Understanding Inertia

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Understanding Inertia:

1. The law of Motion is Newtons First law of Motion:

Newtons First law of Motion

If the total force acting on a body is zero, the body will stay at rest (if it is

stationary), or will continue to move at a steady speed in a straight line (if it

is already moving).

a) Unless a force acts, a body will continue with whatever motion it has.

b) If it is stationary (at rest), it will stay stationary.

c) If it is moving, it will continue moving at a steady speed in a straight

line.

d) This property of all bodies, their tendency to go on doing whatever they

are doing, is called inertia.

2. Inertia and Mass

a) The mass of a body is a measure of its inertia.

b) If a body has a small mass, it will have a small inertia; if its mass is

large, then so is its inertia.

c) An empty shopping trolley is easy to push across a floor; it is easy to

start it moving and to stop again. It hasnt got much inertia. But if the

trolley is full of groceries, it is much harder to start and stop. The inertia

is much larger. The trolley full of groceries has a large mass; the empty

trolley has a small mass.

d) Using an inertial balance, if a moving body has large inertia, it will resist

constant changes of motion, and its vibrations will be slower. If the

inertia is small, the vibrations will be faster. When the mass of the


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moving object is increased by adding a second identical trolley, or a

second metal cylinder, the period of vibration becomes longer.

e) In an inertial balance experiment, the time for 20 oscillations of the

balance is measured for loads of 1, 2 and 3 units of mass (metal

cylinders). The results are shown in Table 1.

Table 1

An object with unknown mass is then placed in the balance. The time for

20 oscillations with this mass is found to be 6.6 s. The first step is to draw

a graph of the results:

From the graph, a mass of 1.6 units would correspond to a time of 6.6. s

for 20 oscillations. The unknown mass is equivalent to 1.6 of the standard

cylindrical masses.

Conclusion:

Inertia of an object is the tendency of an

object to remain at rest or if moving to

continue its motion in a straight line.


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Notes:

1. Stationary objects resist attempts to set them in motion. This can be

seen when you pull a horizontal string fixed to a brick suspended from a

string.2. We are also aware that a moving object tends to maintain its speed and

direction of motion.

3. When a person paddling a bicycle stops paddling, the bicycle will

continue to move forward and does not stop immediately. To stop the

bicycle the person has to apply the brakes.

4. An object has the tendency to continue in its motion in a straight line

unless acted by an external force.

5. This behaviour of stationary objects to resist attempts to move them or

moving objects to continuing moving is called Inertia

6. It is easier to move and stop the empty tin and more difficult to move or

stop the heavier tin. This shows that the heavier tin has more inertia

The concept of inertia in our daily life.

1. Common trick of the coin and the bottle.

2. Dispensing tomato ketchup of a bottle

3. Fixing the head of a hammer into its holder.

4. A lorry carrying logs has strong chains to tie the logs so that when the

lorry brakes suddenly, the logs do not move forward.

5. Oil tankers have the tanks divided into small compartments to reduce

the effects of inertia of the content in it.

6. Safety belt and air bags are used in cars to protect drivers and front

passengers when accidents happen. They prevent the drivers and front

Newtons First Law of Motion

states that an object continues in its

state of rest or of uniform motion in

a straight line unless compelled by

some external force to act otherwise.

An object with

more mass has

more inertia.


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passenger from being thrown forward.

ACTIVITIES : 50 MINUTES

Activity to show inertia:

Activity 1:

1. Put a cardboard on top of a glass bottle.

Put a coin at the centre of the cardboard.

Pull the cardboard quickly.

a) What happen to the coin when the cardboardis pulled quickly?

b) Explain what happened.

Activity 2:

Cylindrical weight

2. Put a cylindrical weight on a trolley and hold the cylindrical weight inposition by placing it on a thin layer of plasticine.

Push the trolley and allow the trolley to knock a wooden block.

What happen to the cylindrical weight?

a) .

.

Trolley


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b) ..

.

Activity to show the relationship between the mass of a body and its

inertia.

Activity1:

3. Suspend 2 tins

Tin A sand Tin B

Fill on of the tins with sand.

Pull the tin with a string and observe which one needs more force to move

it.

Allow both tins to oscillate.

Try stopping the tins.

Which tin needs more force to stop?

(a)

(b)Activity to show everyday situations involving inertia.

4. Activity 1

(a) What happens to the passengers in a stationary bus if the busaccelerates quickly?


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(b) What happen to the passengers in a moving bus if the bus stopsquickly?

5.Activity 2:

(a) Suggest two ways to reduce the negative effects of inertia on the driver

of a car during an accident.

(b) For an object moving with a uniform velocity, the object will continue to

move with that velocity unless acted upon by an ..

(c) A ball rolling on a horizontal surface will eventually stop. Does this

contradict Newtons first Law of motion? Give the reason for your answer.

(d) What happens to a passenger standing in a moving bus when the bus

suddenly stops? Why?

(e)

Smooth surface

A ball rolls down an inclined plane to a smooth horizontal surface. What

will happen to the ball on the horizontal surface?


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(f) An astronaut throws a stone away from him as shown in the figure

below. What will happen to the stone?

(g) What is the purpose of headrests placed in cars?

Assessment :( 40 minutes)(20 marks)

Objective(In 10 minutes time)(5 marks)

1. Inertia of an object depends on itsA massB weightC mass and weight

2. In the Galileos thought experiment, a ball bearing is allowed to roll on acurtain rail of different curvatures as shown in the Figure 1.

Figure 1What is the physics concept that is demonstrated in this experiment?A Gravitational force acting on the ball bearingB Inertia of the ball bearing


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C Action and reaction pair on the ball bearing and on the railD The weight of the ball bearing

3. A person can pull a table cloth with the dishes above remaining on thetable. Which law of motion applies to this popular party tricks?A Newtons third law of motion

B Newtons second law of motionC Newtons first Law of motionD Newtons gravitational law

4. A bus is moving at a uniform speed. A passenger jumps up vertically.Where will he land on the floor?A At a position beside his original positionB At a position in front of his original positionC At a position behind his original positionD At the original position

5 If you are sitting in a bus, what will happen to you when the bus

suddenly brakes?A Suddenly jerksB Bends backwardsC Remains stationaryD Jumps upwardsE Surges forward

Structure (15 minutes)(9 marks)

1. Figure 2(a) Figure 2(b)

Figure 2(a) A car is bumped by another car at the backFigure 2(b) A car stops suddenly

Figure 2 shows two situations of a driver and the decorative accessory thatare hung to the rear-view mirror. Figure 2(a) shows when his car isbumped at the back by another car and Figure 2(b) shows when his carstops suddenly after collision. The car in the two situations experiencesgreat impulsive force. Observe the two situations in Figure 2 carefully.


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Explain what happens to the driver and the decorative accessory that arehung to the rear view mirror when the car is bumped at the back byanother car and when the car stops suddenly after collision. Give aconclusion of the situations that are related to a physics concept.(5 marks)

2. Based on the principle of inertia,

(a) Explain why passengers in a moving car lurch forward when the carsuddenly stops. (2 marks)

(b) State one feature that can lessen the injuries which the driver wouldsustain if an accident occurs (2 marks)

Essay (15 minutes) (6 marks)1.

String X

A B

String Y

A and B are two spheres with weights of 1 kg and 2 kg respectively.Sphere A is suspended from the ceiling by a string X which is similar tostring Y. Both strings are able to withstand a tensile force of 20 N. SphereB is lifted to the same height as sphere A as shown above and sphere B isthen released. Explain what will happen to the system. Provide reasons foryour explanation.

2. Upon arriving at a station, passengers in a train are reminded not tojump off the train while it is still moving. Based on the principle of inertia,explain why this reminder is important.

MARKING SCHEME

1. (a) The coin will fall into the glass.(b) This shows that the inertia of the coin resists anything that tries to

change its state of rest.2. (a) The cylindrical block will fall backward

(b) The cylindrical block will slide forward. The inertia of the cylindricalblock tends to keep its state of motion.

3. (a) Mass B is bigger therefore it is difficult to move because inertia isbigger too


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4. (a) The passengers are thrown backwards(b) The passengers are thrown forwards. This is because the

passengers feet are brought to rest, but the inertia of their bodytends to continue its forward motion.

5. (a) fasten seat belt; install air bag inside the steering(b) external force

(c) No. This is because there is frictional force acting on the ball.(d) The passenger will be thrown forward. Inertia keeps the passenger

in a state of motion.(e) The ball continues to move with uniform velocity indefinitely.(f) The stone moves with uniform speed in a straight line indefinitely(g) To prevent head injuries during rear-end collisions.

Objectives1.A2.B3.C

4 D5 E

Structure1. When the car is bumped at the back with another car, the driver and the

decorative accessory are moved backwards.This shows that the driver and the decorative accessory are reluctant tomove forward.

When the car stops suddenly, the driver and the decorative accessoryare moved forward showing that they are reluctant to stop.The two situations explain the concept of inertia of a body-the tendencyof a body to remain in its state of rest or if moving to continue its state ofmotion

2 (a) When a car is suddenly stopped due to collision, the driver andpassengers of the car will be jerked forward due to inertia. This causes

injury or death to them.(b) seat belts, air bags etc

ESSAY1.The sphere B falls at a high speed when it is released. A sudden forceacts on string Y. The inertia of stationary sphere A prevents the transfer ofthe tension in string Y to string X. As a result, string Y snaps.2.The passengers in a moving train move at the same velocity as the train.If a passenger jumps out of a moving train, the inertia of his body


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maintains his motion in the direction of the train. As a result, the passengerfalls when he lands on the ground.

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Understanding Inertia