Asked by: Adhuay Rao, by email
It depends on the direction. An upwards acceleration of about 5g is enough to overwhelm the ability of your heart to pump blood to your brain. This causes oxygen starvation and you will black out within a few seconds.
Downward, or negative, g-force is even worse. The blood pools in your head, your face swells up and your lower eyelids are forced over your eyes. This is called ‘redout’ because all you see is the light shining through your eyelids. At negative 3g, the blood can’t get back to your lungs to re-oxygenate, so you pass out.