| A | B |
|---|
| friction | a force that stops motion |
| inertia | the property of objects that makes them resist changes in motion |
| gravity | a force that pulls all objects toward the earth |
| gravitational attraction | a force that exists between all objects in the universe |
| tides | the daily changes in the level of the earth's waters |
| flood tide | the incoming or rising tide |
| ebb tide | the outgoing or falling tide |
| spring tides | extra high tides and extra low tides |
| neap tides | tides that are not as high or as low as usual |
| ellipse | an oval shape |
| elliptical orbit | an oval-shaped path |
| perihelion | the point in an orbit at which a planet is closest to the sun |
| aphelion | the point in an orbit at which a planet is farthest from the sun |
| orbital velocity | the speed at which a planet travels along its orbit |
| star trails | lines on photographs of the stars that are caused by the motion of the earth when a time exposure is made |
| pendulum | a weight on a string that swings back and forth |
| solar day | the time it takes for the sun to appear at the same place twice in a row |
| leap year | a year that has 366 days, instead of 365 |
| thrust | the forward force on a rocket |
| escape velocity | the starting speed needed by a rocket to escape the earth's gravity (40,250 kilometers per hour) |
| multistage rocket | a rocket with many stages |
