Is there any kind of incident or force that would accelerate/decelerate the earth?

In fact, the earth does speed up and slow down a little bit though out the year because our orbit is elliptical so, when we are moving closer to the sun, earth is speeding up and vice versa.

As far as enough acceleration to "feel"... The answer about the earth quake is right and the answers about meteor strikes are sort of right (but it would be hard to distinguish between the acceleration of the earth itself from the violent shaking of the earth due to the impact. Recognize that the Fukashima earthquake raised a big section of the Japanese mainland about 8 feet and it "rang" the earth like a bell (the ringing was detected by seismographs all around the world) and that earthquake was really really tiny compared to an impact with a meteor big enough to change earth orbit.

Anything that could, you wouldn't live to see. Or see through.

Only a planetary collision would noticeably speed up or slow down Earth's motion around the sun. While *some* scientists have postulated the notion that billions of years from now one or more of the inner planets could collide such an event, if anything was still alive to witness it, would be the last thing they saw. Because at the very least it would sterilize the planet, and if the angle was head-on, it would shatter the Earth altogether.

Actually, the Earths motion is not undetectable to us because of it's constant motion, but because we are massively attracted to it. We would still be just as attracted to it even if it came to a sudden halt and would therefore not feel a thing.

Of course a massive enough impact needed to affect the Earths movement would also extinguish, at least the vast majority of life, and certainly all complex life on Earth anyway.

Earthquakes, no matter how local, provide the exact feel of acceleration you describe. It may not be the constant acceleration in a single direction, but it fits.

Don't make the mistake of confusing movement with acceleration. Acceleration is a change in tire tigon or intensity of movement.

About the only thing that comes to mind would be a major (and I mean *major*) impact with another body...

Even if a black hole were to swoop through the solar system, and steal Earth from the sun, (providing it didn't come within a certain distance), Earth would be pulled from solar orbit, and we wouldn't "feel" anything... Other than getting colder....

I would have to say a planetary collision. Or if the sun exploded the shock wave might be felt.

Nothing short of that would effect us on earth.

The force of motion is different to the earth. Because of gravity . If it gains enough speed it could mess with the gravity on our planet and could make it hard to breath. Just guessing