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 4 replaced http://physics.stackexchange.com/ with https://physics.stackexchange.com/ edited Apr 13 '17 at 12:39 Spacecraft in orbit around the Earth are constantly accelerated by the gravitational field of Earth. That's why the spacecraft ($$m \ll M$$) is in an (elliptical) orbit around the centre of gravity of the Earth, accelerated by $$\mathbf{g}=-{G M \over r^2}\mathbf{\hat{r}} \, .$$ Plugging in the numbers for a spacecraft orbiting at roughly 420 km (such as the International Space Station), this gives: $$\mathbf{g} = {- 3.986 \cdot 10^{14} \over (6371 + 420) \cdot 10^3 m} = - 8.6 \, \mathrm{m}/\mathrm{s}^2$$ So, an astronaut on-board the ISS is in a reference frame that is constantly accelerating at an acceleration of $$8.6 \, \mathrm{g}/\mathrm{m}^2$$. Yet unlike the astronauts featured in this related questionthis related question, they do in fact feel no gravitational acceleration at all; at most they may feel some centrifugal pseudoforce, but this is considerably less than the gravitational acceleration, and at most at microgravity levels. Is acceleration only perceived when it changes the magnitude of the velocity, as opposed to the direction? What is the fundamental reason for this? Or am I misunderstanding something? Spacecraft in orbit around the Earth are constantly accelerated by the gravitational field of Earth. That's why the spacecraft ($$m \ll M$$) is in an (elliptical) orbit around the centre of gravity of the Earth, accelerated by $$\mathbf{g}=-{G M \over r^2}\mathbf{\hat{r}} \, .$$ Plugging in the numbers for a spacecraft orbiting at roughly 420 km (such as the International Space Station), this gives: $$\mathbf{g} = {- 3.986 \cdot 10^{14} \over (6371 + 420) \cdot 10^3 m} = - 8.6 \, \mathrm{m}/\mathrm{s}^2$$ So, an astronaut on-board the ISS is in a reference frame that is constantly accelerating at an acceleration of $$8.6 \, \mathrm{g}/\mathrm{m}^2$$. Yet unlike the astronauts featured in this related question, they do in fact feel no gravitational acceleration at all; at most they may feel some centrifugal pseudoforce, but this is considerably less than the gravitational acceleration, and at most at microgravity levels. Is acceleration only perceived when it changes the magnitude of the velocity, as opposed to the direction? What is the fundamental reason for this? Or am I misunderstanding something? Spacecraft in orbit around the Earth are constantly accelerated by the gravitational field of Earth. That's why the spacecraft ($$m \ll M$$) is in an (elliptical) orbit around the centre of gravity of the Earth, accelerated by $$\mathbf{g}=-{G M \over r^2}\mathbf{\hat{r}} \, .$$ Plugging in the numbers for a spacecraft orbiting at roughly 420 km (such as the International Space Station), this gives: $$\mathbf{g} = {- 3.986 \cdot 10^{14} \over (6371 + 420) \cdot 10^3 m} = - 8.6 \, \mathrm{m}/\mathrm{s}^2$$ So, an astronaut on-board the ISS is in a reference frame that is constantly accelerating at an acceleration of $$8.6 \, \mathrm{g}/\mathrm{m}^2$$. Yet unlike the astronauts featured in this related question, they do in fact feel no gravitational acceleration at all; at most they may feel some centrifugal pseudoforce, but this is considerably less than the gravitational acceleration, and at most at microgravity levels. Is acceleration only perceived when it changes the magnitude of the velocity, as opposed to the direction? What is the fundamental reason for this? Or am I misunderstanding something? 3 retagged; | link edited Feb 11 '13 at 14:15 Qmechanic♦ 111k1212 gold badges214214 silver badges13141314 bronze badges 2 fixed typo edited Feb 11 '13 at 13:14 gerrit 2,4491313 silver badges2626 bronze badges Spacecraft in orbit around the Earth are constantly accelerated by the gravitational field of Earth. That's why the spacecraft ($$m \ll M$$) is in an (elliptical) orbit around the centre of gravity of the Earth, accelerated by $$\mathbf{g}=-{G M \over r^2}\mathbf{\hat{r}} \, .$$ Plugging in the numbers for a spacecraft orbiting at roughly 420 km (such as the International Space Station), this gives: $$\mathbf{g} = {- 3.986 \cdot 10^{14} \over (6371 + 420) \cdot 10^3 m} = - 8.6 \, \mathrm{g}/\mathrm{m}^2$$$$\mathbf{g} = {- 3.986 \cdot 10^{14} \over (6371 + 420) \cdot 10^3 m} = - 8.6 \, \mathrm{m}/\mathrm{s}^2$$ So, an astronaut on-board the ISS is in a reference frame that is constantly accelerating at an acceleration of $$8.6 \, \mathrm{g}/\mathrm{m}^2$$. Yet unlike the astronauts featured in this related question, they do in fact feel no gravitational acceleration at all; at most they may feel some centrifugal pseudoforce, but this is considerably less than the gravitational acceleration, and at most at microgravity levels. Is acceleration only perceived when it changes the magnitude of the velocity, as opposed to the direction? What is the fundamental reason for this? Or am I misunderstanding something? Spacecraft in orbit around the Earth are constantly accelerated by the gravitational field of Earth. That's why the spacecraft ($$m \ll M$$) is in an (elliptical) orbit around the centre of gravity of the Earth, accelerated by $$\mathbf{g}=-{G M \over r^2}\mathbf{\hat{r}} \, .$$ Plugging in the numbers for a spacecraft orbiting at roughly 420 km (such as the International Space Station), this gives: $$\mathbf{g} = {- 3.986 \cdot 10^{14} \over (6371 + 420) \cdot 10^3 m} = - 8.6 \, \mathrm{g}/\mathrm{m}^2$$ So, an astronaut on-board the ISS is in a reference frame that is constantly accelerating at an acceleration of $$8.6 \, \mathrm{g}/\mathrm{m}^2$$. Yet unlike the astronauts featured in this related question, they do in fact feel no gravitational acceleration at all; at most they may feel some centrifugal pseudoforce, but this is considerably less than the gravitational acceleration, and at most at microgravity levels. Is acceleration only perceived when it changes the magnitude of the velocity, as opposed to the direction? What is the fundamental reason for this? Or am I misunderstanding something? Spacecraft in orbit around the Earth are constantly accelerated by the gravitational field of Earth. That's why the spacecraft ($$m \ll M$$) is in an (elliptical) orbit around the centre of gravity of the Earth, accelerated by $$\mathbf{g}=-{G M \over r^2}\mathbf{\hat{r}} \, .$$ Plugging in the numbers for a spacecraft orbiting at roughly 420 km (such as the International Space Station), this gives: $$\mathbf{g} = {- 3.986 \cdot 10^{14} \over (6371 + 420) \cdot 10^3 m} = - 8.6 \, \mathrm{m}/\mathrm{s}^2$$ So, an astronaut on-board the ISS is in a reference frame that is constantly accelerating at an acceleration of $$8.6 \, \mathrm{g}/\mathrm{m}^2$$. Yet unlike the astronauts featured in this related question, they do in fact feel no gravitational acceleration at all; at most they may feel some centrifugal pseudoforce, but this is considerably less than the gravitational acceleration, and at most at microgravity levels. Is acceleration only perceived when it changes the magnitude of the velocity, as opposed to the direction? What is the fundamental reason for this? Or am I misunderstanding something? 1 asked Feb 11 '13 at 13:00 gerrit 2,4491313 silver badges2626 bronze badges