forcefully accelerate through the effect of gravity

the car would hit the first dip, then slingshot off the second rise

a forked stick, to which an elastic strap (or a pair of elastic bands connected by a small sling) is fastened to the two prongs, typically used for shooting small stones; a catapult

the effect of the gravitational pull of a celestial object in accelerating and changing the course of another object or a spacecraft

stars passing near a black hole might be ejected by a gravitational slingshot effect