Newton’s Laws of Motion - Newton's First Law of Motion

Newton's First Law of Motion is a fundamental principle that describes how objects behave when no external forces act on them. The law essentially states that a body's velocity remains unchanged if no force causes it to accelerate. This means that if an object is not moving (stationary), it will stay at rest. If an object is moving, it will continue to move with the same velocity in the same direction.

"If no force is acting on a body, its velocity does not change, i.e., the body does not accelerate. In other words, if a body is stationary, it will remain stationary. If it is in motion, it will continue moving with the same velocity and in the same direction."

or

"An object continues to remain at rest or in a state of uniform motion along a straight line unless an external unbalanced force acts on it."

or

"Every inanimate object continues to be in its state of rest or of uniform unaccelerated motion unless and until it is acted upon by an external, unbalanced force."

Balanced Force

Forces applied by both sides are equal, causing the centre of the object (or rope) to remain static despite the forces.

Example: The centre of the rope in a tug-of-war when forces are equal.

Unbalanced Force

Applied forces are unequal, resulting in a net force that acts in the direction of the greater force.

Example: The centre of the rope in a tug-of-war when one side pulls harder.

The process described involves the application of a net unbalanced force:

• Initial State: An object is either at rest or in uniform motion along a straight line.

• Forces Present: There may be several forces acting on the object, but they cancel one another, so the net force is zero.
• Action: An external unbalanced force acts on the object.
• Result: The object's state of motion is changed (it accelerates).

• It shows an equivalence between the 'state of rest' and the 'state of uniform motion along a straight line'.
• It defines force as an entity (or a physical quantity) that brings about a change in the 'state of motion' of a body.
• Force is something that initiates a motion or controls a motion.
• It defines inertia as a fundamental property of every physical object.
• Inertia is the property by which an object resists any change in its state of motion.
• Inertia is measured as the mass of the object (more specifically, inertial mass).

\[\vec F\] = m \[\frac{d\vec{\mathrm{v}}}{dt}\] = m\[\vec a\] ... (for constant mass)

Thus, if \[\vec F\] = 0, \[\vec v\] is constant. Hence, if there is no force, velocity will not change. This is nothing but Newton's first law of motion.

Aim: To demonstrate the concept of inertia of rest using a simple experiment involving a coin, cardboard, and a glass filled with sand.

Materials Required:

• A transparent glass (preferably tall)
• Sand (to fill the glass)
• A stiff piece of cardboard (slightly larger than the mouth of the glass)
• A ₹5 coin (or any similar coin)

Procedure:

1. Fill the glass with sand.
2. Place the cardboard flat on top of the glass, covering its mouth evenly.
3. Carefully place the ₹5 coin at the center of the cardboard.
4. Using your index finger, strike the edge of the cardboard sharply and horizontally.
5. Observe the motion of the coin after the cardboard is struck

Observations:

When the cardboard is hit sharply:

• The cardboard moves quickly away due to the external force applied.
• The coin, however, does not move with the cardboard initially.
• Instead, it falls straight down into the glass of sand.

Result:

• The coin remained at rest momentarily and did not follow the cardboard because of the principle of rest.
• Since no direct force was applied to the coin, it resisted the change in motion and maintained its state of rest.
• This caused the coin to fall vertically into the glass when the support (cardboard) was removed suddenly.

• Newton's First Law states that a body at rest or in uniform motion stays that way unless an external unbalanced force acts on it.
• The law explains inertia, which is an object's resistance to changing its state of motion on its own.
• The 'state of rest' and 'state of uniform motion' are considered equivalent; both require a net unbalanced force to change.
• Force is defined by the law as the entity that changes a body’s state of motion.
• A balanced force results in a net force of zero, meaning no acceleration.
• An unbalanced force results in a net force that causes acceleration and changes the state of motion.
• Inertia is quantitatively measured by the object's mass (inertial mass).