Free fall and motion under gravity
Free fall is constant acceleration g toward the ground — with air resistance ignored.
In this lesson
- Free fall is constant acceleration g toward the ground — with air resistance ignored.
A feather and a hammer fall together on the Moon. Gravity accelerates everything equally.
Near Earth, a = g ≈ 9.8 m/s² (take 10 m/s² when stated). Choose the downward direction as +x, then a = +g and the equations of motion apply unchanged.
Classic results: time to fall from height h: t = √(2h/g); speed on impact: v = √(2gh).
Free-fall time and impact speed
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.
Transcript (0 min)
WEBVTT 1 00:00:00.000 --> 00:00:05.000 Free fall and motion under gravity — FemtoLearn. 2 00:00:05.000 --> 00:00:15.000 Free fall is constant acceleration g toward the ground — with air resistance ignored.
Frequently asked
Does heavier fall faster?
In vacuum, no — g is the same for all masses.
Practice
Free · 4 questions with full solutions- Q1 · Numerical · difficulty 2/5
A stone is dropped from rest from a height of 10 m. Taking g = 10 m/s², find the time it takes to reach the ground (neglect air resistance).
- Q2 · Numerical · difficulty 2/5
The acceleration of a body starting from rest is a = 2 m/s² for 4 s, then zero for 2 s. Find its velocity at t = 6 s.
- Q3 · MCQ · difficulty 2/5
A particle moves with constant velocity. Which a–t graph describes it?
- A horizontal line at a = 0
- A horizontal line at a = 2 m/s²
- A straight line through the origin
- A parabola
- Q4 · Numerical · difficulty 2/5
A body moves with v = 10 m/s for 3 s, then v = 5 m/s for 2 s in the same direction. Find the total distance.