Motion And Measurement Of Distances

The distance between two cities like Delhi and Mumbai is large. If it is measured in smaller units like metre or centimetre, then the result will be a large value. So, a bigger unit like kilometre is needed to measure the distance between two cities.

The ‘time interval’ means that it is the difference between two instants of time. The time of the first bell in the school defines a particular instant of time. It is not a difference between two instants of time. Thus, it is not a ‘time interval’.

All other options like a day, a second or a school period describe a period of time. This period of time is called time interval.

The length of the measuring scale is 15 cm or 0.15 m.

Formula used:

1 m = 100 cm

The given distances in cm are:

(a) 0.1 m = 0.1×100 = 10 cm

(b) 0.15 m = 0.15×100 = 15 cm

(c ) 0.2 m = 0.2×100 = 20 cm

(d) 0.05 m = 0.05×100 = 5 cm

Tip: To measure a distance using any scale only once, the distance should be less than or equal to the length of the scale.

The options (a),(b) and (d) are less than or equal to 15 cm. So, they can be measured using the given scale.

But (c) is 0.2 m(20 cm) which is bigger than 15 cm. So, it cannot be measured using the scale only once.

The folded ribbon lies between 2 cm and 28 cm. So its length is the distance between 28 cm and 2 cm. It can be about (28-2) = 26 cm. The length of folded ribbon is nearly 26 cm.

The total length of ribbon is about 5×26 cm = 130 cm = 1.30 m. (100 cm = 1 m)

So, the length of the ribbon is 1.25 m-1.35 m.

The time taken by Paheli to travel same distance is constant from A to B and B to C. So, she is moving

She returns to the same point B and A after fixed interval of time. So, her motion is periodic.

Tip: A periodic motion repeats after a fixed interval of time.

Paheli is travelling in a straight line along ABC. So, her motion is rectilinear too.

Tip: A body in rectilinear motion travels along a straight path.

Tip: The sum of two sides of a triangle is always greater than the third side.

This means that the distance along path 1 is more than path 2.

Bholu travels along two sides of the triangle (path 1). Golu travels along only one side of the triangle (path 2). So, Bholu travels longer distance than Golu.

Both reach at the same time. To cover a longer distance in same time as Golu, Bholu needs faster speed for running. So, Bholu travels at higher speed than Golu.

Using:

length of a stick = Final reading of scale- Initial reading of scale

For the correct placement of the block, both the ends of the block should be on the marked side of the scale. The side of the block to be measured should be parallel to the scale. Thus (c) is the correct option which fulfills both the conditions.

To measure a distance of 10 cm, the length of undamaged scale should be greater than or equal to 10 cm. All the scales given above are not damaged for less than 12 cm. So all can be used.

(a) The motion of a swing is an example of periodic motion.

The to and fro motion of the swing makes it periodic.

(b) 1 m = 100 cm

(i) Motion of an object or a part of it around a fixed point is known as circular. motion.

(ii) A body repeating its motion after certain interval of time is in periodic motion.

(iii) In rectilinear motion, object moves along. a straight line

(iv) SI unit of length is m.

I. Rectilinear: A car moving in straight line

II. Circular: The blades of a fan

III. Periodic: The motion of a swing

IV. Circular and periodic: Tip of minute hand of clock

The grill has both straight and curved rods.

The straight rods are measured with a measuring tape while the curved rods can be measured using a thread. The thread is kept along the curved rods and marked to measure it with tape.

(iv)

Saleem will get the most accurate measurement because he has used a standard instrument to measure the table. The length of the table is 2 m which is more close to the measuring tape readings. So, he doesn’t have to take too many readings like he had to when using 15 cm scale or half a metre thread.

The standard instrument will male it better choice than hand span.

(a) – (ii)

(b) – (i)

(c) – (iii)

(a) The wheel of a sewing machine rotates to do rotational motion. It has fixed axis for rotational motion.

(b) The tip of minute hand rotates around the clock fixed at one end performing circular motion.

(c) A moving swing moves to and fro to do periodic motion.

When there is relative motion between two bodies, each body seems to be moving at a different speed when seen from other body.

For the above condition, there is relative motion between the trees and the train. So when the train moves, the trees near the track too seem to move in opposite direction.

There is no relative motion between the train and the co-passengers. So, they appear to be stationary.

All the measurements that we do have errors. We cannot remove errors completely but reduce it.

There can be three main causes of errors in measurement:

● Defect in instrument can lead to error in measurement. Different instruments can have little differences. So, they can cause errors.

● Incorrect method of measurement: They may not have followed the correct methods of measurement like keeping eyes in straight line to the measuring point.

● Use of different scales: They might have used different scales. So they could have different readings.