Motion and Measurement of Distances

1. Two examples of modes of transport used on land are buses and wheel carts.
   Two examples of modes of transport used on water are ships and boats.
   Two examples of modes of transport used in air are aeroplanes and helicopters.
2. (i) One metre is __100__ cm.
   (ii) Five kilometres is __5000__ m.
   1 km = 1000 m
   5 km = 1000 × 5 = 5000 m
   Hence, the answer is 5000 m.
   (iii) Motion of a child on a swing is __periodic__.
   Periodic motion:
   The motion of a swing repeats itself at a certain time interval. Therefore, it has periodic motion. Hence, a child on a swing is said to have periodic motion.
   (iv) Motion of the needle of a sewing machine is __periodic__.
   Periodic motion:
   The needle of a sewing machine moves up and down repeatedly with a certain time interval. Hence, it is an example of periodic motion.
   (v) Motion of the wheel of a bicycle is __circular__.
   Circular motion:
   The central part of the wheel of a bicycle is attached to a fixed point. The wheel rotates about this fixed point as the bicycle moves. Hence, the wheel has circular motion.
3. The size of the foot varies from person to person. If footsteps of two persons are used to measure the length respectively, then the two distances may not be equal. Thus, a footstep is not a constant quantity. Hence, it cannot be used as a standard unit of length.
4. 1 cm = 10 mm
   1 m = 100 cm = 1000 mm
   Again, 1 km = 1000 m = 100000 cm = 10000000 mm.
   Hence, 1 mm is smaller than 1 cm, 1 cm is smaller than 1 m, and 1 m is smaller than 1 km, i.e.,
   
5. 
   Height of the person = 1.65 m
   1 m = 100 cm
   1.65 m = 100 × 1.65 = 165 cm
   Hence, the height of the person is 165 cm.
   Again, 1 m = 100 cm = 1000 mm
   Therefore, 1.65 m = 1.65 × 10 = 1650 mm
   Hence, the height of the person is 1650 mm.
    
6. 
   The distance between Radha’s home and her school is 3250 m.
   1 km = 1000 m
   i.e., 1000 m = 1 km
   3250 m = 3.25 km
7. 
   The reading of the scale at one end is 3 cm and at the other end is 33. 1 cm.
   Therefore, the length of the knitting needle is given by subtracting both the readings, i.e., (33.1− 3.0) cm = 30.1 cm.
    
8. 
   Similarities between the motion of a bicycle and a ceiling fan:
   (i) The blades of a fan and the wheels of a bicycle are fixed at a point.
   (ii) Both have circular motion about their respective fixed points.
   Differences between the motion of a bicycle and a ceiling fan:
   (i) A bicycle has linear motion, whereas the blades of a ceiling fan do not have linear motion.
   (ii) The motion of the blades of a fan is periodic, whereas the motion of a bicycle is rectilinear motion.
9. An elastic measuring tape is stretchable. It cannot be used to measure distances because the length of the tape may change on stretching. As a result, the measured length would not be correct.
   If you measure the length of an object twice using an elastic tape, then you may get different values of the same length each time. This is because elastic tapes are stretchable.
10. 
    Examples of periodic motion:
    (i) Motion of a pendulum
    The bob of a pendulum repeats itself at a certain time period. This motion is called periodic motion.
    (ii) Motion of a boy sitting on a swing
    The motion of a swing repeats itself at a certain time period. Hence, a boy sitting on a swing has periodic motion.