It sounds like this“The Moon stays up because gravity pulling it in is balanced by a force pushing it out, like a tug of war that ends in a draw.”
Thinking an outward force balances gravity
Why it feels right
You have felt this. In a car going round a bend you are pressed against the door, as if something is pushing you outward. On a roundabout you feel flung out. So when something moves in a circle, it is natural to say the pull toward the centre is balanced by a push away. 'Centrifugal force' is a word everybody has heard, and balancing it against gravity makes the Moon's steadiness look like a tug of war that ends in a draw.
Why it’s wrong
If the forces balanced, the Moon would not turn: it would go in a straight line. The pull is not balanced. It is that unbalanced pull, always at right angles to the Moon's motion, that bends its path into a circle. The 'outward push' people feel in a turn is just their own inertia.
A worked example
Try a test. If the pull in and the push out balanced exactly, the net force on the Moon would be zero. A thing with zero net force moves in a straight line at a steady speed, by Newton's first law. The Moon would sail off in a straight line and never go round.
The Moon does go round. So the force on it is not zero: there is a net pull toward Earth, and that is what turns its path. The pull needed to turn something moving at speed v on a circle of radius r is an acceleration of v squared divided by r. For the Moon that is the 0.0027 m/s² that gravity supplies, so gravity provides exactly the turning and nothing else is needed.
The push you feel in the car isn't a force from outside. It is your body trying to carry straight on while the car turns, and the door pushes you round. Seen from outside, there is only one force: the door pushing in.
How to catch yourself
- You explain orbits by 'gravity balances the centrifugal force'.
- You think an orbiting object must feel no net force.
- You describe the pull as 'cancelling' something.
What to hold instead
The forces on the Moon don't cancel: gravity alone acts, pulling it toward Earth, and that unbalanced pull is what bends its path into a circle. Without it the Moon would travel in a straight line. The 'outward push' you feel when turning is your own tendency to go straight on.
Where you’ll meet it
- Explaining why mud flies off a spinning wheel in a straight line, not outward.
- Understanding why a ball on a string flies off straight when the string is cut.
- Seeing why a car's tyres need grip, a sideways force, to take a bend.
Questions people ask
- Is centrifugal force real?
- It is real as a feeling, and as a convenient idea in a spinning frame of reference, but it isn't a force that some outside object exerts. Seen from outside, there is only the inward force that turns the path, and the body's own tendency to carry straight on.
- What would happen if gravity switched off for the Moon?
- It would fly off in a straight line at about a kilometre a second, along the direction it was moving at that instant, not outward from Earth. Gravity is what keeps bending its path into a circle.