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2.3 Inertia

The meaning of inertia

Inertia  is the property  of an object which resists a change  in its motion.

If it is at rest  it tends to remain at rest , if it is moving it tends to continue moving.


Inertia is the reluctance of an object to move once it is  at rest or the reluctance of an object to stop once it is in uniform velocity.


Explanation of  inertia by Newton’s First Law of Motion


Newton’s First Law of Motion states that “an object will remain at rest or continue with  a constant speed in a straight line unless acted on by an unbalanced force  or if the external force acting on the object is zero”

Relationship between mass and inertia

The mass of a body is a measure of its inertia. If a body has a small mass, it will have a small inertia; if its mass is large, then so is its inersia.


Some  examples of situations involving inertia


Example 1


Observation: The coin drops vertically into the glass .

Explaination : The inertia of the coin causes it remain at rest .

The coin drops vertically into the glass due its weight.


Example 2


Obesrvation : The other blocks fall vertically  onto the table.

Explaination : The inertia of the wooden blocks  causes its   

                        remain at  rest.

                         The weight of other blocks makes them fall

       Vertically onto the table.


Example 3


Observation :     If some ketchup is stuck in a bottle, it can be dislodged by turning the bottle upside down , thrusting it downwards and stopping suddenly.

Explaination :    The inertia of the ketchup causes it maintain its velocity  while the bottle is stopped.


Example 4


Observation:         When a driver inside a car applies brake

Suddenly , the driver and the passengers are

surged forwards.

Similarly ,when the car accelerates , the

occupants will move backwards.


Explaination:     The inertia of the driver and passengers cause they maintain their original motion (original velocity) or maintain at rest.



Example 5


Observation : The pail filled with sand feels more reluctant to

      move and more difficult to stop moving than the  

      empty one.

Explaination:  The pail filled sand has a greater inertia or mass

                                      than the empty one.


Example 6


Observation :  A tanker has to stop its engine 5 km from port.

Explaination :  The tanker has so much inertia , so that it can

                        slow down in time to avoid any acciedent.

Example 7


Observation :     An aeroplane need a long runway for landing and take-off

Explaination :    The aeroplane has a greater mass and also  has so much inertia.

Reducing  the negative side effects of inertia.

(i)            The seat belts are designed to protect the passengers .

If the car is involved in a collision , it will suddenly be brought  to a stop. But because of its inertia, their body will continue traveling at a certain velocity until it hits an obstruction , usually the car’s windscreen.Without a seat-belt , the passengers can be seriously injury.

The belts exerts a force on the passenger’s bodies bringing it to stop without hitting the windscreen or the steering wheel.

(ii)     Head restraints are designed to reduce neck  injury. There are partuculary effective in rear-impact acciedents. As the car is shunted forwards , the back of your seat pushes your body towards. If you do not have a head restraint, the inertia of your head means that it stays behind , while your body moves forward. This can cause ‘whiplash’ injuries.

(iii)    Timbers carried by a lorry  normally are tied up together by a strong iron chain. When the lorry starts to move suddenly , the timbers are more difficult  to fall off due to their inertia because their combined mass has increased.

The positive effects of inertia

(i)      The motion of a spacecraft in deep space is in a straight line with  a constant speed ; since there is no air in space

(the external force acting on the object is zero”) to slow its motion. For this reason, there is no need to make a space probe aerodynamic in shape .

(ii)     A sumo wrestler who has a greater mass also has a greater inertia. He is harder to toppled and normally he is likely to win a championship tournament.


Inertial balance




An inertial balance is used to compare masses or to measure the mass of  an object.

When the mass of the metal cyclinder is increased by adding a second metal cyclinder , the period of vibration become longer.

The square of the  period of vibration ,T2  of a inertial balance is directly proportional to the mass,m of the metal cyclinder.

                                                T2  a  m



Example 1


          In an inertial  balance experiment , the period of oscillations for loads  0.5 kg is  4.0 s. What is the mass of the load if the period of the oscillations is 9.0 s





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