Conservation of Momentum: Steel ball collides with Magnet

A steel ball rolls along a level track at a known speed.  It collides with a neodymium magnet at the end of the track.  The velocity of the attached masses is measured after the collision.

Basic Idea of the Experiment

Students give the ball a consistent x-velocity by rolling it down an incline from a fixed height they choose.  They figure out its x-velocity by seeing how far from the table edge the ball lands on the floor.  Then they put the magnet in the path of the ball — at position M in the drawing — and repeat the process so that the ball and magnet hit and attach as they launch off the table.  Again they find the new distance the pair goes before hitting the floor to measure the velocity after collision.

  • Lab table
  • A light, stable ramp — we used some old tracks from toy “Hot Wheels” taped to a ring stand.
  • A 16 mm steel ball bearing, weighing ≈ 16 grams
  • Neodymium magnet  — ours was 18 mm diameter x 9 mm tall and weighed 19 grams.   But other choices are possible and could be interesting!


To measure the velocity of an object such as the steel ball just before rolling off the table alone, or the steel ball after sticking to the magnet, students must:

  1. Figure out the time of flight by measuring the height of the table and then using h = ½ g t 2
  2. Measure the horizontal distance, on the floor, between the edge of the table and the landing point X.
Student Worksheet Downloadable Word Doc

We used the following word document as a lab procedure in the PHY121 class.  However, it can be used in PHY111 or possibly in PHY101 as well.

PHY121 Worksheet for Momentum Conservation Lab

  • The worksheet asks for predictions about conservation of momentum and energy, and guides the student through the data collection process.  Finally the following questions are posed:
  1. Conclusion: Are the initial and final momentum equal?  Is this what you expected?
  2.  Are the initial and final kinetic energy equal?  Is this what you expected?
  3.  How much heat is produced in the collision?  This heat is what percentage of the original kinetic energy?

Terryl Fender, Prof. of Physics, South Mountain Community College, Phoenix, AZ

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