default_background
✕

Find the best tutors and institutes for Class 11 Tuition

Find Best Class 11 Tuition

Please select a Category.

Please select a Locality.

No matching category found.

No matching Locality found.

Q5.14:
A molecule in a gas container hits a horizontal wall with speed (200 m s^{-1}) and angle (30^{\circ}) with the normal, and rebounds with the same speed. Is momentum conserved in the collision ? Is the collision elastic or inelastic ?

Solution :

1. Analysis of Momentum Conservation

To determine if momentum is conserved, we must define the system. Per [Newton's Third Law of Motion], the collision involves forces acting between the gas molecule and the wall. If we define the system as (Molecule + Wall), the momentum is conserved because the net external force acting on the system is zero [By the Law of Conservation of Momentum].

However, if we define the system as only the molecule, the momentum is NOT conserved. The wall exerts an impulsive force on the molecule, changing its velocity vector. [Per the Second Law of Motion: $\vec{F}_{ext} = \frac{d\vec{p}}{dt}$]. Since an external force acts on the molecule, its change in momentum is non-zero.

2. Analysis of Kinetic Energy and Elasticity

A collision is defined as elastic if the total kinetic energy of the system remains conserved before and after the collision. We must compare the initial kinetic energy ($K_i$) and the final kinetic energy ($K_f$).

Given variables:

  • Mass of the molecule: $m$
  • Initial speed: $v_i = 200 \text{ m s}^{-1}$
  • Final speed: $v_f = 200 \text{ m s}^{-1}$

The kinetic energy is calculated as $K = \frac{1}{2}mv^2$.

Initial Kinetic Energy: $K_i = \frac{1}{2}m(200)^2 = 20,000m \text{ J}$

Final Kinetic Energy: $K_f = \frac{1}{2}m(200)^2 = 20,000m \text{ J}$

Since $K_i = K_f$, the kinetic energy of the molecule is conserved. [Per the definition of an elastic collision], a collision where kinetic energy is conserved is classified as elastic.

3. Visual Representation of Velocity Components

Normal Incident Rebound 30° 30°

4. Mathematical Justification of Momentum Change

Using coordinate geometry where the wall is the x-axis and the normal is the y-axis:

$\vec{v}_i = v\sin(30^\circ)\hat{i} - v\cos(30^\circ)\hat{j}$

$\vec{v}_f = v\sin(30^\circ)\hat{i} + v\cos(30^\circ)\hat{j}$

Change in momentum $\Delta \vec{p} = m(\vec{v}_f - \vec{v}_i) = m(2v\cos(30^\circ)\hat{j})$.

Since $\Delta \vec{p} \neq 0$, the molecule experiences an impulse, confirming that momentum is not conserved for the molecule alone, but the collision remains elastic due to the conservation of speed and mass.

Final Solution: Momentum is conserved if the wall is included in the system; it is not conserved for the molecule alone. The collision is elastic because the kinetic energy remains constant.


More Questions from Class 11 Physics Work EXERCISES


CBSE Solutions for Class 11 Physics Work


Chapters in CBSE - Class 11 Physics


Top Tutors who teach Work

Manveer Singh khangarot Class 11 Tuition trainer in Gandhidham
(10)
Top Tutor
locationImg Sapna Nagar, Gandhidham
experience 3 yrs of Exp students 1 student
greenClock 13 hrs of classes completed
rsIcon 450 per hour
3+ Years of One-on-One Teaching | School & B.Tech CSE | Practical Learning & Concept Clarity
Online Classes Online Classes

I have 3 years of experience providing one-on-one tuition to students from Classes 10 to 12. For Class 11, I focus on building strong fundamentals and ensuring students understand concepts rather than relying on rote learning. I explain topics in a simple, student-friendly manner and use practical, real-life examples wherever possible. I help students with regular studies, homework, assignments, revision, exam preparation, and problem-solving. Being a final-year B.Tech CSE student, I can also help students develop logical thinking and a strong foundation in computer-related subjects. My teaching style is interactive, patient, and adapted to the individual student's pace and learning needs.

Indu Bhati

An excellent teacher who explains Chemistry concepts clearly and makes learning easy and interesting.

Skills: English , Mathematics and more

(12)
Top Tutor
locationImg Pungavadi Puthur, Tindivanam
experience 14 yrs of Exp students 3 students
greenClock 190 hrs of classes completed
rsIcon 333 per hour
Physics teacher with an Engineering background and 14 years of teaching experience
Online Classes Online Classes

I am a highly experienced educator with over 14 years of academic and teaching experience, including 12 years dedicated to undergraduate and postgraduate instruction in physics-based engineering subjects. I have also provided personalised private tuitions for school students, including ICSE 10th standard, with a focus on conceptual clarity and problem-solving. I am currently offering online and home tuition for Physics, especially tailored to high school students. If you're looking for a dedicated, concept-oriented, and motivational tutor who brings both academic depth and real-world applications into lessons, I’d be happy to assist.

Pradeep

I’ve been taking one-to-one online tuition for Class 11 CBSE Physics, and it’s been a really good experience. The teacher explains concepts in a very simple and clear way, which makes even tough chapters easy to understand. Classes are interactive, and I always feel comfortable asking questions—my doubts get cleared right away. The best part is how the teacher connects topics with real-life examples and gives useful tips for solving problems. I’ve started feeling much more confident in Physics now, and my interest in the subject has definitely grown. Totally recommend these classes to anyone who wants personal attention and better clarity in Physics!

Skills: Physics

(378)
Top Tutor
locationImg Madhurawada, Visakhapatnam
experience 20 yrs of Exp students 47 students
greenClock 5175 hrs of classes completed
rsIcon 400 per hour
Professional Trainer with 30 years of teaching experience in PHYSICS for IIT/NEET/CBSE/ICSE etc.
Online Classes Online Classes
homeTutions Student's Home
regularClasses Tutor's home

I mentored several students for CBSE/ICSE/STATE/NIOS 11 th class in physics for the last 24 years.

Sakshi Singh

I recently started taking online physics classes with him, and my experience has been really positive. He is a knowledgeable and dedicated teacher who explains even difficult concepts in a simple and understandable manner. What I appreciate most is his patience and willingness to clear every doubt, ensuring that I have a proper understanding of the topic before moving forward. His teaching approach makes physics much easier to follow and helps build confidence in the subject. He is also very approachable and creates a comfortable learning environment where asking questions never feels difficult. I would definitely recommend him to students looking for a supportive and effective physics tutor.

Skills: Physics
Also teaches: Class 12 Tuition, Class 9 Tuition and more

(165)
Top Tutor
locationImg Rajajinagar Mahaganapathi Nagar, Bangalore
experience 15 yrs of Exp students 52 students
greenClock 5141 hrs of classes completed
rsIcon 400 per hour
Professional GMAT, GRE & SAT Math trainer with 15 years of experience.
Online Classes Online Classes
regularClasses Tutor's home

I have done Post Graduation in Mathematics, Presidency college, Chennai University. I was a topper and a gold medalist in MSc Math Presidency College. I have been teaching math from last 15 years. I have taken tuitions for 11th Maths for students across different boards like CBSE, ICSE, IGCSE, ISC and State Board. I have an expertise in Math with an excellent track record. My focus is students' success. Conceptually , I make students strong so that they can handle any question in the exam. I give challenging problems in the class so that you can become competetive. Your future is very promising with me.

Srikar

Mala ma'am makes me understand concepts with clarity and clears all my doubts with lots of patience making to solve problems with ease. Thank you ma'am for always being there to help and support me.

Skills: Mathematics , Applied Mathematics
Also teaches: Class 9 Tuition, GRE Coaching and more

(11)
Top Tutor
locationImg Peera Garhi, Delhi
experience 8 yrs of Exp students 2 students
greenClock 72 hrs of classes completed
rsIcon 700 per hour
Experience &Certified Computer Science/IP instructor
Online Classes Online Classes
homeTutions Student's Home
regularClasses Tutor's home

I am having more than 8+ years of experience to teach the Computer Science/ informatic Practice subject to the students of 11th class. I can teach all boards students.

Skills: Computer Science

Other Subjects in CBSE - Class 11

Worksheet Icon

Download free CBSE - Class 11 Physics Work EXERCISES worksheets

Download Now

Find Best Class 11 Tuition ?

Find Now »