Page 95 - Understanding NCERT Science 09

Activity _____________7.11 If you carefully note, on being released
the stone moves along a straight line
• Take a piece of thread and tie a small tangential to the circular path. This is
piece of stone at one of its ends. Move because once the stone is released, it
the stone to describe a circular path continues to move along the direction it has
with constant speed by holding the
thread at the other end, as shown in been moving at that instant. This shows that
Fig. 7.9. the direction of motion changed at every point
when the stone was moving along the circular
path.
When an athlete throws a hammer or a
discus in a sports meet, he/she holds the
hammer or the discus in his/her hand and
gives it a circular motion by rotating his/
her own body. Once released in the desired
Fig. 7.9: A stone describing a circular path with
direction, the hammer or discus moves in
a velocity of constant magnitude.
the direction in which it was moving at the
• Now, let the stone go by releasing the time it was released, just like the piece of
thread. stone in the activity described above. There
• Can you tell the direction in which are many more familiar examples of objects
the stone moves after it is released?
moving under uniform circular motion,
• By repeating the activity for a few
times and releasing the stone at such as the motion of the moon and the
different positions of the circular earth, a satellite in a circular orbit around
path, check whether the direction in the earth, a cyclist on a circular track at
which the stone moves remains the constant speed and so on.
same or not.

What

you have

learnt

• Motion is a change of position; it can be described in terms
of the distance moved or the displacement.
• The motion of an object could be uniform or non-uniform
depending on whether its velocity is constant or changing.

• The speed of an object is the distance covered per unit time,
and velocity is the displacement per unit time.
• The acceleration of an object is the change in velocity per
unit time.
• Uniform and non-uniform motions of objects can be shown
through graphs.
• The motion of an object moving at uniform acceleration can
be described with the help of the following equations, namely
v = u + at
s = ut + ½ at 2
2
2as = v – u 2
84 SCIENCE

Rationalised 2023-24