An experiment to measure the properties of the collisions between two
small spheres or between a small sphere and a semi-infinite flat wall
are described. The apparatus releases the particles in a free-fall wit
hout initial spin. The impacts are modeled in terms of three coefficie
nts. The first is the coefficient of normal restitution. The second re
presents the frictional properties of the contact surfaces. The last c
haracterizes the restitution of the tangential components of the veloc
ity of the contact point for impacts that do not involve sliding. The
coefficients are calculated from stroboscopic photographs of the balli
stics of the particles near the collision. The results establish that
the collision model provides an accurate description of the dynamics o
f the impacts.