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5.1 Inertia
Why do we fasten our Mass and inertia
seat belts and ensure Physical bodies/objects have a tendency to remain in their state of motion.
children are secured If an object is not moving, it tends to stay still. If an object is moving, it
in a safety seat when tends to keep on moving, unless an external force is exerted on it.
textbooks nis edu kz
travelling in a car? This idea of objects not changing in their state of motion is described as
inertia. Of course, the inertia of different objects will be dependent on
how heavy an object is (its mass), as we learnt in Unit 4.
INV Investigate the effect of forces on objects of different masses.
Measure the mass of different objects and then use a newton meter
(spring scale) to measure the force needed to pull the object along a
surface. Can you identify any pattern in your results?
Inertia and the change of speed
The interaction of forces with objects can cause the speed of the
object to change, the direction of motion to change or deformation
of this object. If two objects interact with each other, the change in their
speed and direction will depend on their masses.
The heavier an object is, the less speed it gains (and the object is said to
have more inertia). The lighter an object is, the greater the speed it gains
(and the object is said to have less inertia). Bullets and shells have less
inertia than rifles and cannons.
If objects have the same speed after an interaction, it means their masses
are equal. If objects have different speeds after an interaction, it means their
masses are different. By comparing the speeds of two interacting objects
we can determine the masses of objects that cannot be measured.
OVER TO YOU!
1. In previous centuries,
armies used cannons to
fight in wars. Why does
the cannon shell move
forward but the cannon
move backward when it
is fired? Why is the shell’s
speed greater than the
cannon’s speed?
2. Why do we use seat belts?
Why are passengers in
a car projected forward
when the driver applies
the brakes?
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