We can shield a charge from electric fields by putting it inside a hollow conductor. Can we shield a body from

NCERT Exemplar · Q23

Q. We can shield a charge from electric fields by putting it inside a hollow conductor. Can we shield a body from the gravitational influence of nearby matter by putting it inside a hollow sphere or by some other means?

Uttarakhand UbseShort· 2mImportance★★★★★est

78% · 52/67 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Gravitational Field Direction

Gravitational Field Direction: From Intuition to Precision

Imagine you're holding a ball in your hand. The moment you let go, it falls straight down toward the floor. Not sideways, not up — straight down. That "down" direction is the direction of the gravitational field at that point.

Now picture yourself on the other side of the Earth — in Australia, for instance. If you drop a ball there, it also falls "down" — but from your perspective, that "down" is toward the ground beneath your feet, which points toward the centre of the Earth. So the direction of the gravitational field is always toward the centre of the Earth.

This is the core intuition: gravity pulls things toward the source of the field.

The Precise Statement

Important

The gravitational field at any point in space points directly toward the mass that creates it. For a point mass $M$ , the field direction is radially inward — along the line joining the point to $M$ , pointing from the point toward $M$ .

Mathematically, if you place a test mass $m$ at a position $\vec{r}$ relative to a source mass $M$ , the gravitational field $\vec{g}$ at that point is:

\vec{g} = -\frac{GM}{r^2} \hat{r}

Here:

$\hat{r}$ is a unit vector pointing away from $M$ (radially outward).
The minus sign flips that direction: the field points toward $M$ (radially inward).

So the direction of $\vec{g}$ is always toward the source mass.

Why "Toward the Source"?

Think of a single massive object — say, the Sun. A small rock anywhere near the Sun feels a pull straight toward the Sun's centre. If you place the rock to the left of the Sun, it gets pulled right. If you place it above, it gets pulled down. The pull is always along the line connecting the rock to the Sun's centre, and it always points toward the Sun.

This is because gravity is a central force — it acts along the line joining two masses, and it's always attractive. There's no repulsive gravity.

What About Multiple Masses?

If you have two or more masses (like the Earth and the Moon), the net gravitational field at a point is the vector sum of the fields from each mass. The direction of the net field is the direction of that sum — it points toward the combined effective centre of all the masses, weighted by their distances and sizes.

For example, on the surface of the Earth, the Moon's gravity also pulls on you, but it's much weaker than Earth's. So the net field points almost exactly toward Earth's centre, with a tiny tilt toward the Moon.

Note

The direction of the net gravitational field is not necessarily toward the centre of the nearest large mass — it's the vector sum of all contributions. But in most everyday situations, Earth's field dominates, so "down" is toward Earth's centre.

A Common Misconception …

Start your 14-day free trial to unlock the full solution →

The Equivalence Principle tells us gravity cannot be shielded — unlike electric charge, gravitational “charge” (mass) is inseparable from inertia, so a hollow sphere offers no gravitational protection. The answer is no.

The question touches on a deep difference between electromagnetism and gravity. In electrostatics, a hollow conductor works as a Faraday cage because free charges in the conductor rearrange themselves to cancel the internal field. Gravity has no such “free” counterpart — there is no negative mass to rearrange. But the real reason goes deeper.

The core idea: the Equivalence Principle

Einstein’s Equivalence Principle states that gravitational mass and inertial mass are the same thing. This is not a coincidence — it is the foundation of general relativity. Because of this, any attempt to “shield” gravity runs into a fundamental problem: if you put an object inside a hollow sphere, the sphere’s gravity acts on the object, and the object’s inertia responds identically. There is no way to cancel one without cancelling the other.

Think of it this way: inside a hollow spherical shell, the net gravitational force from the shell is zero (Newton’s shell theorem). But that does not shield the object from other masses outside. The shell itself is transparent to gravity — it does not block or absorb gravitational field lines the way a conductor blocks electric field lines.

Step-by-step reasoning

Newton’s shell theorem

For a uniform spherical shell, the gravitational field inside the shell is exactly zero. This is a mathematical result: the contributions from all parts of the shell cancel. So if you place a body inside a hollow sphere, the sphere itself exerts no net force on it.

But this is not shielding — it is just cancellation due to symmetry. The body still feels the gravity of every other mass outside the sphere (the Moon, the Sun, distant galaxies) because those fields pass straight through the shell.
Why electric shielding works

In electrostatics, a hollow conductor shields because free electrons in the metal rearrange to create an opposing field that exactly cancels the external field inside. This is possible because charge can be separated — positive and negative charges can move independently. Gravity has no negative mass; there is no “gravitational charge” of opposite sign to rearrange. So no cancellation can occur.
The Equivalence Principle forbids it

Suppose you tried to build a gravitational shield. You would need a material that somehow alters the gravitational field of a nearby mass without affecting the mass itself. But the Equivalence Principle says that gravitational effects are indistinguishable from inertial effects in a local frame. Any device that “blocks” gravity would also have to block inertia — which is impossible, because inertia is a property of all matter.
What about a hollow sphere around the Earth?

If you put a hollow sphere around the Earth, the sphere’s gravity would add to the Earth’s gravity outside, but inside the sphere (between the sphere and the Earth) the field would be the sum of both. There is no region where the Earth’s field is cancelled — the sphere cannot “absorb” the Earth’s gravitational influence. …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ22 — Is it possibe for a body to have inertia but no weight?→Next questionQ24 — An astronaut inside a small spaceship orbiting around the earth cannot…

Q.We can shield a charge from electric fields by putting it inside a hollow conductor. Can we shield a body from the gravitational influence of nearby matter by putting it inside a hollow sphere or by some other means?

The core idea: the Equivalence Principle

Step-by-step reasoning

🎓Unlock everything free for 14 days

Unlock everything free for 14 days