# Free fall Motion with only gravity acting. Near Earth's surface every object gains about 9.8 m/s of speed each second, ignoring air. Physics (university) · Part 01: Motion and vectors · Card 5 of 8 · Last updated 2026-10-19 ## Analogy A feather and a hammer drop together on the Moon because there is no air. ## Example **Drop from 20 m** d = 1/2 g t^2 20 = 4.9 t^2 t = 2.0 s v = g t = 19.8 m/s ## In practice A stone dropped from a 20 m bridge takes about 2 seconds to land. ## Practice ### Flashcards - **What value of g do we use near Earth's surface?** About 9.8 m/s^2, pointing down. ## Questions people ask ### Do heavier objects fall faster? Without air, no. All objects accelerate at the same rate, about 9.8 m/s^2, because a heavier object has more weight but also more inertia. In real air, a feather falls slowly because air drag is large compared with its weight, not because it is light. ### Why do we use 9.8 m/s^2 for gravity? It is the average acceleration of falling objects near Earth's surface. The value changes slightly with place, from about 9.78 at the equator to 9.83 at the poles. For most problems 9.8 m/s^2 is accurate enough, and some courses round it to 10. ## Related concepts - [Displacement](https://studyvisually.com/learn/science/physics-university/physics-university-part-1/physics-university-part-1-concept-1) - [Velocity](https://studyvisually.com/learn/science/physics-university/physics-university-part-1/physics-university-part-1-concept-2) - [Acceleration](https://studyvisually.com/learn/science/physics-university/physics-university-part-1/physics-university-part-1-concept-3) - [Constant-acceleration equations](https://studyvisually.com/learn/science/physics-university/physics-university-part-1/physics-university-part-1-concept-4) Web page: https://studyvisually.com/learn/science/physics-university/physics-university-part-1/physics-university-part-1-concept-5 Study this card in the app: https://studyvisually.com/app/learn?category=physics-university&card=physics-university-part-1-concept-5