Let Us YOU Solve Physics JEE (MAIN) 2018 Physics Paper
We continue with JEE (Main) 2018, Paper-1, Part-A: Physics. All questions are of MCQ type, with one option correct. Only after you finish a question, compare and confirm your method and answer with the video solution given at the bottom. The video is given for checking purpose alone. You are strongly advised not to see it before you solve the question yourself.
Question 11: The mass of a hydrogen molecule is 3.32 x 10-27 kg. If 1023 hydrogen molecules strike, per second, a fixed wall of area 2 cm² at an angle of 45° to the normal, and rebound elastically with a speed of 103 m/s, then the pressure on the wall is nearly:
(1) 2.35 x 103 N/m² (2) 4.70 x 103 N/m² (3) 2.35 x 102 N/m² (4) 4.70 x 102 N/m²
For an oblique elastic collision in two dimensions, the component of velocity vector parallel to the wall does not change, while the component perpendicular to the wall reverses direction without change in magnitude. Interested students may study in details from: Chapter Name – Mechanics; Category – Basic; Topic Name – Impulse, Collision, and Centre of Mass; Video Name – Collision in Two Dimensions.
What is the change in linear momentum of a molecule after a collision? What is change in linear momentum of the wall after the collision? For a timely reminder, watch: Chapter Name – Mechanics; Category – Basic; Topic Name – Impulse, Collision, and Centre of Mass; Video Name – Principle of Conservation of Linear Momentum, and Some Applications.
What is the momentum change of the wall per second? What is the force on the wall? You find the connection here: Chapter Name – Mechanics; Category – Basic; Topic Name – Newton’s Laws of Motion; Video Name – Newton’s Second Law.
What is the pressure on the wall?
By the way, the question is modelled on a similar theoretical derivation, here: Chapter Name – Heat and Thermodynamics; Category – Basic; Topic Name – Kinetic Theory of Gases and Properties of Vapour; Video Name – Pressure Exerted by an Ideal Gas.
Question 12: A silver atom in a solid oscillates in simple harmonic motion with a frequency of 1012/s. What is the force constant of the bonds connecting one atom with the other? Molecular mass of silver is 108, and Avogadro’s number is 6.02 x 1023 /mol.
(1) 6.4 N/m (2) 7.1 N/m (3) 2.2 N/m (4) 5.5 N/m
The bond connecting two atoms behaves like an ideal spring. Therefore, a silver atom at the end of a bond is a prototype of spring-block system. What is the force constant of a spring? ? If you need the basics, study from here: Chapter Name – Mechanics; Category – Basic; Topic Name – Work and Energy; Video Name – Work Done by Spring Force.
What is the formula for frequency of the simple harmonic motion? For reference: Chapter Name – Mechanics; Category – Advanced; Topic Name – Gravitation; Video Name – Advanced-Level Problems on Gravitation II (problem 2).
Using given data, what is the mass of a silver atom in standard SI unit? Unless your Chemistry is strong, you will need this: Chapter Name – Heat and Thermodynamics; Category – Basic; Topic Name – Thermal Expansion; Video Name – Expansion of Gases II.
Substituting known values will lead you to the correct choice.