Motion under gravity: all cases
Dropped, thrown up, thrown down, from towers and balloons — one framework: a = ±g with signs.
In this lesson
- Dropped, thrown up, thrown down, from towers and balloons — one framework: a = ±g with signs.
Every gravity problem is the same problem with different signs and initial velocities.
Pick down positive. Dropped: u = 0. Thrown down: u > 0. Thrown up: u < 0. From a balloon rising at w: the released object keeps u = +w (up!) before gravity turns it.
The 'released from a moving balloon' case is the classic trap: the object inherits the balloon's velocity.
Gravity motion with down positive
Worked example
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.
- Area under the a–t graph = change in velocity.
- Area = 2 m/s² × 4 s = 8 m/s (the last 2 s add nothing).
Answer: 8
Transcript (0 min)
WEBVTT 1 00:00:00.000 --> 00:00:05.000 Motion under gravity: all cases — FemtoLearn. 2 00:00:05.000 --> 00:00:15.000 Dropped, thrown up, thrown down, from towers and balloons — one framework: a = ±g with signs.
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.