text: Assume the forearm is pointed straight forward, so the force is directed along its long axis. Calculate the force on the arm from F=ma, with m 3.0kg, and a the acceleration 80kph/.0047sec (fix the units so it's all m/sec first). Then you have the force, and you have the cross-sectional area that's bearing that load, so F/A=P to get the pressure or stress. Maybe divide by two if there's two bones.