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A Classic Conservation of Momentum Experiment with PocketLab

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Submitted by Rich on Wed, 06/28/2017 - 03:26

A well-known conservation of momentum experiment that has been around for many years involves dropping a brick onto a horizontally moving cart.  With PocketLab and the VelocityLab app, your students can perform this experiment easily with the cart from the PocketLab Maker Kit and a small block of wood.  The snapshot below shows the setup with the author about to drop the block of wood onto the cart coming from the left.  A pair of rails, with inside separation just a little larger than the axle of the carts, was constructed with thin balsa wood sticks.  This is optional but does help to keep the cart on "the straight and narrow".

Ready to Drop Wood Block on Cart


The video below shows what a typical run of the experiment looks like.  The cart is given an initial push at the far left end of the track, receives the wood block near the middle of the track, and then the combined cart and wood block hit the styrofome bumper at the right end of the track.


The figure below shows position, velocity, and accelertion vs. time graphs that were constructed in Excel from data obtained from a .csv file created by the VelocityLab app.  The graphs are marked up in red, noting how the motion of the cart applies to the graphical interpretation of the data.

The two most important pieces of information needed to verify the law of conservation of momentum are identified with stars in the velocity graph: (1) the speed of the cart just before receiving the dropped wood block, and (2) the speed of the combined cart and wood block just after the receiving the wood block.  (Note that the mass of cart is 0.144 kg and the mass of the wood block is 0.096 kg.)

Position, Velocity, and Acceleration Graphs


With all of the above information, it is quite easy to compute the momentum of the system right before the cart receives the wood block [p = mv = 0.144 kg x 0.841 m/s = 0.121 kg-m/s], and right after the wood block is dropped [p = mv = (0.144 kg + 0.096 kg) x 0.524 m/s = 0.126 kg-m/s].  The momentum before and after agree within about 4%, providing good verification of the law of conservation of momentum.

(A VelocityLab.csv file for a typical run of this experiment is attached for those who may be interested in viewing it.)

Ready to Drop Block on Cart
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