Free fall: dropping a ball from a tower
Drop a ball from towers of different heights, on the Earth and on the Moon, with a light and a heavy ball. Use the equations of motion to find the time it takes and how fast it lands.
Core concept
A body falling freely speeds up with the acceleration due to gravity, g (9.8 m/s² on the Earth); from rest, h = ½gt² and v = gt, so v² = 2gh, and the time does not depend on the mass
Topic: Force and motion
When a body falls with only gravity: The pull that makes things fall towards the Earth. All key words acting on it (no air resistance: How much something slows down the flow of electricity, measured in ohms (Ω). All key words), it is in free fall. It starts from rest (u = 0) and gains speed: How far something travels in a certain time, like kilometres per hour. All key words steadily: on the Earth its velocity grows by 9.8 m/s every second, so its acceleration is g = 9.8 m/s². The equations of motion v = u + at, s = ut + ½at² and v² = u² + 2as then become v = gt, h = ½gt² and v² = 2gh. So the time to fall is t = √(2h ÷ g) and the landing speed is v = √(2gh). Galileo showed that, without air, a heavy body and a light one fall together. On the Moon, g is only 1.6 m/s², so things fall much more slowly. In this lab there is no air resistance.
What you will learn
- Define free fall and the acceleration due to gravity.
- Use the equations of motion to find the time of fall and the final velocity.
- Explain why the time of fall does not depend on mass.
- Compare free fall on the Earth and on the Moon.
Before you start
- Know the equations of motion v = u + at and s = ut + ½at².
Materials
- Two balls of different mass but the same size
Equipment
- A stopwatch or a phone's slow-motion video, a measuring tape
Procedure
- Drop the 1 kg ball from three or more heights on the Earth, and record each.
- Drop the 5 kg ball from the same height.
- Drop the ball from 20 m on the Moon.
How this simulation is simplified
- There is no air resistance.
- g is 9.8 m/s² on the Earth and 1.6 m/s² on the Moon.