The simplest example illustrating the use of the impact parameter is in the case of scattering from a sphere. Here, the object that the projectile is approaching is a hard sphere with radius R {\displaystyle R} . In the case of a hard sphere, U ( r ) = 0 {\displaystyle U(r)=0} when r > R {\displaystyle r>R} , and U ( r ) = ∞ {\displaystyle U(r)=\infty } for r ≤ R {\displaystyle r\leq R} .