# Inertia without gravity

Is there inertia in absence of gravity? If I was in a region of space with zero gravity, would I have to apply some force to accelerate a massive body?

• This can be answered in 4 characters, but comments can't be that short: $F=ma$ Commented Nov 24, 2020 at 16:42
• Equivalence_principle states that inertial mass and gravitational mass is the same thing, as per $ma=m~GM/r^2$. Commented Jan 17 at 11:43

Is there inertia in absence of gravity?

Inertia is just another word for mass. So yes, there is mass in absence of gravity.

If I was in a region of space with zero gravity, would I have to apply some force to accelerate a massive body?

Yes, of course. This it what Newton's second law says: $$\vec{F}=m\vec{a}$$

You need a force to accelerate a mass. This law is independent of gravity. Gravity is just one possible cause of a force. But there are many other possible causes for a force.

• Accelerate wrt what? Commented Nov 25, 2020 at 17:06
• @user45664 Literally anything. Acceleration is just a change in velocity, which can be measured in any reference frame you want. Commented Nov 30, 2020 at 21:36
• @NuclearHoagie That's incorrect and highly misleading. The acceleration that is present in Newton's second law is w.r.t. a very special class of reference frames, namely, inertial frames (or local inertial frames in general). You can't just measure acceleration w.r.t. "[l]iterally anything".
– user87745
Commented Jan 6, 2022 at 10:05
• @DvijD.C. Acceleration in any frame requires a force in that frame, whether real or fictitious. An object in orbit in an inertial frame accelerates inward due to a real centripetal force. If we instead treat the object at rest in a non-inertial rotating frame, it does not accelerate and has no forces acting on it, but when released from orbit accelerates outward due to a fictitious centrifugal force. Acceleration certainly can be measured with respect to anything, it just gets more complicated in non-inertial frames. Show me two objects between which you cannot measure acceleration. Commented Jan 6, 2022 at 14:36
• @NuclearHoagie Obviously I am not saying that acceleration cannot be measured w.r.t. whichever frame you want to measure it. Newton's second law, however, is physically sensible only in inertial frames. Once you have got the physics right with your footing in inertial frames, you can use fictitious forces as a trick if you want to use non-inertial frames. However, that's just a trick, all calculations in a non-inertial frame have to refer to the acceleration of the said non-inertial frame w.r.t. the inertial frames in order to make correct predictions.
– user87745
Commented Jan 6, 2022 at 18:06

Inertia is a property of mass, and it exists in the absence of gravity and you would have to apply a force to accelerate a body even if the object is massive.

• "Inertia is a property of mass"? Inertia is a property of physical objects, mass is the concrete formulation/description of inertia.
– user87745
Commented Jan 6, 2022 at 10:14

Every answer here is wrong. Inertia and gravity are NOT independent, they are two aspects of the same phenomena. You cannot have one without the other (or without mass, which again is another way of looking at the same phenomena).

In your hypothetical scenario, the massive body in question would necessarily have its own gravitational field so there couldn't be 'zero gravity'. If there were no gravitational field then the body could not have mass, in which case it couldn't be accelerated because it would always be traveling at $$c$$.

• I agree with your objection to the other answers but the issue you raise is essentially moot. The point mass in the absence of external gravitational field would be effectively in zero gravity because it won't be exerting a non-zero force on itself (unless it has some complicated mechanism inside it which makes it radiate gravitational waves).
– user87745
Commented Jan 6, 2022 at 10:08
• Also, it is not true that you can have zero gravity with no mass. Even massless particles create a gravitational field, e.g., light. :)
– user87745
Commented Jan 6, 2022 at 10:09

In order for us to better understand the inertial force and answer this question, can someone prove that this statement is true? Imagine an object with mass is in zero gravity. Applying a force to this object at a point other than its center of gravity would rotate the object. If an inertial force didn't exist, the object would accelerate forward, but because of the inertial force occurring at its center of mass in the opposite direction of the applied force, a force couple occurs which rotates the object. Is this true?