2.3 mass and inertia

7/28/2019 2.3 Mass and Inertia

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Massand

Inertia


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Mass

Mass is defined as amount of

matter in a body.

SI unit = kilogram (kg)

It is a scalar quantity.


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Inertia

Inertia is the tendency of an object to

remain in its state of rest or of uniform

motion in a straight line.

Law of inertia is also known as Newton's

First Law of Motion


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Newton's First Law of motion

state that

In the absence of external forces, anobject at rest will remains at rest and an

object in motioncontinues in motion with a

constant velocity (that is, with a constantspeed in a straight line).


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Object at rest will remains at rest. Inertia of

an object will resist any tendency to

change it state of rest.

For an object in motion, inertia of that

object will remains the object in its state of

motion with uniform velocity.


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Situations involving INERTIA

1) Jerking a cardWhen the cardboard is jerkedquickly, the coin will fall into the

glass.

Explanation:The inertia of the coin resists the

change of its initial state, which is

stationary.

As a result, the coin does not

move with the cardboard and falls

into the glass because of gravity.


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2) Pulling a book

When the book is pulled

out, the books on top will

fall downwards.

Explanation:

Inertia tries to oppose the

change to the stationary

situation, that is, when thebook is pulled out, the

books on top do not follow.


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3) Pulling a Thread 1. Pull slowly - Thread A willbreak.

Explanation:

Tension of thread A is higher

than string B.

Tension at A = Weight of the

load + Pulling Force

2.Yank quickly - Thread B will

break/ snap.

Explanation:

The inertia of the load prevents

the force from being transmitted

to thread A, hence causing

thread B to break/snap.


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Larger Mass -> Greater Inertia

Bucket filled with sand is

more difficult to be moved.It's also more difficult to be

stopped from swinging.

Explanation:Object with more mass

offers a greater resistance to

change from its state of

motion.Object with larger mass has

larger inertia to resist the

attempt to change the state

of motion.


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Another example is a high speed lorry will

takes longer time to stop compared to a

car with same speed because the inertiaof the lorry is much larger than the car.

Larger Mass => The Larger inertia

Smaller mass => the smaller the inertia


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Relationship between Mass &

Inertia

A shopper in a supermarket observes that it is

always easier to start moving an empty trolley

than a full trolley.

Similarly, it is easier to stop an empty trolley

than the full one if both are moving at the

same speed.

Thus, the resistance to change its velocity i.e.

the inertia, is related to the mass of the

object.


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Effects of inertia

THE POSITIVE EFFECT OF INERTIA

1. The head of hammer can be fitted

tightly onto the wooden handle by

hitting the bottom of the handleagainst a hard surface. The head

of hammer has big mass and will

remain in its state of motion, thus

fitting it tighter on the handle.


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POUR OUT CHILLI SAUCE

2. - Chilli sauce in the bottle can be easily

poured out if the bottle is moved down fastwith a sudden stop.

- The sauce inside the bottle moves

together with the bottle.- When the bottle stops suddenly, the

sauce continue in its state of motion due to

the effect of its inertia.


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Ways of reducing the negative

effect of inertia

Situation :

If a car crashes, inertia of the passengers causes

them to continue it motion. This is a

dangerous situation. Upon impact, the

passengers will crash into part of the car

immediately in front of them and suffer

injuries.


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solution

A) Safety belt

Safety belt secure the driver and passengers

to their seats. When the car stops suddenly,

the seat belt provides the external force that

prevents the driver or passengers from being

thrown forward.


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B) Air bag system

Air bags mounted on the dashboard or steering

wheel expand automatically when a collision

occurs. This prevents the driver or passengers

from crashing into the dashboard.

2.3 mass and inertia

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