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Welcome back! Today we’re taking a look at a popular demonstration related to the concept of relativity.

 We’re accustomed to thinking about the motion of a projectile from a perspective outside of its motion, the generally safer option in real life! The PhET collection of physics simulations has a lovely one for seeing how different parameters like mass, gravity, and air resistance affect the motion of a projectile; try it out here: https://phet.colorado.edu/en/simulations/projectile-motion

 When we observe and measure motion, we are inevitably making the measurement against some frame of reference. An inertial reference frame is the technical term for a frame of reference in which an object is observed to have no outside forces acting on it, so that it is moving freely in space. Sometimes we have to go to great lengths to determine what such a frame of reference might be – and in the case of this demonstration, it is literally a metal frame!

 Demonstration P1-02: The Inertial Reference Frame, a large aluminum framework with a mounted winch to lift it.

In demonstration P1-02 in our collection, two spring-powered cannon have been pointed so that if a projectile came out of either of them and moved in a straight line, the projectile would pass through a hole in a transparent barrier and then land in a sophisticated projectile catchment mechanism, also known as a sock. But of course, if we just launch a ball out of the cannon, that doesn’t happen! As soon as the ball leaves the cannon, it starts to fall due to the acceleration of gravity, following a parabolic path, so it slams into the transparent barrier far below the hole.

But, if we raise up the whole aluminum frame that holds the cannon, barrier, and catchment, and then drop it, we can fire the cannon while the aluminum frame is falling. Now, from the perspective of the frame, there’s no separate acceleration pulling the ball down, because the frame is falling at the same rate that the ball is! So the projectile moves “straight” across the frame, through the hole, and lands in the catchment. Meanwhile, from our own perspective outside the experiment, we see the ball following a parabolic path just like always, while the whole experiment falls down.

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