Page 107 -

Physics
Types of equilibrium

When an object is not moving we say it is in static equilibrium. This equilibrium depends
on the relationship between the point of support on a surface, the point of support on the
object and the centre of mass (i.e. the pivot).

The lower the centre of gravity, the more stable the equilibrium. For example, low-slung
cars have the lowest risk of rollover. Or think of a tightrope walker who uses a long rod
textbooks nis edu kz
that bends to the level of the rope. This means that the tightrope walker’s centre of gravity
moves lower, making it easier to walk safely because the system becomes more stable. But
this only happens when the centre of mass is below the pivot, or the balancing point. Any
deviation from the pivot will cause an object to become unstable and rotate.

ACT Make a sphere of plasticine 4 cm in diameter. Stick a fork
in the sphere. Put a second fork in the sphere at an angle of
45 degrees to the first one. Next, insert a toothpick into the
sphere, between the two forks. Rest the toothpick on the edge of
a glass and gently move it towards the centre of the glass until it
finds equilibrium. Explain the result.

OVER TO YOU!

1. Explain how to determine the
centre of mass of a flat (2D)
object. Can you do the same
for a 3D object?

2. Prove that the centre of
gravity of regular figures
coincides with their
geometrical centre.

3. Why is it impossible to get
up from a chair without
bending forwards?

4. How do objects maintain
static equilibrium? What
happens when they don’t?

107