That footballer has a mass of 70.0 kg and accelerates at 3.20 m/s^2 with air resistance negligible. What average force did he exert backward on the ground?

1. With air resistance negligible, the force he exerts backward equals the net external force on him. 2. Newton's second law gives the net force as the mass times the acceleration. 3. Substitute the values: (70.0 kg)(3.20 m/s^2). 4. That gives 224 N. 5. Quoting 686 N uses the acceleration due to gravity and returns his weight instead. _Source: OpenStax College Physics (CC BY 4.0), Ch 4 "Dynamics: Force and Newton's Laws of Motion", section 4.7 Further Applications of Newton's Laws of Motion_