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Force and Laws of Motion - Newton’s First Law of Motion

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Newton’s First Law of Motion states that an object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.

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This law is also known as the Law of Inertia.

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Inertia: The natural tendency of an object to resist a change in its state of rest or of uniform motion is called inertia.

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Mass as a Measure of Inertia: The inertia of an object is measured by its mass. A heavier object has more inertia than a lighter object. Mathematically, Inertia∝Mass\text{Inertia} \propto \text{Mass}.

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Balanced Forces: When the net force acting on a body is zero (Fnet=0F_{net} = 0), the forces are balanced, and there is no change in the state of motion.

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Unbalanced Forces: When the net force acting on a body is non-zero (Fnet≠0F_{net} \neq 0), the forces are unbalanced, resulting in a change in speed or direction (acceleration).

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Inertia of Rest: The tendency of a body to remain at rest. For example, when a bus starts suddenly, passengers fall backwards.

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Inertia of Motion: The tendency of a body to remain in uniform motion. For example, when a moving bus stops suddenly, passengers lean forward.

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Inertia of Direction: The tendency of a body to resist a change in its direction of motion.

📐Formulae

Fnet=0  ⟹  v=constant (or v=0)F_{net} = 0 \implies v = \text{constant} \text{ (or } v=0 \text{)}

∑F⃗=0  ⟹  a⃗=0\sum \vec{F} = 0 \implies \vec{a} = 0

Inertia∝m\text{Inertia} \propto m

💡Examples

Problem 1:

Which has more inertia: a 5 kg5\text{ kg} iron ball or a 5 kg5\text{ kg} bag of cotton? Also, compare a 2 kg2\text{ kg} stone with a 10 kg10\text{ kg} stone.

Solution:

  1. Both the 5 kg5\text{ kg} iron ball and the 5 kg5\text{ kg} bag of cotton have the same inertia because their masses are equal (5 kg=5 kg5\text{ kg} = 5\text{ kg}). 2. The 10 kg10\text{ kg} stone has more inertia than the 2 kg2\text{ kg} stone because 10 kg>2 kg10\text{ kg} > 2\text{ kg}.

Explanation:

Inertia is purely dependent on the mass of the object. Since m1=m2m_1 = m_2 in the first case, inertia is equal. In the second case, the object with the larger mass requires more force to change its state, thus it has higher inertia.

Problem 2:

A car is moving with a constant velocity of 20 m/s20\text{ m/s}. What is the net force acting on the car?

Solution:

The net force acting on the car is 0 N0\text{ N}.

Explanation:

According to Newton's First Law, if an object is moving with constant velocity (v=constantv = \text{constant}), its acceleration is a⃗=0\vec{a} = 0. This implies that the vector sum of all external forces must be zero: ∑F⃗=0\sum \vec{F} = 0

Problem 3:

Why does a person hit the top of a car if it suddenly stops while they are jumping inside?

Solution:

This is due to the Inertia of Motion.

Explanation:

When the car is moving, the person inside is also moving at the same velocity. When the car stops suddenly, the person's body tends to continue moving forward at the same velocity due to inertia, leading to a potential collision with the car's interior if not restrained.