Smash Together—Half the Speed? Exploring Momentum with Colliding Carts!
I’m Ken Kuwako, your Science Trainer. Every day is an experiment!
“CRASH!” Two objects collide and start moving together as one. We see these “collisions” all the time in our daily lives—whether it’s a fender bender or a contact sport. But hidden within these moments are the beautiful yet strict “Rules of Physics” that hold true anywhere in the universe. Today, I’ll show you an experiment using two dynamic carts to reveal the mystery of how Momentum changes before and after a crash.
Tools for Measuring the Invisible Force of Motion
To investigate Momentum—which in physics represents the “oomph” or “power” of an object’s motion—we are using two handy speed sensors called “BeeSpi.” These little devices are a scientist’s best friend, instantly measuring the speed of any object that passes through them.
「Bee-Spi」
The dynamic carts used in this experiment are calibrated to exactly 1kg. By conducting the experiment on this larger scale, we can see the “trends” of physical laws clearly, without being distracted by tiny errors like friction. First, let’s check out the experiment in this video:
Can Dollar Store Items Power Science?
The method is simple, but we use one “magic item.” To make the two carts instantly “fuse” upon impact, we use double-sided suction cups. By attaching these to one of the carts, they stick together the moment they collide and continue moving as a single unit. Believe it or not, these suction cups are common items you can find at a local 100-yen shop or dollar store.
The “Half-Speed” Truth Hidden in the Data
First, we collided two carts with no extra weights (meaning they were the same mass). Looking at the results from one of the student groups, some surprisingly clean numbers appeared.
Notice that the speed after the collision is almost exactly half of the initial speed. When the mass doubled (two carts joined together), the speed was cut in half… This carries a very important message: momentum is conserved.
Next, we added a 0.5kg weight to Cart B to make it a bit heavier and repeated the experiment.
When the weight changes, how does the change in speed shift?
Deciphering the Deep Link Between Mass and Velocity
When we plot the initial speed (horizontal axis) against the post-collision speed (vertical axis) for these two experiments, a beautiful straight line emerges.
The slope of this graph actually tells the story of the relationship between the carts’ masses. Take a close look at how the slope changes between the weighted and unweighted trials. The key to this successful experiment was that tiny tool: the double-sided suction cup. Using everyday items to touch the fundamental laws of the universe, like the Law of Conservation of Momentum—that is the true joy of science!
https://phys-edu.net/wp/?p=31705
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