An adhesive for a tennis ball of the present disclosure includes base rubber and a filler. The base rubber contains, as a main component thereof, liquid rubber having a number average molecular weight of not less than 10,000. The filler has an average nitrogen specific surface area of not less than 40 m.sup.2/g. A tennis ball of the present disclosure includes a hollow core made of a rubber material. The core includes two hemispherical half cores. The two half cores are adhered to each other by using the adhesive for a tennis ball.

An adhesive for a tennis ball of the present disclosure includes base rubber and a filler. The base rubber contains, as a main component thereof, liquid rubber having a number average molecular weight of not less than 10,000. The filler has an average nitrogen specific surface area of not less than 40 m.sup.2/g. A tennis ball of the present disclosure includes a hollow core made of a rubber material. The core includes two hemispherical half cores. The two half cores are adhered to each other by using the adhesive for a tennis ball.

An adhesive for a tennis ball includes base rubber. The base rubber contains, as a main component thereof, liquid rubber having a number average molecular weight of not less than 10,000. The adhesive has a shear viscosity of not greater than 2,000 Pa.Math.s. A tennis ball includes a hollow core made of a rubber material. The core includes two hemispherical half cores. The two half cores are adhered to each other by using the adhesive for a tennis ball.

An adhesive for a tennis ball includes base rubber. The base rubber contains, as a main component thereof, liquid rubber having a number average molecular weight of not less than 10,000. The adhesive has a shear viscosity of not greater than 2,000 Pa.Math.s. A tennis ball includes a hollow core made of a rubber material. The core includes two hemispherical half cores. The two half cores are adhered to each other by using the adhesive for a tennis ball.

Systems and methods for a ball game comprising a playing field having a modified soccer goal wherein said goal having a foldable stand placed above its center holding a rigid basket and at least one playing ball wherein said ball is spherical in shape and having extruded ribs, wherein said ball have at least one sensor is embedded in it to allow detection of said ball location in said field.

A table tennis ball which has, on the spherical surface thereof, a marking to make a ball rotation measurable. The marking contains a number of marking points, which are distributed on the ball surface in such a way that the standard deviation of the lengths of the orthodromes between each of the marking points and the three nearest neighboring points thereof is at least 12% of the average value of these lengths, and the minimum length of the orthodromes between each of the marking points and the three nearest neighboring points thereof is at least 40% of the average value of these lengths.

A table tennis ball which has, on the spherical surface thereof, a marking to make a ball rotation measurable. The marking contains a number of marking points, which are distributed on the ball surface in such a way that the standard deviation of the lengths of the orthodromes between each of the marking points and the three nearest neighboring points thereof is at least 12% of the average value of these lengths, and the minimum length of the orthodromes between each of the marking points and the three nearest neighboring points thereof is at least 40% of the average value of these lengths.

A pitching training tool includes: a hollow ball; a shaft; and a wing. The shaft has one end attached to the ball, and another end attached to the wing. The ball and the wing are spaced apart from each other by a length equal to or greater than a sum of a diameter of the ball and a length of the wing with the shaft interposed between the ball and the wing.

A non-spherical training ball for training a pitcher to throw a ball with proper technique is provided. The training ball has a hollow body having a spherical surface forming a spherical cap having a height greater than the radius of the sphere from which the spherical cap is formed and a substantially planar surface forming the spherical cap plane. Sidewalls form the hollow cavity of the hollow body, the sidewalls have a spherical surface sidewall portion forming the spherical surface of the hollow body and a planar surface sidewall portion forming the substantially planar surface of the hollow body. The sidewall thickness of the planar surface sidewall portion has a thickness greater than the sidewall thickness of the spherical surface sidewall portion.

A non-spherical training ball for training a pitcher to throw a ball with proper technique is provided. The training ball has a hollow body having a spherical surface forming a spherical cap having a height greater than the radius of the sphere from which the spherical cap is formed and a substantially planar surface forming the spherical cap plane. Sidewalls form the hollow cavity of the hollow body, the sidewalls have a spherical surface sidewall portion forming the spherical surface of the hollow body and a planar surface sidewall portion forming the substantially planar surface of the hollow body. The sidewall thickness of the planar surface sidewall portion has a thickness greater than the sidewall thickness of the spherical surface sidewall portion.