Class11 Physics Chapter3 One shot | Motion in straight line full chapter one shot | CBSE NEET JEE

Brief Summary

This video provides a comprehensive overview of motion in a straight line, covering key concepts such as motion, distance, displacement, speed, velocity, acceleration, and relative velocity. It explains these concepts with examples and includes problem-solving techniques, graph analysis, and equations of motion. The video also offers tips for solving numerical problems and emphasizes the importance of understanding the underlying principles.

Motion and its types (straight line, plane, three-dimensional)
Distance and displacement with real-world examples
Speed and velocity, including average and instantaneous values
Acceleration and its relationship to velocity
Graph analysis (position-time, velocity-time, acceleration-time)
Relative velocity and its applications
Equations of motion and problem-solving techniques

Introduction

The video introduces the topic of motion in a straight line, also known as rectilinear motion or one-dimensional motion, where movement occurs along a single axis (x, y, or z). It clarifies that this type of motion involves only one coordinate to define the position of an object at any given time. The video contrasts this with motion in a plane (two-dimensional) and motion in space (three-dimensional), providing examples to differentiate them.

Motion

Motion is defined as the state of a body in which its position changes with time. If the position of a body does not change with time, it is considered to be at rest. The video specifies that it will focus on straight-line motion, where movement occurs along a single axis. This is also referred to as one-dimensional motion.

Distance / Displacement

Distance is the actual length of the path traveled by an object, while displacement is the shortest distance between the initial and final positions. Displacement is calculated as the final position minus the initial position. The video uses the example of traveling from home to a hotel to illustrate the difference. Distance can be equal to or greater than displacement, but displacement can never be greater than distance. Displacement can be positive, negative, or zero, while distance is always positive or zero.

Speed / Velocity

Speed is the rate at which distance changes with time, while velocity is the rate at which displacement changes with time. Speed is a scalar quantity (magnitude only), while velocity is a vector quantity (magnitude and direction). Speed is always positive or zero, while velocity can be positive, negative, or zero.

Seed(average/ instantaneous)

Speed can be further classified into average speed and instantaneous speed. Average speed is the total distance traveled divided by the total time taken. Instantaneous speed is the speed at a particular instant in time, often read from a speedometer.

Velocity(average/ instantaneous)

Similarly, velocity can be average or instantaneous. Average velocity is the total displacement divided by the total time taken. Instantaneous velocity is the velocity at a specific moment, calculated as the limit of the change in displacement over the change in time as the time interval approaches zero.

Question 1

A car travels 50 km from A to B and then returns from B to A, covering 50 km again. The trip from A to B takes 2 hours, and the return from B to A takes 1 hour. The average speed is calculated as total distance (100 km) divided by total time (3 hours), resulting in 33.33 km/hr. The average velocity is zero because the total displacement is zero (the car returns to its starting point).

Question 2

The video explains how to find instantaneous velocity given a position function x(t). To find the velocity, differentiate the position function with respect to time (dx/dt). For example, if x(t) = 6t² + 2t, then v(t) = 12t + 2. To find the velocity at a specific time, substitute the time value into the velocity function. For instance, at t = 2 seconds, v(2) = 26 m/s, and at t = 3 seconds, v(3) = 38 m/s.

Acceleration

Acceleration is defined as the rate of change of velocity. It is calculated as the change in velocity divided by the change in time. If the time interval is very small, it is represented as dv/dt.

Position time Graph

The video discusses the interpretation of position-time graphs. The slope of a position-time graph represents velocity. A steeper slope indicates a higher velocity. If the slope is constant, the velocity is constant. If the position-time graph is a horizontal line, the object is at rest. The video also explains how to determine if the slope is positive, negative, or zero based on the angle the line makes with the x-axis.

Question 3

The video discusses which graphs are not possible in real-world scenarios. A graph where distance decreases with time is not possible because distance always increases or remains constant.

Velocity time Graph

The video explains velocity-time graphs, where the slope represents acceleration. A constant slope indicates uniform acceleration. The area under the velocity-time curve represents displacement.

Acceleration time Graph

The video discusses acceleration-time graphs. The area under the acceleration-time curve represents the change in velocity.

Question 4

The video presents a problem involving a velocity-time graph and asks to calculate the displacement between 2 and 5 seconds. The displacement is equal to the area under the velocity-time graph between these times. The video also asks to find the acceleration from the same graph, which is the change in velocity over the change in time.

Relative velocity

The video introduces the concept of relative velocity, explaining that motion is not absolute but relative. The observed velocity depends on the frame of reference of the observer. The video uses examples of trains and cars moving in the same or opposite directions to illustrate how relative velocity is calculated. The formula for relative velocity of A with respect to B is given as V_AB = V_A - V_B.

Question 5

Two trains are moving on parallel tracks. Train A moves north at 54 km/hr, and train B moves south at 90 km/hr. The relative velocity of train A with respect to train B is calculated as 144 km/hr. The relative velocity of the ground with respect to train B is -90 km/hr. A monkey moves on train A opposite to its motion at 18 km/hr with respect to train A. The velocity of the monkey with respect to the ground is calculated as 36 km/hr.

Equation of Motion

The video introduces the three equations of motion:

v = u + at
s = ut + (1/2)at²
v² = u² + 2as

These equations relate initial velocity (u), final velocity (v), acceleration (a), time (t), and displacement (s). The video explains how to apply these equations to solve various problems.

Question 6

A car starts from rest and attains a velocity of 180 km/hr in 0.25 minutes (15 seconds). The distance covered during this time is calculated using the equations of motion. First, the acceleration is found to be 3.33 m/s², and then the distance is calculated as 375 meters.

Question 7

An object moves with uniform acceleration. Its velocity after 5 seconds is 25 m/s, and after 8 seconds, it is 34 m/s. The distance traveled by the object in the 12th second is calculated using the equations of motion. The initial velocity (u) and acceleration (a) are first determined, and then the distance is calculated.