Motion in Daily Life

Motion in Daily Life

Motion

When an object shifts its position in relation to a reference point within a specific time frame, it is said to be in motion. The change in a body's position with respect to a frame of reference as time changes is measured and expressed mathematically in terms of displacement, distance, velocity, acceleration, speed, and frame of reference.

Scalar and Vector Quantities

Scalar Quantities

• Physical quantities that have magnitude only and no direction are called scalar quantities.

Example: Mass, speed, volume, work, time, power, energy, etc.

Vector Quantities

• Physical quantities that have both magnitude and direction and obey the triangle law are called vector quantities.

Example: Displacement, velocity, acceleration, force, momentum, torque, etc.

Electric current, though it has a direction, is a scalar quantity because it does not obey the triangle law.

Note: Moment of inertia, refractive index, and stress are tensor quantities.

Distance and Displacement

Distance

• Distance is the length of the actual path covered by a moving object in a given time interval.

Displacement

• The shortest distance covered by a body in a definite direction is called displacement.

• Distance is a scalar quantity, whereas displacement is a vector quantity, both having the same unit (meter).

• Displacement may be positive, negative, or zero, whereas distance is always positive.

• In general, the magnitude of displacement < distance

Speed and Velocity

Speed

The distance travelled by the moving object in unit time intervals is called speed.

• It is a scalar quantity.

• SI unit: metre/second (m/s)

If an object covers half of the distance with uniform speed V1 and the other half with uniform speed V2, then the average speed of the object is ...

Velocity

The velocity of a moving object is defined as the displacement of the object in a unit time interval.

• It is a vector quantity.

• SI unit: metre/second (m/s)

Acceleration

Acceleration of an object is defined as the rate of change of velocity of the object.

• It is a vector quantity.

• SI unit: metre/second² (m/s²)

If velocity decreases with time, then acceleration is negative and is called retardation.

Equations of Motion (One Dimension)

1. Motion with uniform velocity: S = vt

2. Motion with uniform acceleration

In case of motion with uniform acceleration:

• average velocity = ...

• u = initial velocity

• v = final velocity

• a = acceleration

• t = time

• s = distance in a straight line/displacement

Circular Motion

If an object describes a circular path, its motion is called circular motion. If the object moves with uniform speed, its motion is uniform circular motion.

• Uniform circular motion is an accelerated motion because the direction of velocity changes continuously, though the magnitude of velocity (speed) remains unchanged.

Angular Velocity

The angle subtended by the line joining the object from the origin of the circle in a unit time interval is called angular velocity.

If T = time period and n = frequency, then ...

• In one revolution, the object travels a 2πr distance.

• Linear Speed = angular speed × radius

Newton’s Laws of Motion

First Law of Motion

Everybody maintains its initial state of rest or motion with uniform speed on a straight line unless an external force acts on it.

• Also called the Law of Galileo or the Law of Inertia.

Inertia

Inertia is the property of a body by virtue of which it opposes change in its initial state of rest or motion.

Types of inertia:

1. Inertia of rest

2. Inertia of motion

Examples of Inertia:

• Passengers bend backward when a car starts suddenly.

• A rider bends forward when a horse stops suddenly.

• Dust comes out of the blanket when beaten.

The first law defines force.

Force

• Force is that external cause that changes or tries to change the state of rest or motion of a body.

Examples: push, pull, tension, friction, action, reaction, normal reaction.

Momentum

Momentum = mass × velocity

• Vector quantity

• SI unit: kg·m/s

Second Law of Motion

The rate of change of momentum is proportional to the applied force.
F = ma

• Gives the magnitude of force.

Third Law of Motion

To every action, there is an equal and opposite reaction.

Examples:

• Recoil of the gun

• Motion of a rocket

• Swimming

• Drawing water from a well

• Driving a nail into wood

Principle of Conservation of Momentum

If no external force acts, the total momentum remains constant.

• Explains the working of rockets.

Impulse

Impulse = force × time = change in momentum

• Vector quantity

• SI unit: newton second (Ns)

Force in a Spring

Force in a spring: F = kx

If cut into n equal parts, the force constant of each part is nk.

Friction

The force that opposes motion between two bodies in contact.

Types of Friction

1. Static Friction: Self-adjusting force, maximum value = limiting friction.

2. Kinetic Friction: Acts when a body slides; less than limiting friction.

3. Rolling Friction: Much smaller than kinetic friction.

Centripetal and Centrifugal Force

Centripetal Force

Force towards the center is required for circular motion.

Examples:

• Tension in the string for the whirling stone

• Gravitational force for planets

• Electrostatic force for electrons

• Banking of roads for cyclists

Centrifugal Force

• A pseudo force, equal and opposite to the centripetal force.

Examples: cream separator, centrifugal dryer.

Moment of Force

Moment of force = Force × moment arm

• Vector quantity

• SI unit: Newton metre (Nm)

Centre of Gravity

The point through which the entire weight acts.

• Does not change with orientation.

• Acts downward.

Equilibrium

If the resultant of all forces = 0, the body is in equilibrium.

• Static equilibrium

• Dynamic equilibrium

Types of Static Equilibrium

1. Stable: Returns to the original position.

2. Unstable: Moves away from the position.

3. Neutral: Stays in new position.

Conditions for Stable Equilibrium:

• Center of gravity at minimum height.

• A vertical line through the center of gravity must pass through the base.

Final Thoughts

Motion is a fundamental concept in physics that explains how objects change their position with time. Every moving body can be described using terms like distance, displacement, speed, velocity, and acceleration. Quantities in physics are either scalars, which have only magnitude, or vectors, which have both magnitude and direction. Newton’s laws of motion form the basis of understanding forces, inertia, and momentum, showing how bodies react to pushes, pulls, and interactions.

Friction, centripetal force, and centrifugal force play an important role in daily life, from walking and driving to the motion of planets and rockets. The concepts of equilibrium and center of gravity explain why some objects are stable while others topple easily.

Circular motion, impulse, and moment of force add deeper insight into how things move in real-world scenarios. These principles not only shape our understanding of science but also explain common experiences like why we bend forward when a bus stops suddenly or how a cyclist leans while turning.