Rest and Motion

Rest and motion describe the state of objects in relation to their surroundings. Whether an object is at rest or in motion, these states can be analyzed and understood through the principles of physics. When an object changes its position with respect to a stationary object with the passage of time, the object is said to be in motion; otherwise, the object is said to be in rest. In this article, we will learn about rest and motion, frames of reference, types of motion, laws of motion, and equations of motion.

Rest and Motion Definition

Rest refers to the state of an object when it is not changing its position with respect to its surroundings with the passage of time. In other words, an object at rest remains stationary and does not undergo any change in its location.

Motion, on the other hand, refers to the state of an object when it is changing its position with respect to its surroundings. Objects in motion can exhibit various types of motion, including translational, rotational, and oscillatory motion.

What is Frame of Reference?

The frame of reference is defined as a set of axes or a coordinate system relative to which an observer measures the position, orientation, and other properties of objects.

A practical example of a frame of reference is observing a train from different perspectives. If we are standing on a platform, the train appears to be moving relative to us. But if we are sitting inside the train, the train appears stationary relative to us, but the platform and the surrounding scenery appear to be moving. This shows how motion can be described differently depending on the observer’s frame of reference.

Terms Related to Rest and Motion

Various physical terms used to describe rest and motion are discussed below:

Distance and Displacement

Distance refers to the total length of the path traveled by an object. It is a scalar quantity, meaning it only involves magnitude. It can never be negative.

Displacement, on the other hand, refers to the change in position of an object from its initial position to its final position. It is a vector quantity, meaning it involves both magnitude and direction. Displacement can be positive, negative, or zero, depending on the direction of movement.

• Distance measures how much ground an object covers through its journey. It does not account for the direction taken.
• Displacement measures the shortest path between start and end points. It considers the direction of that path.
• Thus, distance is a scalar, and displacement is a vector.
• If we return to the start, displacement is zero, but distance is not.

Speed and Velocity

Speed refers to how quickly an object is moving regardless of its direction. It is a scalar quantity, which means it only has magnitude. Speed is calculated by dividing the distance traveled by the time taken to travel that distance.

Velocity, on the other hand, not only describes how fast an object is moving but also includes the direction of its motion. It is a vector quantity, which means it has both magnitude and direction. Velocity is calculated by dividing the displacement (change in position) by the time taken.

• Speed indicates how fast an object moves. It only looks at how much distance is covered over time.
• Velocity includes both speed and the direction of travel. This makes velocity a vector quantity.
• Constant velocity means steady speed in a straight line.
• Changes in speed or direction affect velocity.

Acceleration and Retardation

Acceleration refers to the rate at which an object’s velocity changes over time. It can be thought of as how quickly an object speeds up, slows down, or changes direction. Mathematically, acceleration is calculated by dividing the change in velocity by the time taken for that change to occur.

Retardation is also known as deceleration or negative acceleration. It refers to the rate at which an object’s velocity decreases over time. It is actually the opposite of acceleration and describes how quickly an object slows down. Like acceleration, retardation is calculated by dividing the change in velocity by the time taken.

• Acceleration is how quickly velocity changes with time.
• It can involve increasing speed, decreasing speed, or changing direction.
• Retardation specifically refers to reducing speed.
• It’s also called negative acceleration.
• Both are measured in meters per second squared.
• Acceleration can be caused by various forces.
• Retardation is often due to friction or resistance.

Centripetal Acceleration

Centripetal acceleration is the acceleration experienced by an object moving in a circular path. It is directed towards the center of the circle and is responsible for keeping the object moving in a curved path rather than in a straight line.

• It is always directed towards the center of the circle.
• This acceleration keeps the object on its circular path.
• It’s necessary for any kind of circular motion.
• Its magnitude depends on the object’s speed and the circle’s radius.
• Faster speeds or tighter circles increase centripetal acceleration.
• It is distinct from the object’s forward speed.

Types of Motion

The different types of motion are mentioned below:

Rectilinear Motion

In rectilinear motion, objects move along a straight path. This type of motion is characterized by either constant velocity or varying velocity.

• Directional changes do not occur in rectilinear motion.
• An example of rectilinear motion is a train moving on straight tracks.
• The acceleration, if any, is in the direction of motion or opposite to it.

Circular Motion

Objects in circular motion travel along a circular path. This motion is always accelerated because of the constant change in direction.

• Even if the speed remains constant, the velocity direction changes continuously.
• A central force, known as centripetal force, keeps the object on its path.
• Examples include the motion of electrons in orbits and cars in roundabouts.
• The speed can be constant (uniform circular motion) or variable.

Uniform and Non-Uniform Motion

Uniform motion occurs when an object moves at a constant speed while in non-uniform motion, the speed of the object changes over time.

• Uniform motion implies zero acceleration, while non-uniform motion does not.
• Examples of uniform motion include a car moving at a steady speed.
• Non-uniform motion is seen in vehicles accelerating or braking.

Projectile Motion

Projectile motion is the motion of an object or particle that is thrown near the Earth’s surface and moves along a curved path under the action of gravity only.

• This type of motion has both a horizontal component and a vertical component.
• Gravity affects the vertical motion, causing a parabolic trajectory.
• The horizontal motion remains uniform as there is no acceleration in that direction.
• Common examples are a ball thrown in the air or a fountain’s water stream.

Graphs of Motion

Graphs of motion are used to represent how an object moves over time. Different types of graphs can show various aspects of motion, such as speed, velocity, and acceleration. Let’s understand the main types of motion graphs:

Graph Type	Key Characteristics
Distance-Time Graph	Slope indicates speed
Displacement-Time Graph	Slope indicates velocity. Flat sections mean no motion
Speed-Time Graph	Area under the curve represents distance travelled
Velocity-Time Graph	Slope indicates acceleration; area under curve is displacement
Acceleration-Time Graph	Provides a measure of how acceleration changes over time

Laws of Motion

The laws of motion are three rules given by Sir Isaac Newton that explain how objects move. These laws are important for understanding how everything in our universe behaves, from tiny particles to massive planets.

Newton’s First Law

The first law of motion states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. It is also known as the law of inertia.

This means that unless something causes a change, an object will keep doing what it’s currently doing. It will stay still or keep moving at a constant speed in the same direction.

For example, if we roll a ball on a flat surface, it will continue to roll unless it hits something or friction from the surface slows it down.

Newton’s Second Law

The second law of motion states that the acceleration of an object is dependent upon two variables – the net force acting upon the object and the mass of the object. It is expressed mathematically as:

F = ma

where,

• F represents the net force applied to the object,
• m is the mass of the object, and
• a is the acceleration that results.

This formula means that the force required to accelerate an object is equal to the mass of the object multiplied by the desired acceleration. This means that heavier objects need more force to move or stop than lighter ones. So, if we push a shopping cart, it takes more effort to start it moving or to stop it than to push a small toy car.

Newton’s Third Law

The third law of motion states that for every action, there is an equal and opposite reaction. This means that whenever one object exerts a force on another object, the second object exerts a force of equal magnitude in the opposite direction on the first object.

This law explains why forces always occur in pairs that are equal in strength and opposite in direction. For example, when a swimmer pushes against the water with their arms and legs, the water pushes back against the swimmer with equal force, moving them forward through the water.

Equations of Motion

There are three equations that are used to describe motion of an object. These equations are called equations of motion. The three equations of motion are

First Equation of Motion: v = u + at

Second Equation of Motion: s = ut + 1/2at²

Third Equation of Motion: v² – u² = 2as

In the above three equations, u is initial velocity, v is final velocity, t is time, a is acceleration, s is distance covered.

FAQs on Rest and Motion

Define Rest and Motion.

“Rest” and “motion” are terms used to describe the state of an object in relation to a frame of reference. An object is considered to be at “rest” if it does not change its position relative to a given frame of reference. An object is in “motion” if it changes its position over time when observed from that same frame of reference.

How do distance and displacement differ?

Distance is the total path traveled regardless of direction, while displacement measures the straight-line distance from start to end point, considering direction.

What is the difference between speed and velocity?

Speed is the rate of covering distance which is a scalar quantity. Velocity is speed with direction considered, making it a vector quantity.

What does acceleration refer to in physics?

Acceleration is the rate of change of velocity over time. It can be an increase or decrease in speed or a change in direction.

What is retardation in terms of motion?

Retardation, or deceleration, specifically refers to the negative acceleration change that decreases the velocity of an object.

What role does centripetal acceleration play in circular motion?

Centripetal acceleration is necessary for circular motion, directed towards the circle’s center, keeping the object on the circular path.