Ncert Books Class 11 Physics Chapter 3 Free Download
FAQs on Ncert Books Class 11 Physics Chapter 3 Free Download
1. What are some expected 5-mark important questions from Chapter 3, Motion in a Straight Line, for the CBSE Class 11 exams?
For the 2025-26 session, important 5-mark questions from this chapter often test a combination of derivation and application. Students should focus on:
Deriving the three equations of uniformly accelerated motion (v = u + at, s = ut + ½at², v² = u² + 2as) using the graphical method. This is a frequently asked question.
Solving complex numericals involving motion under gravity, where an object is thrown upwards or dropped from a height. These questions might ask for time of flight, maximum height, and final velocity.
Questions involving relative velocity in one dimension, for example, problems about two trains or cars moving on parallel tracks.
2. What types of numerical problems are important from this chapter for Class 11 Physics exams?
The most important numericals from Motion in a Straight Line test the application of kinematic equations. Key types include:
Calculating distance and displacement: Problems where an object moves along a non-linear path (e.g., forward and then backward) to test the difference between the two concepts.
Uniformly Accelerated Motion: Using the three equations of motion to find variables like initial velocity, final velocity, acceleration, time, or displacement.
Motion under Gravity: These are a special case of uniform acceleration where a = g. Expect questions on objects dropped from a height or thrown vertically upwards.
Relative Velocity: Problems involving two objects moving with different velocities, often asking for the time and position where they meet or overtake each other.
3. Can a body have zero velocity but still have non-zero acceleration? What is an important example of this?
Yes, a body can have zero instantaneous velocity while still experiencing acceleration. This is a key conceptual question (HOTS). The most common example is an object thrown vertically upwards. At the highest point of its trajectory, the object is momentarily at rest, so its velocity is zero. However, the acceleration due to gravity (g ≈ 9.8 m/s²) is still acting on it, pulling it downwards. Therefore, it has zero velocity but non-zero acceleration.
4. How can we distinguish between average speed and average velocity? In what specific scenario are their magnitudes equal?
This is a fundamental concept frequently tested in 1 or 2-mark questions. The key differences are:
Average speed is a scalar quantity, defined as the total path length travelled divided by the total time taken. It is always positive.
Average velocity is a vector quantity, defined as the total displacement (shortest distance between initial and final points) divided by the total time taken. It can be positive, negative, or zero.
The magnitude of average speed and average velocity are equal only when an object moves along a straight line in a single direction without reversing its course. In this case, the path length is equal to the magnitude of the displacement.
5. What is the significance of the slope and the area of a velocity-time (v-t) graph in kinematics?
Understanding velocity-time graphs is critical for solving many problems. The two most important interpretations are:
Slope of the v-t graph: The slope (change in velocity / change in time) of a velocity-time graph represents the acceleration of the object. A positive slope indicates positive acceleration, a negative slope indicates negative acceleration (retardation), and a zero slope (a horizontal line) indicates zero acceleration (constant velocity).
Area under the v-t graph: The area enclosed between the velocity-time graph and the time axis gives the displacement of the object during that time interval.
6. Why is the concept of a 'frame of reference' considered a foundational topic in the study of motion?
The concept of a 'frame of reference' is crucial because motion itself is relative. An object's state of rest or motion, its velocity, and its displacement can only be described with respect to an observer or a coordinate system. For example, a person sitting on a moving train is at rest with respect to the train (the frame of reference), but is in motion with respect to a person standing on the ground (a different frame of reference). Without defining a frame of reference, any description of motion is incomplete and ambiguous.
7. How should a student approach graph-based questions from this chapter in an exam?
Graph-based questions are a very important part of this chapter. To solve them effectively:
Step 1: Identify the Axes. First, check if it is a position-time (x-t), velocity-time (v-t), or acceleration-time (a-t) graph. This determines what the slope and area represent.
Step 2: Analyse the Slope. For an x-t graph, the slope gives velocity. For a v-t graph, the slope gives acceleration.
Step 3: Calculate the Area. For a v-t graph, the area under the curve gives displacement. For an a-t graph, the area gives the change in velocity.
Step 4: Look for Turning Points. Points where the slope changes sign are critical. For instance, in an x-t graph, a peak or trough indicates a point where velocity is momentarily zero.
8. What are some important 1-mark questions that can be asked from Motion in a Straight Line?
For the 2025-26 CBSE board pattern, 1-mark questions test core definitions and concepts. Important ones include:
Define instantaneous velocity.
When is the displacement of a particle zero even if the distance covered is not zero? (When it returns to its starting point).
What does the odometer of a car measure: distance or displacement?
Give an example of a body with uniform speed but non-uniform velocity. (Uniform circular motion).
