Across

1. 2noun The force that slows down two surfaces that rub. (8)
2. 6noun Speed in a given direction. (8)
3. 7noun The amount of matter in an object. (4)
4. 9noun The force that pulls objects toward each other. (7)
5. 11verb To increase speed. (10)
6. 14noun The ability to do work. (6)
7. 15noun An icy object in space with a bright tail. (5)

Down

1. 1noun A push or a pull. (5)
2. 3adjective Impossible to see. (9)
3. 4noun An instrument to see objects far away in space. (9)
4. 5verb To travel around a planet or a star. (5)
5. 8adjective Able to attract iron. (8)
6. 9noun A huge system of stars, like the Milky Way. (6)
7. 10noun A large body that moves around a star. (6)
8. 12noun It travels at about 300,000 km per second. (5)
9. 13noun A movement that carries energy, like sound. (4)