From the schematic:

1. Set the DSO for a single sweep triggered by channel 1 going high.

2. Pull M1 away from M2 and M3, which are just barely touching,
and let them settle for a few seconds.

3. Arm the DSO.

4. Release M1 so that it hits M2 directly across from where M3 is
touching M2.

5. Read the time it took for channel 2 to go low after channel 1 went
high. That'll be the time it took for the momentum accumulated by
M1 to be transferred to M3.

--
JF