## Topic outline

• ### Motion and Measurement of distances

##### Q30. What is the need of measurement?

Ans. It is difficult to make correct judgment about an object without making actual measurement. It is not always easy to find out the measurement of different objects just by looking at them. Guessing may lead to incorrect measurement. Therefore, to make measurement more reliable, accurate, actual measurement is taken.

##### Q31. Why a tailor uses a tape whereas a cloth merchant uses a metre rod for measurement?

Ans. A tailor uses a tape, whereas a cloth merchant uses a metre rod because for measuring the length of an object, we must choose a suitable device. Tailor cannot measure the size of your chest using a metre scale, for instance. He needs a flexible device for this. That’s why Measuring tape is more suitable for this kind of measurement.

##### Q32. Write two examples where objects undergo combinations of different types of motion.

Ans.  Examples:

i.     The ball is rolling on the ground – rotating as well as moving forward on the ground. Thus, the ball undergoes a rectilinear motion as well as rotational motion.

ii.    In sewing machine, its wheel moves in a circular motion and its needle moves up and down continuously, as long as the wheel rotates in a periodic motion.

##### Q33. Write the similarity and dissimilarity between motion of a bicycle and a ceiling fan that has been switched on.

Ans.

 Similarity Bicycle Ceiling fan 1. Wheels have circular motion about their respective fixed point. 1. Blades have circular motion about their respective fixed point. Dissimilarity 1. Bicycle has linear/rectilinear motion. 1. Ceiling fan do not have linear /rectilinear motion. It is fixed at one place.

##### Q34. How can we measure the length of a curved line?

Ans. We can use a thread to measure the length of a curved line.

Steps to measure the length of a curved line

i.     Put a knot on the thread near one of its ends and place this knot on the one of its ends.

ii.    Now, holding the thread at one end with one hand, place the thread along the curved line, keeping it taut using your fingers and thumb.

iii.  Now make a mark on the thread where it reaches the other end.

iv.  Finally, place the thread along a metre scale and measure the length between the knot and the marked point. This gives the length of the curved line.

##### Q35. Diya is using a scale which is broken at one end. She is not able to see the zero mark. What precaution should she take while measuring the length of a book using this scale?

Ans. When one of the ends of the scale is broken and zero mark is not clearly visible. In such case, she can use any other full mark of the scale and subtract the reading of this mark from the reading at the other end.

For example, the reading at one end is 1.0 cm and at the other end it is 15 cm. Therefore, the length of the object is (15-1.0) cm = 14 cm.

##### Q36. Write a note on ancient method of measurement.

Ans. In ancient times, the length of a foot, the width of a finger, hand span (the length from tip of the thumb to the tip of the little finger), cubit (the length from the elbow to the finger tips), an angul (finger) or a mutthi (fist), length of fore arm and the distance of a step were commonly used as different units of measurements.

##### Q37. What is the need of common system of measurement?

Ans. Everyone's body parts could be of slightly different sizes. So, non-standard units such as cubit, hand span, foot etc. causes confusion in measurement. That’s why for the sake of uniformity, scientists all over the world have accepted a set of standard units of measurement. The system of units now used is known as the International System of Units (SI units).