Notes on Inertia and Momentum | Grade 9 > Science > Force | KULLABS.COM

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The property of a body due to which it remains or tends to remain in the state of rest or uniform motion in a straight line unless an unbalanced forces act on it is called inertia. Only unbalanced force can change the state of rest to the state of motion and vice versa. Inertia may show itself in any of the following three forms: inertia of rest, the inertia of motion and inertia of direction.

#### Inertia of rest

It is the property of the body by virtue of which it remains or tends to remain in the state of rest unless an unbalanced force acts on it. Examples:

• When the bus suddenly starts people in a bus jerks backwards in the bus as the lower parts of our body also sets into the motion with the bus but the upper parts tend to remain at rest due to the inertia of rest.
• When a blanket is given a sudden jerk, the dust particle falls off as blanket sets in motion but dust particles tend to remain at rest due to the inertia of rest.

#### Inertia of Motion

It is the property of a body by virtue of which it remains are tends to remain in the state of uniform motion in a straight line unless unbalanced forces act on it. Examples:

• When the bus suddenly stops people in a bus jerks forwards in the bus as the lower parts of our body comes to rest with the bus but the upper parts tend to remain in motion due to the inertia of motion.
• A coin thrown vertically upwards in moving bus comes back in the hands of a thrower because the coin continues to move with the bus. This is due to the inertia of motion.

#### Inertia of direction

It is the property of a body by virtue of which it maintains or tends to maintain its direction of motion unless unbalanced forces act on it. Examples:

• When a running bus suddenly takes a turn, passengers experiences a jerk in the outward direction as the passenger tends to maintain their original direction of motion due to the inertia of direction.
• When the wheel rotates at a high speed, the mud sticking to the wheel flies off tangentially. This is due to the inertia of direction.

#### Momentum

Momentum is the product of mass and velocity. It is a vector quantity. The momentum of a system is the vector sum of the momenta of the objects which make up the system. If the system is an isolated system, then the momentum of the system is a constant of the motion and subject to the principle of conservation of momentum.The symbol of momentum is p and its SI unit for momentum is kg m/s.
Momentum of a body is calculated by the given formula,

p = m $$\times$$ v

where,
p = momentum
m = mass
v = velocity

The differences between momentum and inertia are given below,

 Momentum Inertia It can be calculated by using formulae. It cannot be calculated by using formulae. Momentum is conserved in some cases. Inertia doesn’t have to be conserved in any case. Momentum comes in the forms of linear momentum and angular momentum The inertia comes only in one form. Momentum is the product of mass and velocity. The property of a body due to which it remains or tends to remain in the state of rest are uniform motion in a straight line unless an unbalanced forces act on it is called inertia.

• Inertia is the property of a body due to which it remains or tends to remain in the state of rest are uniform motion in a straight line unless an unbalanced forces act on it.
• Inertia of rest is the property of the body by virtue of which it remains or tends to remain in the state of rest unless unbalanced force acts on it.
• Inertia of motion is the property of a body by virtue of which it remains are tends to remain in the state of uniform motion in a straight line unless unbalanced forces act on it.
• Inertia of direction is the property of a body by virtue of which it maintains or tends to maintain its direction of motion unless unbalanced forces act on it.
• Momentum is the product of mass and velocity.
• Momentum is a vector quantity.
.

#### Click on the questions below to reveal the answers

The property of a body by which it remains or tends to remain in the state of rest or uniform motion in a straight line unless an external force acts on it is known as inertia.

The mass of the body is the measure of its inertia. It means greater the mass of the body, greater is its inertia and vice versa.

As we know that larger the mass larger will be the inertia and vice versa. Here, the mass of the chalk is less than that of duster. Hence, chalk starts to move but duster does not move even same force is applied on both of them.

It is the property of a body by virtue of which it remains or tends to remain in the state of rest unless any external force is applied on it. For example: Fruits of mango fall when we shake the tree.

It is the property of a body by virtue of which it remains or tends to remain in the state of uniform motion in a straight line unless any external unbalanced force is applied on it. For example: A coin thrown up in moving bus returns to the thrower's hand.

It is the property of a body by virtue of which it maintains or tends to maintain its direction of motion unless an unbalanced force acts on it. For example:
When a running bus suddenly takes turn, then the passengers tend to move tangentially outwards.

When the car is at rest, the person is also at rest. When the car is set in motion, the lower portion of the person comes into motion but the upper portion tends to be at rest. Due to inertia of rest, person falls backward.

When we beat the blanket then the blanket sets in motion but the dust particles continue to be at rest. Due to inertia of rest, dust starts to fall.

Before shaking the branches, both the branches and the fruits are at rest. After shaking, branches set in motion but fruits still tend to be at rest. Due to inertia of rest, fruits fall down.

Initially both the coin and the card are at rest. When the card is pulled suddenly, the card sets in motion but the coin tends to remain in rest. Finally, due to inertia of rest, the coin falls on the glass.

When a bus is moving, the passengers are also moving. If a bus suddenly stops, the lower part of the body sets at rest but the upper part is still in motion. Due to inertia of motion, the passengers fall forward.

When a person is inside a moving bus, he is also is motion. If he jumps from the moving bus, his lower part immediately comes at rest but the upper part still tends to be in motion due to which he falls and may get injured. So, it is dangerous to jump out of a moving bus.

When a bicycle is moving, it has inertia of motion according to which it tends to be in motion until external force (brake or friction) is acted on it. Hence, the bicycle runs for sometimes even if paddling is stopped.

If an athlete runs for a distance, he gains inertia of motion. This makes him take a longer jump. So, he runs a certain distance before jumping.

The coin which is thrown up through a moving train has same horizontal velocity to the train. When a coin is thrown up, it continues to move with velocity of the train due to inertia of motion. Hence, coin thrown upward from a moving train returns back to the thrower's hand.

While firing a bullet from a gun, it exerts large force in short time. The time interval is so small that the glass near the hole is unable to share the motion state of the bullet, only the portion of the glass which is of the same size of the bullet is able to share and carry the glass with it. Hence, it makes a clear hole in the glass window.

When a person jumps from a moving bus, his feet come to rest but the upper part is still in motion. So, to give inertia of motion for whole body, he should run in the direction of the bus.

While switch is on, the fan is in motion due to which it gains inertia of motion and it continues in the state of motion even if switch is off. Hence, fan continues to rotate even if the switch is off.

As we know that inertia and mass of a body are directly related, the truck has a larger mass than that of the car. So, the truck has higher inertia of motion than that of car and due to it, it takes a longer time to stop than the car.

When the bus is moving in a fixed direction and suddenly changes its direction, passengers inside it tends to be in the original direction. Due to inertia of direction, they jerk outward.

Solution:
Here,
Mass (m) = 40,000 kg
Initial velocity (u) = 60 m/s
Final velocity (v) = 0 m/s
Time (t) = 10s
We have,
a = or, a = or, a = -6 m/s
Similarly, force (F) = ma = 40,000 x 6 = 24,0000 N
Hence, force of 24,0000N is applied on the brake.

String is same for the above and downward portion of the stone. If string is pulled suddenly downwards, due to inertia of rest the stone force is more in downward and less in top due to which the downward string breaks.

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3rd law

1st law

2nd law

4th law

• ### Which object has more inertia and why?

Soccer ball. Because it is moving with a high velocity.
Bowling ball. Because it is more massive
Bowling ball. Because it is not moving.
Soccer ball. Because it has a greater volume than the bowling ball.
• ### Which of the following statements is true about friction?

Friction always opposes the motion or attempted motion.

Static friction is less than kinetic friction.

Friction exists only when an object is in motion.

Friction never opposes the motion or attempted motion

• ### ___________is the property of a body by virtue of which it maintains or tends to maintain its direction of motion unless unbalanced forces act on it.

Inertia of Motion
Inertia of gravity
Inertia of rest
Inertia of direction

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