its length is changed because the time period is dependent on the length of a simple pendulum.
Q.11. The _________ in the vehicle gives the distance travelled in kilometres.
Ans: The odometer in the vehicle gives the distance travelled in kilometres.
Q.12. In the SI system, standard units for length, mass, time and temperature are fixed. What are these units?
Ans: The SI unit for mass, length, time and temperature are as follows:
(i) Mass = Kilogram (kg)
(ii) Length = meter (m)
(iii) Time = second (s)
(iv) Temperature = Kelvin (K)
Q.13. Measuring time is more difficult than measuring length. Do you agree? Give reasons.
Ans: No, measuring length is more difficult than measuring time. Because to measure time, we just need the time interval. i.e., the starting value and the end value of the time. But to measure the length, we need the actual path and the direction in which the length has to be measured. That's why it is more difficult to measure.
Q.14. The time period of a pendulum varies with its length. If the length is increased how does the time period change?
Ans: The time period of a pendulum is directly proportional to the square root of the length of the pendulum. So, if the length increases, the time period of the pendulum increases accordingly.
Q.15. Speed of a moving object depends on two factors–distance moved and time taken. What is the relation between the three quantities–speed, distance moved and time taken?
Ans: Speed of a moving object is given by the rate of change of distance travelled by the object. i.e.,
Q.16. If a body travels in a straight line and covers equal distances in equal intervals of time, what kind of motion is it said to be in?
Ans: If a body travels in a straight line and travels equal distances in equal intervals of time then the body is said to be in a uniform motion.
Q.17. For uniform motion, what kind of graph will you get if you plot distance vs time?
Ans: In a uniform motion, a body travels equal distances in equal intervals of time. Hence, if we plot a graph between distance and time for uniform motion, the graph would be a straight line with a constant slope.
Q.18. What are the quantities that can be measured called?
Ans: The quantities which can be measured are called physical quantities.
For example, the length of an object can be determined by comparing it to an object of known length, such as a ruler.
Q.19. An object moves to-and-fro about its position of rest. What type of motion is this?
Ans: When an object moves to and fro about its position of rest, the motion is known as the periodic motion.
Q.20. In a mechanical wristwatch, which part works in the same way as a pendulum in a clock?
Ans: In a mechanical wristwatch, the balance wheel works in the same way as a pendulum in a clock.
Q.21. What kind of watch will you use to accurately measure short intervals of time?
Ans: We will use stopwatch for measuring short intervals of time accurately. This watch is designed to measure the amount of time elapsed from a particular time when activated to until it is deactivated.
Q.22. What is the term used for 'distance covered by a body in unit time'?
Ans: The distance covered by a moving body in unit time is known as 'speed'.
speed = distance/time
Q.23. If the direction of motion and the speed of a body do not change with time, what kind of motion does it have?
Ans: If the direction of motion and the speed of a body do not change with time, the body is said to be in uniform motion.
Q.24. Why are standard units used in measurements?
Ans: Standard units are used in measurements because these are reliable and can be uniformly used by everyone. Other units like the footstep and hand span are unreliable because they vary from person to person.
Q.25. The discovery of which principle made the accurate measurement of time possible? What is the principle?
Ans: The discovery of the simple pendulum made the accurate measurement of time possible. According to the principle of simple pendulum, if a weight hung from a string is made to swing, it always completes one to and fro motion in exactly the same time.
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