An Astronaut Floating in Orbit Is Actually Falling the Whole Time
Learn to calculate the gravitational force between two masses, use the value of G and predict how the force changes, separate mass from weight properly, and explain free fall, weightlessness and how g varies.
Why does an astronaut float if gravity still reaches orbit?
An astronaut on a space station drifts across the cabin with no apparent weight. The usual explanation — there is no gravity up there — is wrong.
Gravity reaches everywhere, and at the height of a space station it is not far short of its value on the ground. The Earth is pulling on that astronaut with almost full strength.
What is missing is not gravity but something to push back. On the ground you feel your weight because the floor pushes up on you; take the floor away and the sensation goes with it. In orbit, the station and the astronaut are both falling under gravity at exactly the same rate, so the cabin floor never catches up to press on the astronaut's feet. There is no upward push, so there is no feeling of weight.
The same thing happens much closer to home. Cut the cable of a lift and everyone inside floats for the few seconds before it lands, for precisely the same reason.
So weightlessness means no reaction force, not no gravity — and the whole of this page is about keeping such things apart: the force between two masses, the constant that sets its size, the difference between mass and weight, and how itself changes from place to place.
This page covers the third part of the ICSE Class 9 Physics chapter on the laws of motion — universal gravitation, mass and weight, free fall and weightlessness.
Gravity reaches everywhere, and at the height of a space station it is not far short of its value on the ground. The Earth is pulling on that astronaut with almost full strength.
What is missing is not gravity but something to push back. On the ground you feel your weight because the floor pushes up on you; take the floor away and the sensation goes with it. In orbit, the station and the astronaut are both falling under gravity at exactly the same rate, so the cabin floor never catches up to press on the astronaut's feet. There is no upward push, so there is no feeling of weight.
The same thing happens much closer to home. Cut the cable of a lift and everyone inside floats for the few seconds before it lands, for precisely the same reason.
So weightlessness means no reaction force, not no gravity — and the whole of this page is about keeping such things apart: the force between two masses, the constant that sets its size, the difference between mass and weight, and how itself changes from place to place.
This page covers the third part of the ICSE Class 9 Physics chapter on the laws of motion — universal gravitation, mass and weight, free fall and weightlessness.
Formula
How do you calculate the gravitational force between two masses?
Every two bodies attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Here is the distance between their centres, and is the universal gravitational constant. The force acts along the line joining them, and each pulls the other equally — an action-reaction pair from the previous part of this chapter.
Worked example 1 — two small masses. Two bodies of kg each, m apart, with :
An utterly negligible force — far too small to notice between everyday objects.
Worked example 2 — larger masses, closer together. Masses of kg and kg, m apart:
Worked example 3 — why the Earth's pull is different. The Earth's mass is enormous, so the same formula gives a force of N on a kg body at its surface. Gravitation is a very weak force that only becomes noticeable when one of the bodies is astronomically large — which is why you feel the Earth pulling you and not the person sitting next to you.
Predicting changes without recalculating. The two dependences make this quick:
- Double one mass — the force doubles
- Double both masses — the force becomes four times
- Double the separation — the force falls to one quarter
- Triple the separation — the force falls to one ninth
- Halve the separation — the force becomes four times
Worked example 4 — a combined change. Two masses attract with N. One mass is tripled and the separation is doubled. Then
Worked example 5 — another. Both masses are doubled and the separation is halved:
sixteen times the original.
The distance goes in SQUARED, so it dominates. Doubling a mass gives a factor of , while doubling the distance gives a factor of — the separation has twice the leverage. Halving the distance is worth more than doubling both masses, and that asymmetry is what "inverse square" means in practice.
**The value and unit of .**
Its unit follows from rearranging the formula as , which has units — the derivation trick from the measurements chapter.
** and are entirely different quantities.** is a universal constant, the same at every point in the universe, with units . is the acceleration due to gravity, about at the Earth's surface, and it changes from place to place and from planet to planet. Confusing the two is the standard error in this chapter, and the units alone tell them apart.
Here is the distance between their centres, and is the universal gravitational constant. The force acts along the line joining them, and each pulls the other equally — an action-reaction pair from the previous part of this chapter.
Worked example 1 — two small masses. Two bodies of kg each, m apart, with :
An utterly negligible force — far too small to notice between everyday objects.
Worked example 2 — larger masses, closer together. Masses of kg and kg, m apart:
Worked example 3 — why the Earth's pull is different. The Earth's mass is enormous, so the same formula gives a force of N on a kg body at its surface. Gravitation is a very weak force that only becomes noticeable when one of the bodies is astronomically large — which is why you feel the Earth pulling you and not the person sitting next to you.
Predicting changes without recalculating. The two dependences make this quick:
- Double one mass — the force doubles
- Double both masses — the force becomes four times
- Double the separation — the force falls to one quarter
- Triple the separation — the force falls to one ninth
- Halve the separation — the force becomes four times
Worked example 4 — a combined change. Two masses attract with N. One mass is tripled and the separation is doubled. Then
Worked example 5 — another. Both masses are doubled and the separation is halved:
sixteen times the original.
The distance goes in SQUARED, so it dominates. Doubling a mass gives a factor of , while doubling the distance gives a factor of — the separation has twice the leverage. Halving the distance is worth more than doubling both masses, and that asymmetry is what "inverse square" means in practice.
**The value and unit of .**
Its unit follows from rearranging the formula as , which has units — the derivation trick from the measurements chapter.
** and are entirely different quantities.** is a universal constant, the same at every point in the universe, with units . is the acceleration due to gravity, about at the Earth's surface, and it changes from place to place and from planet to planet. Confusing the two is the standard error in this chapter, and the units alone tell them apart.
What is the real difference between mass and weight?
Mass is how much matter a body contains; weight is the force with which gravity pulls on it. One is a property of the body, the other a fact about where the body is.
The full comparison.
- Mass — the quantity of matter. A scalar. SI unit the kilogram. Constant everywhere, including in space. Measured by a beam balance. It is also the measure of inertia
- Weight — the gravitational force on the body. A vector, directed towards the Earth's centre. SI unit the newton. Varies with location. Measured by a spring balance. Can be zero in free fall
Worked example 1 — on Earth. A body of mass kg:
which is also , using the gravitational unit from the previous part of this chapter.
Worked example 2 — on the Moon. The same body taken to the Moon, where :
**The mass is still kg. Nothing was removed from the body; only the pull on it changed — to roughly a sixth.
Worked example 3 — working backwards.** A body weighs N on Earth, so
Worked example 4 — a spring balance in a lift. A person of kg stands on a weighing machine in a lift. The machine reads the normal reaction, which is the apparent weight.
- Lift at rest or moving at a steady speed: N
- Lift accelerating upward at : N — the person feels heavier
- Lift accelerating downward at : N — lighter
- Lift in free fall, : N — weightless
**Throughout all four cases the mass stayed kg and the true weight stayed N. What changed was the reaction force, which is what a machine actually measures.
A beam balance works anywhere; a spring balance does not. A beam balance compares two masses, and since both are affected by gravity equally, the comparison holds on the Moon unchanged. A spring balance measures force, so its reading would fall to a sixth on the Moon. That is why a shopkeeper's beam balance is trustworthy and a spring balance has to be calibrated for where it is used.
Everyday speech uses "weight" for mass and physics does not.** A bag described as *weighing kg* has a mass of kg and a weight of N. Both statements are about the same bag, and only one of them is in the unit of a force.
The full comparison.
- Mass — the quantity of matter. A scalar. SI unit the kilogram. Constant everywhere, including in space. Measured by a beam balance. It is also the measure of inertia
- Weight — the gravitational force on the body. A vector, directed towards the Earth's centre. SI unit the newton. Varies with location. Measured by a spring balance. Can be zero in free fall
Worked example 1 — on Earth. A body of mass kg:
which is also , using the gravitational unit from the previous part of this chapter.
Worked example 2 — on the Moon. The same body taken to the Moon, where :
**The mass is still kg. Nothing was removed from the body; only the pull on it changed — to roughly a sixth.
Worked example 3 — working backwards.** A body weighs N on Earth, so
Worked example 4 — a spring balance in a lift. A person of kg stands on a weighing machine in a lift. The machine reads the normal reaction, which is the apparent weight.
- Lift at rest or moving at a steady speed: N
- Lift accelerating upward at : N — the person feels heavier
- Lift accelerating downward at : N — lighter
- Lift in free fall, : N — weightless
**Throughout all four cases the mass stayed kg and the true weight stayed N. What changed was the reaction force, which is what a machine actually measures.
A beam balance works anywhere; a spring balance does not. A beam balance compares two masses, and since both are affected by gravity equally, the comparison holds on the Moon unchanged. A spring balance measures force, so its reading would fall to a sixth on the Moon. That is why a shopkeeper's beam balance is trustworthy and a spring balance has to be calibrated for where it is used.
Everyday speech uses "weight" for mass and physics does not.** A bag described as *weighing kg* has a mass of kg and a weight of N. Both statements are about the same bag, and only one of them is in the unit of a force.
What exactly is free fall, and when are you weightless?
A body is in free fall when gravity is the only force acting on it, and it is weightless when nothing pushes back against that fall — so there is no reaction force to feel.
Free fall. With only weight acting, the acceleration is
The mass cancels, so every body in free fall accelerates at regardless of what it is made of — the result the motion chapter established from the same two lines.
Weightlessness. Your sensation of weight comes from the normal reaction of whatever supports you. Remove the support and the sensation goes, even though gravity is unchanged.
Worked example 1 — the falling lift. A person of kg in a lift whose cable snaps. Both person and lift accelerate downward at , so the floor cannot press on the feet:
The person floats inside the lift. **Their weight is still N — the Earth is pulling as hard as ever — but the apparent weight is zero.
Worked example 2 — an astronaut in orbit. The space station and everything in it are in continuous free fall around the Earth, moving sideways fast enough that the curve of the fall matches the curve of the Earth. Everything falls together, so nothing presses on anything, and everything appears weightless.
Worked example 3 — a diver. From the moment a diver leaves the board until entering the water, gravity alone acts, so the diver is in free fall and feels weightless for that second or so.
Weightlessness is not the absence of gravity. At the orbit of a space station the Earth's pull is still most of its surface value. If gravity were absent the station would fly off in a straight line instead of circling, which is the first law applied to an orbit. So gravity is not merely present in orbit — it is the very thing keeping the station there.
How varies from place to place.
With altitude** — decreases as you go up, because you are further from the Earth's centre and the force falls as . On a high mountain it is measurably less than at sea level.
With depth — also decreases as you go down below the surface, and becomes zero at the centre of the Earth. Only the mass in the sphere below you pulls you inward, and that shrinks as you descend.
**So is greatest AT the surface** and falls off in both directions — upward and downward. That is a genuinely surprising shape, and it is the point most often got wrong: many students expect to be largest at the centre, where the whole Earth is around you, and in fact the pulls from all sides cancel there exactly.
With latitude — is least at the equator and greatest at the poles, for two reasons acting together. The Earth is not a perfect sphere but bulges at the equator, so the equatorial surface is further from the centre. And the Earth's rotation reduces the effective value at the equator, where the spin is fastest, while having no such effect at the poles.
Worked consequence. A pendulum clock or a spring balance calibrated at a pole would read slightly differently at the equator, because changed. A beam balance would be unaffected, which is the distinction the previous section drew.
Free fall. With only weight acting, the acceleration is
The mass cancels, so every body in free fall accelerates at regardless of what it is made of — the result the motion chapter established from the same two lines.
Weightlessness. Your sensation of weight comes from the normal reaction of whatever supports you. Remove the support and the sensation goes, even though gravity is unchanged.
Worked example 1 — the falling lift. A person of kg in a lift whose cable snaps. Both person and lift accelerate downward at , so the floor cannot press on the feet:
The person floats inside the lift. **Their weight is still N — the Earth is pulling as hard as ever — but the apparent weight is zero.
Worked example 2 — an astronaut in orbit. The space station and everything in it are in continuous free fall around the Earth, moving sideways fast enough that the curve of the fall matches the curve of the Earth. Everything falls together, so nothing presses on anything, and everything appears weightless.
Worked example 3 — a diver. From the moment a diver leaves the board until entering the water, gravity alone acts, so the diver is in free fall and feels weightless for that second or so.
Weightlessness is not the absence of gravity. At the orbit of a space station the Earth's pull is still most of its surface value. If gravity were absent the station would fly off in a straight line instead of circling, which is the first law applied to an orbit. So gravity is not merely present in orbit — it is the very thing keeping the station there.
How varies from place to place.
With altitude** — decreases as you go up, because you are further from the Earth's centre and the force falls as . On a high mountain it is measurably less than at sea level.
With depth — also decreases as you go down below the surface, and becomes zero at the centre of the Earth. Only the mass in the sphere below you pulls you inward, and that shrinks as you descend.
**So is greatest AT the surface** and falls off in both directions — upward and downward. That is a genuinely surprising shape, and it is the point most often got wrong: many students expect to be largest at the centre, where the whole Earth is around you, and in fact the pulls from all sides cancel there exactly.
With latitude — is least at the equator and greatest at the poles, for two reasons acting together. The Earth is not a perfect sphere but bulges at the equator, so the equatorial surface is further from the centre. And the Earth's rotation reduces the effective value at the equator, where the spin is fastest, while having no such effect at the poles.
Worked consequence. A pendulum clock or a spring balance calibrated at a pole would read slightly differently at the equator, because changed. A beam balance would be unaffected, which is the distinction the previous section drew.
Exam tip
Exam tip: state whether a quantity is mass or weight before calculating
Write which one you have. *Mass kg and weight N* are different lines, and a question giving "weight kg" means a mass of kg.
** with , and the answer is in newtons. Never give a weight in kilograms.
and are different.** is universal; is local. Quote both with their units.
Use centre-to-centre distance in , and square it.
For "what happens if" questions, reason with factors rather than recalculating: tripling one mass and doubling gives , so N becomes N.
Remember the distance dominates — it is squared, so halving beats doubling both masses.
**For a lift, use **: up gives , down gives , free fall gives zero.
Weightlessness means no reaction force, not no gravity. Say it in those words — the orbiting station is held in orbit by gravity.
** is maximum at the surface and decreases with both altitude and depth, reaching zero at the centre. It is least at the equator and greatest at the poles.
Give two reasons for the latitude variation: the equatorial bulge and the Earth's rotation.
And note a beam balance works anywhere, a spring balance does not** — one compares masses, the other measures a force.
** with , and the answer is in newtons. Never give a weight in kilograms.
and are different.** is universal; is local. Quote both with their units.
Use centre-to-centre distance in , and square it.
For "what happens if" questions, reason with factors rather than recalculating: tripling one mass and doubling gives , so N becomes N.
Remember the distance dominates — it is squared, so halving beats doubling both masses.
**For a lift, use **: up gives , down gives , free fall gives zero.
Weightlessness means no reaction force, not no gravity. Say it in those words — the orbiting station is held in orbit by gravity.
** is maximum at the surface and decreases with both altitude and depth, reaching zero at the centre. It is least at the equator and greatest at the poles.
Give two reasons for the latitude variation: the equatorial bulge and the Earth's rotation.
And note a beam balance works anywhere, a spring balance does not** — one compares masses, the other measures a force.
Did you know
Why the pull between two people is real but hopeless to feel
Two friends of kg each sit half a metre apart on a bench. The law of gravitation says they attract each other, so work out how hard:
Under a millionth of a newton. For comparison, each of them weighs N — so the Earth pulls each one about seven hundred million times harder than they pull each other.
The reason is the size of . At it is one of the smallest constants in physics, which makes gravitation by far the weakest of the fundamental forces. A small magnet can lift a pin against the pull of the entire Earth, and a rubbed comb can lift paper the same way — electric and magnetic forces beat gravity easily at everyday scales.
So why does gravity dominate the universe?
Because it never cancels. Electric charges come in two signs and ordinary matter is almost exactly neutral, so the huge electric forces inside a stone add up to nothing measurable outside it. Gravitational attraction has no opposite — every kilogram pulls every other kilogram, always inward — so it just keeps accumulating as bodies get bigger.
Put the Earth's mass into the same formula instead of kg and the force on a person becomes N. Nothing about the law changed; only one of the masses did.
That is the whole reason gravitation looks like two different phenomena. Between two people it is a curiosity too small to detect; between a planet and a person it holds you to the floor. One law, one constant, and a factor of a hundred thousand million million million in one of the masses.
Under a millionth of a newton. For comparison, each of them weighs N — so the Earth pulls each one about seven hundred million times harder than they pull each other.
The reason is the size of . At it is one of the smallest constants in physics, which makes gravitation by far the weakest of the fundamental forces. A small magnet can lift a pin against the pull of the entire Earth, and a rubbed comb can lift paper the same way — electric and magnetic forces beat gravity easily at everyday scales.
So why does gravity dominate the universe?
Because it never cancels. Electric charges come in two signs and ordinary matter is almost exactly neutral, so the huge electric forces inside a stone add up to nothing measurable outside it. Gravitational attraction has no opposite — every kilogram pulls every other kilogram, always inward — so it just keeps accumulating as bodies get bigger.
Put the Earth's mass into the same formula instead of kg and the force on a person becomes N. Nothing about the law changed; only one of the masses did.
That is the whole reason gravitation looks like two different phenomena. Between two people it is a curiosity too small to detect; between a planet and a person it holds you to the floor. One law, one constant, and a factor of a hundred thousand million million million in one of the masses.
Exam relevance
How is gravitation tested in JEE Main and NEET?
Because the inverse-square law reappears for electric charges, and the mass-against-weight distinction underlies every problem involving a lift, an orbit or another planet.
This is the foundation for Class 11 Physics Gravitation, examined in JEE Main and NEET. That chapter derives from this page's single formula the expression for itself,
where and are the Earth's mass and radius. Every variation described on this page then becomes a calculation: at height falls as , and at depth falls in proportion to the remaining radius. Numericals asking for on another planet, or at a stated height, are standard and use nothing but this formula.
Orbital velocity and escape velocity follow directly, and both are recurring JEE Main topics. The orbital case is the astronaut of the opening section made quantitative — a satellite is in free fall with enough sideways speed to keep missing the Earth, and equating the gravitational pull to the required centripetal force gives its speed. The conceptual point made here, that gravity is what holds the orbit rather than being absent from it, is exactly what that derivation says in symbols.
The inverse-square form transfers to electrostatics. Class 12 Electric Charges and Fields gives Coulomb's law as
the identical shape with charges in place of masses. So the factor reasoning practised on this page — double the separation and the force quarters — carries over unchanged, and questions in both chapters are solved the same way. The one difference worth knowing is the one raised in the previous section: charges cancel and masses do not.
Weightlessness and apparent weight are examined as lift problems in Class 11 Laws of Motion, using from this page, and as assertion-reason items about satellites in both exams.
Kepler's laws are added in Class 11 and shown to follow from the inverse-square law, which is why the chapter is placed where it is.
What the questions look like. For board work, expect state the law and calculate a force, **give the value and unit of , predict the change when masses or distance alter, distinguish mass from weight in a table, calculate weight on Earth and on the Moon, and explain weightlessness and the variation of . For JEE Main and NEET**, expect on another planet, satellite speed, escape velocity, apparent weight in a lift, and Coulomb's law problems using the same algebra.
How board and competitive emphasis differ. A board paper rewards the table of differences between mass and weight and a carefully worded explanation of weightlessness. A competitive paper assumes both and tests whether the inverse-square scaling can be applied in one step, without recomputing from the constant.
The single trap that costs the most marks. Saying that an astronaut is weightless because there is no gravity in space. There is gravity, and it is what keeps the station in orbit — the astronaut is weightless because nothing pushes back, both station and occupant falling together. The defence is to write the reaction force explicitly, with , which gives zero and shows precisely which quantity vanished and which did not.
This is the foundation for Class 11 Physics Gravitation, examined in JEE Main and NEET. That chapter derives from this page's single formula the expression for itself,
where and are the Earth's mass and radius. Every variation described on this page then becomes a calculation: at height falls as , and at depth falls in proportion to the remaining radius. Numericals asking for on another planet, or at a stated height, are standard and use nothing but this formula.
Orbital velocity and escape velocity follow directly, and both are recurring JEE Main topics. The orbital case is the astronaut of the opening section made quantitative — a satellite is in free fall with enough sideways speed to keep missing the Earth, and equating the gravitational pull to the required centripetal force gives its speed. The conceptual point made here, that gravity is what holds the orbit rather than being absent from it, is exactly what that derivation says in symbols.
The inverse-square form transfers to electrostatics. Class 12 Electric Charges and Fields gives Coulomb's law as
the identical shape with charges in place of masses. So the factor reasoning practised on this page — double the separation and the force quarters — carries over unchanged, and questions in both chapters are solved the same way. The one difference worth knowing is the one raised in the previous section: charges cancel and masses do not.
Weightlessness and apparent weight are examined as lift problems in Class 11 Laws of Motion, using from this page, and as assertion-reason items about satellites in both exams.
Kepler's laws are added in Class 11 and shown to follow from the inverse-square law, which is why the chapter is placed where it is.
What the questions look like. For board work, expect state the law and calculate a force, **give the value and unit of , predict the change when masses or distance alter, distinguish mass from weight in a table, calculate weight on Earth and on the Moon, and explain weightlessness and the variation of . For JEE Main and NEET**, expect on another planet, satellite speed, escape velocity, apparent weight in a lift, and Coulomb's law problems using the same algebra.
How board and competitive emphasis differ. A board paper rewards the table of differences between mass and weight and a carefully worded explanation of weightlessness. A competitive paper assumes both and tests whether the inverse-square scaling can be applied in one step, without recomputing from the constant.
The single trap that costs the most marks. Saying that an astronaut is weightless because there is no gravity in space. There is gravity, and it is what keeps the station in orbit — the astronaut is weightless because nothing pushes back, both station and occupant falling together. The defence is to write the reaction force explicitly, with , which gives zero and shows precisely which quantity vanished and which did not.
Key takeaways
Gravitation, mass, weight and weightlessness: quick revision
- Universal law of gravitation: , with the centre-to-centre distance, acting along the line joining them as an action-reaction pair.
- **, a universal constant**; its unit follows from .
- Two kg masses m apart attract with only N; kg and kg at m give N.
- ** is a constant, is a local acceleration — different quantities with different units.
- Scaling**: double one mass, force doubles; double both, ; double , ; triple , ; halve , .
- N with one mass tripled and doubled becomes N; both masses doubled and halved gives .
- The separation is squared, so it dominates — halving beats doubling both masses.
- Mass: quantity of matter, scalar, in kg, constant everywhere, measured by a beam balance, and the measure of inertia.
- Weight: the gravitational force, vector, in newtons, varies with place, measured by a spring balance, and can be zero in free fall.
- ****: kg weighs N on Earth and N on the Moon, with the mass unchanged at kg. A body weighing N has a mass of kg.
- Apparent weight in a lift, for a kg person: at rest or steady N; accelerating up at , N; down at , N; in free fall, N.
- Throughout, the mass and the true weight never changed — only the reaction force did.
- A beam balance works anywhere; a spring balance does not, since one compares masses and the other measures a force.
- Free fall means gravity is the only force, so and the mass cancels.
- Weightlessness means no reaction force, not no gravity. A falling lift, an orbiting station and a diver in mid-air are all weightless.
- Gravity is what holds an orbit — without it the station would fly off in a straight line, by the first law.
- ** is maximum AT the surface, decreasing with both altitude and depth, and reaching zero at the centre, where the pulls from all sides cancel.
- is least at the equator and greatest at the poles, for two reasons: the equatorial bulge and the Earth's rotation**.
- Two people of kg half a metre apart attract with only N, against a weight of N each — gravitation is the weakest fundamental force, and it dominates the universe only because it never cancels.
Work out the gravitational pull between yourself and your school bag on the desk beside you, then compare it with the bag's weight — and see how many zeros separate the two.
- **, a universal constant**; its unit follows from .
- Two kg masses m apart attract with only N; kg and kg at m give N.
- ** is a constant, is a local acceleration — different quantities with different units.
- Scaling**: double one mass, force doubles; double both, ; double , ; triple , ; halve , .
- N with one mass tripled and doubled becomes N; both masses doubled and halved gives .
- The separation is squared, so it dominates — halving beats doubling both masses.
- Mass: quantity of matter, scalar, in kg, constant everywhere, measured by a beam balance, and the measure of inertia.
- Weight: the gravitational force, vector, in newtons, varies with place, measured by a spring balance, and can be zero in free fall.
- ****: kg weighs N on Earth and N on the Moon, with the mass unchanged at kg. A body weighing N has a mass of kg.
- Apparent weight in a lift, for a kg person: at rest or steady N; accelerating up at , N; down at , N; in free fall, N.
- Throughout, the mass and the true weight never changed — only the reaction force did.
- A beam balance works anywhere; a spring balance does not, since one compares masses and the other measures a force.
- Free fall means gravity is the only force, so and the mass cancels.
- Weightlessness means no reaction force, not no gravity. A falling lift, an orbiting station and a diver in mid-air are all weightless.
- Gravity is what holds an orbit — without it the station would fly off in a straight line, by the first law.
- ** is maximum AT the surface, decreasing with both altitude and depth, and reaching zero at the centre, where the pulls from all sides cancel.
- is least at the equator and greatest at the poles, for two reasons: the equatorial bulge and the Earth's rotation**.
- Two people of kg half a metre apart attract with only N, against a weight of N each — gravitation is the weakest fundamental force, and it dominates the universe only because it never cancels.
Work out the gravitational pull between yourself and your school bag on the desk beside you, then compare it with the bag's weight — and see how many zeros separate the two.