4 ms·

I think best way to look at this is m = F/a. Then mass is basically objects resistance to force.

saagarjha 7y ago
Inertia is a property of matter…
Anon84 7y ago
This is a perfectly fine way of looking at it. But here's an interesting question: - We know that Force is proportional to acceleration. Let's call the proportionality constant, m_1=F/a. - We also know that objects are attracted to the Earth with a proportionality constant: m_2= F r^2/(GM) and we can certainly set: - F == m_1 a == G m_2 M/r^2 to calculate the acceleration of the gravitational force. But why should m_2==m_1? In other words, why is "inertial mass" (m_1) the same as gravitational mass (m_2)?
- saagarjha 7y ago
  Follow up question: is there an “electrical” inertia? Why does gravity have only one relevant quantity but electromagnetic forces need two?
  - Anon84 7y ago
    Well, it really doesn't. The "problem" is that charge is always associated with mass, with the charge generating electromagnetic fields and the mass being responsible for the gravitational one
- cygx 7y ago
  Because gravity is a pseudo-force: Free fall is inertial motion, tracing out a straight path through a curved spacetime. In a way, it's more like m·(a - a_grav) = F_nongrav
- p1mrx 7y ago
  When you drop a hammer, its freefall follows a geodesic, which could be considered a "straight line" through spacetime... at least until the ground gets in the way. In some sense, you could say that the ground is accelerating upward at a constant 9.8 m/s². When we feel weight, that's just regular inertia, pushing back against the ground's acceleration. (I'm not sure to what extent this follows the mainstream interpretation of Einstein's equivalence principle. It at least requires a different definition of "acceleration", given that the Earth's radius is not increasing.)
kps 7y ago
Yes, but… why does this resistance to force determine gravitational force? It's not trivial that inertial mass = gravitational mass.

Inertia is a property of matter…

This is a perfectly fine way of looking at it. But here's an interesting question: - We know that Force is proportional to acceleration. Let's call the proportionality constant, m_1=F/a. - We also know that objects are attracted to the Earth with a proportionality constant: m_2= F r^2/(GM) and we can certainly set: - F == m_1 a == G m_2 M/r^2 to calculate the acceleration of the gravitational force. But why should m_2==m_1? In other words, why is "inertial mass" (m_1) the same as gravitational mass (m_2)?

saagarjha 7y ago
Follow up question: is there an “electrical” inertia? Why does gravity have only one relevant quantity but electromagnetic forces need two?
- Anon84 7y ago
  Well, it really doesn't. The "problem" is that charge is always associated with mass, with the charge generating electromagnetic fields and the mass being responsible for the gravitational one
cygx 7y ago
Because gravity is a pseudo-force: Free fall is inertial motion, tracing out a straight path through a curved spacetime. In a way, it's more like m·(a - a_grav) = F_nongrav
p1mrx 7y ago
When you drop a hammer, its freefall follows a geodesic, which could be considered a "straight line" through spacetime... at least until the ground gets in the way. In some sense, you could say that the ground is accelerating upward at a constant 9.8 m/s². When we feel weight, that's just regular inertia, pushing back against the ground's acceleration. (I'm not sure to what extent this follows the mainstream interpretation of Einstein's equivalence principle. It at least requires a different definition of "acceleration", given that the Earth's radius is not increasing.)

kps 7y ago

Yes, but… why does this resistance to force determine gravitational force? It's not trivial that inertial mass = gravitational mass.