Independence of vertical and horizontal motions
Practical Activity for 14-16
This is a simple experiment showing a difficult idea: that time of fall is unaffected by any horizontal motion.
Apparatus and Materials
Health & Safety and Technical Notes
Whether the activity is done inside or outside, only do this experiment with students that you can trust using stones as projectiles.
Do this in a place where stones will land on soft ground and not shatter or bounce.
There are many ingenious trigger devices for launching horizontal and vertical balls at the same time. They are normally based on one ball rolling down a ramp to hit a stationary ball at the end of the ramp. Both balls have the same mass and so their momentum is exchanged and the rolling ball falls vertically downwards and the once stationary ball follows a parabolic path.
Do the experiment in the open, if possible.
- Drop two stones vertically from the same height, one from each hand. Practice releasing them at the same time. Listen for them hitting the ground. They should land at the same time.
- Repeat, but give one of the stones a horizontal motion by moving one of your hands sideways as you let go.
- Repeat, but throw both stones out sideways with different velocities.
- Try it with stones of unequal size.
- Try dropping both stones from lower down or even from an open window or staircase.
- Try increasing the horizontal speed of the stone.
- The motion could be recorded using a video camera.
- Different masses fall in the same time, with the same vertical acceleration.
- The key point is that the fall of a stone from a particular height to the ground takes the same time whether or not it also has horizontal motion.
- The independence of the horizontal and vertical motion enabled Galileo to examine the horizontal motion of projectiles free from any accelerating force. This gave him an indirect way of observing motion when there was no force acting on the object. Hence he understood what later became known as Newton's first law.
This experiment was safety-tested in March 2005
- A different experiment also shows this, Monkey and Hunter: