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Published: 28 August 2020

• physics
• mechanics

Dropped and Shot Masses: A Kinematics Experiment

Today we’re taking a look at a popular demonstration of how Newton’s Laws of Motion apply to falling objects. This device has a spring-loaded launcher on top of a tall stand. Two identical aluminum cubes are placed on it. The cube to your left is supported by the spring-loaded rod; the cube to your right is resting on the platform in front of the rod. When the spring is released, the rod will abruptly push to the right; it will release the cube on the left, allowing it to drop straight down, while it slams into the cube on the right, launching it out horizontally.

 Consider: Once the two cubes have left the launcher, what forces are acting on them? How will they accelerate?

 Try to predict which of the cubes will reach the ground first. The one the drops straight down, or the one launched out to the side? Or will they both reach the ground at once?

There are two key concepts to remember here: that a force is a vector that acts in a particular direction, and Newton’s principle of inertia: that a mass’s velocity (or lack of velocity!) remains the same until acted upon by an outside force.

 The cube on the right was given an initial velocity in the horizontal direction when it was struck by the rod, but not in the vertical direction. The cube on the left had no initial velocity in either the horizontal or vertical direction.

 The force of gravity is acting equally on the two identical cubes, pulling them down at the same rate. They will have the same vertical acceleration. This has no effect on the right-hand cube’s horizontal movement, and likewise its horizontal movement has no effect on gravity pulling it down.

So as a result, even though one has moved some distance away horizontally and the other has not, their vertical movement is identical, and they strike the ground at the same time! Check it out in this slow-motion video.