Lesson 9, Topic 6
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# Inertia & Inertia Mass

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### Topic Content:

• Objects in a Lift
• When the lift is ascending with acceleration, a
• When the lift is descending, a is less than g
• When the lift is stationary or moving up with uniform acceleration or constant velocity
• When the lift is falling under gravity or falling freely, g = a, T = 0

Inertia is the reluctance of a body to change its state of rest or uniform motion in a straight line. The more massive a body is, the greater the force required to change its state of rest or of uniform motion and give acceleration to the body, hence the greater the inertia. Inertia is determined by the quantity of matter contained in a body.

Since F = ma.

### Objects in a Lift:

When an object is in a lift or elevator, there are two forces acting on the object.

The forces are:
i. The true weight of the object which acts vertically downward (W = mg)
ii. The reaction force T of the floor of the lift on the man, which acts upward.

We are going to take a close look at different cases when an object is in a lift.

### When the lift is ascending with acceleration, a:

In this case, when the lift is ascending with acceleration, there is an unbalanced force acting on the object in the lift. The upward reaction force of the floor, T, is greater than the downward force of the object’s weight on the floor of the lift, W. (T > W)

From Newtonâ€™s second law of motion,
Force F = ma

In this case the effective force, F = T – W

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