Free fall and motion under gravity
Free notes · JEE Main Physics
Formulas
Free-fall time and impact speed
Key points
- Sign convention: if up is positive, g is NEGATIVE — get the sign wrong and the whole answer flips.
Worked example
A stone is dropped from a 45 m tower (g = 10 m/s²). How long does it take to hit the ground?
- u = 0, a = g = 10, s = 45.
- s = ½gt² → t = √(2×45/10) = √9 = 3 s.
Answer: t = 3 s.