Measurement of Length and Motion: questions with answers

A standard unit is needed so measurements of the same length made by different people do not differ. Body-part units such as handspan, foot, fist, or fingers can differ from person to person. The system of units now used is called the International System of Units or SI units.

When zero is not clear, the scale can still be used from another full mark. The length is found by subtracting the starting reading from the reading at the other end. Here, 7.4 cm - 2 cm = 5.4 cm.

A scale can still be used when zero is not clear if other full marks can be read. Start from a clear full mark and subtract that reading from the reading at the other end. For these readings, 7 cm - 3 cm = 4 cm.

A measuring scale needs a suitable material so that length marks remain fixed. A metre scale cannot directly measure the girth of a tree. For that kind of measurement, a flexible measuring tape, such as a tailor's tape, is more suitable.

The measured length of the outer boundary tells how far the bicycle moves in one complete turn of the wheel. To find the total distance, multiply this length by the number of turns. Here, 9 multiplied by 5 gives 45 length units.

One complete turn covers 4 length units. So count along the travelled path in steps of 4 length units. Each step is one complete turn. You reach 28 length units after 7 steps, so the wheel made 7 complete turns.

The scale measures the straight distance between the ends, not the length along the curve. Place a thread along the curve, then straighten it and measure it with a scale.

The caption rejects two observations made from different houses. It wrongly treats the observations as if they used one reference point. Both observations can be correct.

Each observation uses a different house as its reference point. The figure supports both observations.

Both readings give positions measured from Delhi. Since 40 km is less than 70 km, the later position is nearer to Delhi than the first position.

An object is said to be in motion if its position changes with respect to the reference point with time. If its position is not changing with respect to the reference point with time, it is said to be at rest. Here the object's position changes with respect to the reference point from time 1 to time 2, so it is motion. How the object looks does not decide it.

An object is in motion if its position changes with respect to a reference point with time. Here the reference point is the tree. The girl stays in the same seat, but the bus carries her along. So her distance from the tree at time 2 is not the same as at time 1. Her position with respect to the tree changes with time, so she is in motion.

The portion marked AB is straight. Motion along a straight line is linear motion. Motion along a circular path is circular motion. Each portion of a track can have its own type of motion. So the motion on portion AB is linear motion.

On the portion CD the ball moves to and fro about a fixed position. Motion to and fro about some fixed position is called oscillatory motion. So the motion of the ball on CD is oscillatory motion.

A straight part has linear motion. A circular path part has circular motion. A part that moves to and fro about a fixed position has oscillatory motion. Only the label that does not match its own part is changed. So the change needed is BC: circular motion.