A ball is released from stationary at a great height. Explain how the forces acting on it change before it hits the ground and how these forces affect the velocity of the ball.

The initial velocity of the ball is 0, and so the only relevant force acting on it is weight pulling it downwards, so, the ball accelerates towards the ground. As the ball falls it hits air particles which causes an upwards force in the opposite direction to its fall. The resolved force is still downwards so the ball still accelerates but at a slower rate. As the ball speeds up it hits more air particles per second, increasing the air resistance until the upwards force is equal to the downwards force of weight causing acceleration to be 0. The velocity it is now falling at is called 'terminal velocity' and the ball will fall at this constant until reaching the ground. It is important to note that this assumes no other external forces act upon the ball during its fall which in a real life situation is unlikely due to wind and changes in pressure or temperature during the fall. If the ball is irregularly shaped, any turning during the fall would also affect the air resistance as it is very reliant on surface area. It is assumed that the ball does actually reach terminal velocity before hitting the ground as it is said to have been dropped from a great height, if this was not specified, it wouldn't be assumed.