Page 107 - Understanding NCERT Science 09

Fig. 8.12: As the sailor jumps in forward direction,
the boat moves backwards.

Activity ______________ 8.4 Fig. 8.13

• Request two children to stand on two Now, place two children on one cart and
separate carts as shown in Fig. 8.13. one on another cart. The second law of motion
• Give them a bag full of sand or some can be seen, as this arrangement would show
other heavy object. Ask them to play a different accelerations for the same force.
game of catch with the bag.
The cart shown in this activity can be
• Does each of them experience an constructed by using a 12 mm or 18 mm thick
instantaneous force as a result of plywood board of about 50 cm × 100 cm with
throwing the sand bag?
• You can paint a white line on two pairs of hard ball-bearing wheels (skate
cartwheels to observe the motion of the wheels are good to use). Skateboards are not
two carts when the children throw the as effective because it is difficult to maintain
bag towards each other. straight-line motion.

What

you have
learnt

• First law of motion: An object continues to be in a state of
rest or of uniform motion along a straight line unless acted
upon by an unbalanced force.
• The natural tendency of objects to resist a change in their
state of rest or of uniform motion is called inertia.
• The mass of an object is a measure of its inertia. Its SI unit
is kilogram (kg).
• Force of friction always opposes motion of objects.
• Second law of motion: The rate of change of momentum of
an object is proportional to the applied unbalanced force in
the direction of the force.

96 SCIENCE

Rationalised 2023-24