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Instead of hunting for a PDF of answers, use:
A ball rolls off a 20 m high cliff with a horizontal velocity of 15 m/s. How far from the base of the cliff does it land?
In projectile motion, up is usually positive, so gravity ($a$) is ($-9.8 \text m/s^2$). If you use $+9.8$, your time or height will be incorrect (often a negative square root). Compare your work to the textbook answer; if they got a positive time but you got an imaginary number, check your signs.
Keep this formula sheet next to you while you check your answers:
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