Motion Class 9

Motion

Motion refers to the change in an object's position over a certain period of time. When an object moves from one place to another, it is said to be in motion. This change in position can be observed by comparing the object's location at different points in time. For example, if you see a car moving along the road, you can tell that it's in motion because its position changes as time goes by.

Rest

An object is considered to be at rest when it remains in the same position relative to its surroundings over a specific duration. In other words, if an object isn't changing its location with respect to the things around it, it is said to be at rest. For instance, a book sitting on a table without moving is at rest since its position isn't altering in relation to the table.

Reference Point

1. A reference point is a crucial concept used to describe the position of an object. It's like a fixed point that we use as a comparison to determine how an object's position changes. Imagine you're sitting in a moving car. The trees, buildings, and other cars outside the window are your reference points. By observing how these reference points change in relation to the car, you can understand your own motion. Notably, an object's position can be described differently based on the reference point you choose. Different reference points give different perspectives on the same object's motion or rest.
2. For instance, consider a person walking on a train. To the person on the train, they're simply walking forward. However, to someone watching from outside the train, the person's motion is a combination of their walking and the train's motion. The train itself might be moving forward, but the person's steps are what determine their position relative to the train's interior.

Distance, Displacement and Path Taken

Distance

Distance refers to the total length of the path covered by an object during its entire journey from its initial position to its final position. It's a scalar quantity, which means it only has magnitude (size) and no direction. Imagine a winding road that curves and loops. The distance covered by a car along this road is the sum of all the individual segments it travels.

Displacement

Displacement is the shortest straight-line distance between the object's initial position and its final position. It's a vector quantity, which means it has both magnitude and direction. In the case of the car on the winding road, the displacement would be the straight-line distance from where it started to where it ended up.

Differences between Distance and Displacement

Relationship between Distance and Displacement

1. If an object travels in a straight line without changing direction, the displacement will be equal to the distance travelled. This happens when the object moves in a single direction from start to finish.
2. If an object changes direction during its motion, the displacement can be less than the distance travelled. This occurs when the object moves back and forth or follows a curved path.

Examples:

1. Straight Line Motion: Imagine a person walking 10 metres to the east. In this case, the distance covered and the displacement are both 10 metres to the east.
2. Circular Path: Now consider a person walking in a circle with a radius of 5 metres. After completing the circle, the distance covered is the circumference of the circle (about 31.42 metres), but the displacement is zero because the person returns to the starting point.

Types of Motion

Uniform Motion

Uniform motion refers to the type of motion in which an object covers equal distances in equal intervals of time. In other words, the speed of the object remains constant throughout its journey. Some key characteristics of uniform motion are:

1. Constant Speed: In uniform motion, the object maintains the same speed throughout its movement. This means that it covers the same amount of distance in each unit of time.
2. Equal Intervals: The time taken to cover each segment of the journey is the same. This results in a linear relationship between distance and time, leading to a straight-line distance-time graph.
3. Examples: A car moving along a straight highway at a steady speed, a person walking at a consistent pace on a straight path, or a train travelling without changing its speed.

Non-uniform Motion

Non-uniform motion, on the other hand, occurs when an object covers unequal distances in equal intervals of time or when its speed changes at different points during its journey. Some characteristics of non-uniform motion are:

1. Changing Speed: In non-uniform motion, the object's speed is not constant. It can either increase, decrease, or change direction, causing it to cover varying amounts of distance in equal time intervals.
2. Unequal Intervals: The time intervals remain the same, but the distances covered during each interval are different. This results in a non-linear relationship between distance and time, leading to a curved or non-linear distance-time graph.
3. Examples: A car accelerating or decelerating, a person jogging and then slowing down, a roller coaster ride with loops and turns, or a swinging pendulum.

Speed and Velocity

Speed

Speed is a fundamental concept in physics that measures how fast an object is moving. It quantifies the rate at which an object covers a certain distance in a given amount of time. In simpler terms, speed tells us how quickly an object changes its position relative to a reference point. Speed is a scalar quantity, which means it has magnitude (numerical value) but no direction. It only tells you how fast something is moving, not the direction in which it's moving.

Calculation of Speed

The speed of an object is calculated by dividing the distance it travels by the time it takes to cover that distance. Mathematically, the formula for calculating speed is:

Where:

Speed (v): The rate of change of distance with respect to time.
Distance (s): The length of the path covered by the object during its motion.
Time (t): The duration for which the object is in motion.

SI Unit of Speed

The International System of Units (SI) unit of speed is metres per second (ms^-1). This unit indicates the distance covered in metres during each second of motion. It's important to use consistent units for both distance and time to obtain the correct SI unit for speed.
For example, if the distance is measured in metres and the time in seconds, then the speed will be in metres per second (ms^-1).

Velocity

Velocity is defined as the rate of change of displacement with respect to time.
While speed tells us how fast an object is moving, velocity goes a step further by incorporating the direction of motion. In other words, velocity not only tells us the rate of change of position but also specifies the direction in which the object is moving.

Calculation of Velocity

The velocity of an object is calculated by dividing the displacement it undergoes by the time it takes to undergo that displacement. Mathematically, the formula for calculating velocity is:

Where:

Velocity: The rate of change of displacement with respect to time, including direction.
Displacement: The change in position of the object, considering both magnitude and direction.
Time: The duration for which the object experiences the displacement.

SI Unit of Velocity

The International System of Units (SI) unit of velocity is metres per second (ms^-1), just like the unit of speed. However, velocity is a vector quantity, which means it has both magnitude and direction. The unit "metres per second" specifies the distance covered in metres during each second of motion, along with the direction of motion.

Average Speed and Velocity

Average Speed

1. Average speed is defined as the total distance travelled by an object divided by the total time taken to cover that distance. It gives a general idea of how fast an object is moving on average over a given period, considering all the changes in speed that might have occurred.
2. Average speed is valuable in scenarios where an object's speed is not constant. It provides a single value that represents the overall motion, making it easier to compare different journeys or scenarios.
3. Average speed is also useful when analysing motion in real-world situations, such as in transportation, sports, and everyday activities.

Calculation of Average Speed

The formula for calculating average speed is:

Where:

Average Speed: The overall rate of motion of an object over a given time interval.
Total Distance Travelled: The sum of all the distances covered by the object during its motion.
Total Time Taken: The total duration of the object's motion.

Average Velocity

1. Velocity can be either uniform (constant) or variable (changing). When an object moves in a straight line with a constant velocity, its speed and direction remain unchanged. However, if the speed or direction changes, then the velocity is variable.
2. Average velocity is used to describe the overall motion of an object that might have variable velocity. It's calculated similarly to average speed but considers direction as well. If the velocity changes at a uniform rate, the average velocity is given by the arithmetic mean of the initial velocity (starting velocity) and the final velocity (ending velocity) over a specific time period.

Calculating Average Velocity for Uniform Motion

The formula for average velocity is:

Where:

v_av is the average velocity
u is the initial velocity
v is the final velocity

Calculating Average Velocity for Non-Uniform Motion

For non-uniform motion, where the velocity (speed and direction) of an object is changing, we can calculate the average velocity by dividing the total displacement by the total time taken. The formula for average velocity is:

It's important to note that in non-uniform motion, the total distance travelled and the total displacement can be different, leading to a distinction between average speed and average velocity.

Units of Velocity

Both speed and velocity are measured in the same units, which are commonly expressed as metres per second (ms^-1) or, equivalently, kilometres per hour (kmh^-1).

Acceleration

Acceleration is the rate at which an object's velocity changes over time. It indicates how quickly an object's speed or direction of motion changes. If an object is accelerating, it means that its velocity is changing, either by speeding up, slowing down, or changing direction.

Formula for Acceleration

The formula for calculating acceleration is:

Where:

Acceleration (a): Rate at which velocity changes with time.
Final Velocity (v): The velocity of the object at the end of the time interval
Initial Velocity (u): The velocity of the object at the beginning of the time interval
Time Taken (t): The duration over which the velocity changes

SI Unit of Acceleration

The SI unit of acceleration is metres per second squared (ms^-2). This unit indicates that acceleration is measured in terms of how many metres per second the velocity changes over each second.

Direction of Acceleration

Acceleration is a vector quantity, which means it has both magnitude and direction. The direction of acceleration is determined by whether the velocity is increasing or decreasing and in which direction this change is occurring.

1. Positive Acceleration: If an object's velocity is increasing in the same direction as its motion, the acceleration is considered positive. This typically indicates that the object is speeding up.
2. Negative Acceleration (Retardation or Deceleration): If an object's velocity is decreasing or changing in the opposite direction to its motion, the acceleration is considered negative. This is often referred to as deceleration or retardation and indicates that the object is slowing down.

Uniform and Non-uniform Acceleration

1. Uniform Acceleration: When an object's velocity changes by the same amount in equal intervals of time, the object is said to have uniform acceleration. This could occur when an object is moving in a straight line and its velocity changes by a consistent value over equal time intervals.
2. Non-uniform Acceleration: When an object's velocity changes by unequal amounts in unequal intervals of time, the object is said to have non-uniform acceleration. This could happen when an object's motion is more complex, and its velocity changes irregularly over time.

Equations of Uniformly Accelerated Motion

Uniformly accelerated motion refers to the motion of an object when its velocity changes at a constant rate (uniform acceleration). In such cases, there are a set of equations that relate various quantities, such as initial velocity, final velocity, acceleration, time, and displacement. These equations are derived from the definitions of acceleration and velocity, and they help us predict the motion of objects undergoing constant acceleration. Here are the three fundamental equations of uniformly accelerated motion:

First Equation of Motion

This equation relates displacement (s), initial velocity (u), final velocity (v), acceleration (a), and time (t):

This equation relates the final velocity (v), initial velocity (u), acceleration (a), and time (t) of an object. It expresses how an object's velocity changes over time due to constant acceleration.

Second Equation of Motion

This equation connects the displacement (s), initial velocity (u), acceleration (a), and time (t) of an object. It allows us to calculate the distance covered by an object under uniform acceleration during a specific time interval.

Third Equation of Motion

This equation relates displacement (s), initial velocity (u), final velocity (v), and acceleration (a). It eliminates the time variable and provides a relationship between an object's initial and final velocities and the distance it travels.

It's important to note that these equations apply specifically to situations where the acceleration is constant and the motion is along a straight line.

Graphical Representation of Motion

1. Displacement-Time Graphs

A displacement-time graph, also known as a position-time graph, depicts how an object's displacement changes over a specific period. The x-axis represents time, and the y-axis represents displacement. The slope of the graph at any point represents the object's instantaneous velocity at that moment.

1. A horizontal line indicates that the object is at rest (constant displacement) during that time interval.
2. A straight line with a positive slope indicates constant positive velocity (constant displacement in the positive direction).
3. Curved sections of the graph indicate changing velocity (acceleration or deceleration).
4. A straight line with a negative slope indicates constant negative velocity (constant displacement in the negative direction).

2. Velocity-Time Graphs

A velocity-time graph illustrates how an object's velocity changes over time. The x-axis represents time, and the y-axis represents velocity. The slope of the graph at any point represents the object's acceleration at that moment.

1. A horizontal line indicates that the object is moving at a constant velocity.
2. An upward-sloping line indicates that the object's velocity is increasing over time (acceleration).
3. Curved sections of the graph indicate that the object is moving with increasing acceleration.
4. A downward-sloping line indicates that the object's velocity is decreasing over time (deceleration or negative acceleration).

3. Acceleration-Time Graphs

An acceleration-time graph shows how an object's acceleration changes over time. The x-axis represents time, and the y-axis represents acceleration.

1. A horizontal line indicates constant acceleration.
2. An upward-sloping line indicates that acceleration is increasing over time.
3. A downward-sloping line indicates that acceleration is decreasing over time.

These graphs help in understanding the motion of objects in different scenarios. By analysing the slopes and shapes of these graphs, we can gather information about an object's motion, whether it's moving uniformly, accelerating, decelerating, or undergoing more complex changes.

Uniform Circular Motion

Uniform circular motion is a specific type of motion where an object travels in a circular path with a constant speed. This type of motion has several interesting characteristics that distinguish it from other forms of motion:

1. Uniform Speed: In a uniform circular motion, the object maintains a constant speed throughout its journey along the circular path. This means that the distance it covers in each unit of time remains the same.
2. Changing Velocity: While the speed remains constant, the velocity of the object changes at every point along the circular path. This is because velocity is a vector quantity that takes into account both the speed and the direction of motion. As the object moves along the circular path, its direction changes, resulting in a continuous change in velocity.
3. Acceleration in Uniform Circular Motion: Despite the constant speed, uniform circular motion is considered a form of accelerated motion. This is because acceleration is defined as any change in velocity, which includes changes in direction. In a uniform circular motion, the object's velocity changes due to the change in direction, resulting in acceleration even if the speed remains constant.
4. Centripetal Acceleration: The acceleration experienced by an object in uniform circular motion is called centripetal acceleration. It is directed toward the centre of the circle and is responsible for keeping the object in its circular path. The formula for centripetal acceleration is:

   

   Where a_c is the centripetal acceleration, v is the velocity of the object, and r is the radius of the circular path.
5. Velocity in Uniform Circular Motion: The velocity of an object undergoing uniform circular motion is given by the formula:

   

   Where v is the velocity, r is the radius of the circle, and t is the time taken by the object to complete one full revolution around the circle. This formula indicates that the velocity is directly proportional to the radius and inversely proportional to the time taken.

Applications of Motion Concepts

1. Transportation

1. Cars and Planes: Engineers use motion concepts to design efficient engines, brakes, and aerodynamics for vehicles.
2. Traffic Management: Concepts like speed and distance help optimise traffic flow and design roadways.

2. Sports

1. Athletic Performance: Athletes use motion principles to improve techniques, like adjusting stride length for better speed.
2. Equipment Design: Engineers apply motion physics to create high-performing sports equipment.

3. Engineering

1. Robotics: Motion knowledge is crucial for programming robots to move accurately and perform tasks.
2. Structural Engineering: Architects use motion principles to design stable and safe structures against forces like wind and earthquakes.

Frequently Asked Questions

1. Can an object be in motion and at rest at the same time? Explain.

Yes, an object can be in motion relative to one reference point and at rest relative to another. For example, a passenger sitting in a moving bus is at rest relative to the bus but in motion relative to the ground.

2. Can displacement be zero even if the distance travelled is not? Give an example.

Yes, displacement can be zero even if distance is not. For example, if a person walks 5 metres forward and then 5 metres back to the starting point, the distance travelled is 10 metres, but the displacement is zero because the initial and final positions are the same.

3. What does a velocity-time graph show, and how can you determine acceleration from it?

A velocity-time graph shows how the velocity of an object changes over time. The slope of the velocity-time graph represents the acceleration of the object. A straight line with a positive slope indicates uniform acceleration, while a flat line indicates constant velocity.

4. How does understanding motion help in designing safer vehicles?

Understanding motion helps engineers design vehicles with features like braking systems, airbags, and seat belts that reduce the effects of rapid changes in motion (acceleration and deceleration). This ensures safer stops and protects passengers in case of a collision.

5. What role does circular motion play in satellite orbits?

Satellites in orbit around the Earth are in uniform circular motion. The centripetal force needed to keep the satellite in orbit is provided by the gravitational pull of the Earth. This allows the satellite to maintain a stable orbit without falling to the surface.