# Gravity and acceleration near to the speed of light

1. May 3, 2010

### Smarky

It is well known that a mass and acceleration can have the same "gravity effect" on an object.
Let's assume that an astronaut is being accelerated in a spaceship for a long time by 9.8m/s, which I presume means that he will feel the same gravity as near earth.
But when approaching the speed of light, the effect of energy turning into mass is strongly visible.
Which mean that the change in speed will decrease?
Although the same amount of energy is accelerating the spaceship.
Will the astronaut feel less "gravity" when approaching the speed of light?

2. May 3, 2010

### tiny-tim

Hi Smarky!
The 9.8 m/s2 will be as felt by the astronaut, in the astronaut's (non-inertial) frame of reference …

he will not approach the speed of light, he will remain stationary inside his spaceship, exactly as if it was resting on the Earth's surface.

3. May 3, 2010

### Nickelodeon

Relatively speaking, although the acceleration will be decreasing his observed mass will increase and ensure the pressure he feels on the soles of his feet remains the same.