Class 11 Physics, Chapter 7 (NCERT). 15 questions by concept, 3 with pictures. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
A comet aphelion distance is 2 times its perihelion distance. What is aphelion speed divided by perihelion speed?
(a) 1/2
At the two apsides radius and velocity are perpendicular. Conserved angular momentum gives rp vp = ra va, so the speed ratio equals the inverse distance ratio.
Two negligible-mass satellites have the same circular orbit radius about planets of masses M and 9M. What is period about the heavier planet divided by period about the lighter planet?
(c) 1/3
At fixed radius T = 2π√(r³/(GM)). The greater central mass reduces the period by its square-root factor.
A balanced torsion experiment has zero net force on its rod. Then the equal opposite end forces are both reversed. What happens to the gravitational torque?
(a) It reverses direction while retaining its magnitude.
Zero net force does not mean zero torque: forces at different positions can form a couple.
The stated change is: the equal opposite end forces are both reversed.
Each end force reverses direction while its magnitude and lever arm remain unchanged. Each moment therefore changes sign only, so the couple reverses with the same magnitude.
Therefore, the requested answer is It reverses direction while retaining its magnitude.
In an ideal torsion balance, two equal opposite gravitational forces of magnitude F act perpendicularly at the ends of a rod of length L. If F = 2 N and L = 4 m, what is the gravitational torque magnitude?
(a) 8 N m
The two forces have different lines of action and form a couple. Each acts at lever arm L/2, giving total torque FL.
Two fixed masses attract with force 81 N at separation r. What is their force magnitude at separation 3r?
(c) 9 N
Keep both masses fixed and apply F proportional to 1/r². Divide the original force by the square of the distance multiplier.
The drawing shows an axial section of a uniform hemispherical shell. C is the centre of its circular opening. What is the direction of the field at C due to this shell?
(b) Vertically upward.
For a complete sphere the two halves cancel. For one hemisphere, in-plane components cancel and the remaining field points toward the existing half-shell.
The drawing shows a point P away from the centre but within the flat opening of a uniform hemispherical shell. Which statement about the field at P is correct?
(c) It is perpendicular to the opening plane and directed upward.
Completing the shell to a sphere gives zero interior field. Reflection across the opening makes the two halves share in-plane components, forcing those components to vanish; the surviving component points toward the existing hemisphere.
Two identical source masses are fixed on opposite sides of a test mass, each exerting gravitational force 7 N on it. What is the net force?
(a) 0 N
The individual forces are equal and oppositely directed. Their vector sum is zero.
A body weighs 80 N on a spherical planet surface. At altitude R, where R is the planet radius, what is its gravitational weight?
(b) 20 N
The inverse-square distance is measured from the planet centre. Use W(h)=W0 R²/(R+h)², giving 20 N.
An isolated system contains 4 point masses. How many distinct pair terms belong in its gravitational potential energy?
(c) 6
An interaction pair is unordered: i,j and j,i are the same pair. There are n(n−1)/2 distinct pairs.
Potential is zero at infinity. At radius 4R outside a spherical mass M, which expression is gravitational potential?
(c) −GM/(4R)
Attractive gravity with zero at infinity gives Φ=−GM/r. The potential is negative and varies as inverse distance, not inverse square.
At the same radius outside an isolated spherical planet, two projectiles have masses m and 5m. With no atmosphere, what is heavy escape speed/light escape speed?
(b) 1
The escape energy equation has projectile mass in both terms. Cancelling it gives ve=√(2GM/r), independent of projectile mass.
A circularly orbiting satellite has kinetic energy 50 MJ. With gravitational potential zero at infinity, what is its total mechanical energy?
(c) -50 MJ
For a circular gravitational orbit U=−2K. Therefore E=K+U=−K.
A satellite moves from a circular orbit of radius r to another circular orbit of radius 9r about the same planet. What is final speed divided by initial speed?
(b) 1/3
Circular speed is √(GM/r). A larger orbit has a lower speed by the square root of the radius ratio.
For a fixed distant source mass, its distance from a small laboratory is multiplied by 2. How does the tidal acceleration difference across a fixed radial laboratory length scale?
(a) 1/8
Across a short length L, the radial tidal difference is approximately 2GML/r³. At fixed M and L it scales inversely with the cube of source distance.
Wrong
Astronauts float because gravity has stopped acting.
Why it feels right. I associate floating with the absence of a downward pull.
Right
Orbiting objects are in continuous free fall under gravity.
Worked example
