Net Force

Which forces act on the ball in this picture?

The football is in the air, nobody is touching it. The only forces that can act on it are gravity and possibly a drag force due to the air. Gravity is the dominant force, and it is pointing straight down towards the ground. The ball is accelerating towards the ground, its velocity is changing, pointing more and more towards the ground. An unbalanced force is acting on the ball.


The player is in the air, nobody is touching him. The only forces that can act on him are gravity and possibly a drag force due to the air. Gravity is the dominant force, and it is pointing straight down towards the ground. The player is accelerating towards the ground. An unbalanced force is acting on the player.	The player is standing on the ground, nobody is touching him. The forces that act on him are gravity and a force exerted by the ground. Gravity is pointing straight down towards the ground. The force exerted by the ground is pointing up. The two forces cancel each other, resulting in no net force. The player does not accelerate upward or downward. The forces are balanced.

To find the total force acting on a player, we have to add the all the individual forces acting on him. But when adding forces, we must take care of the directions.

	When adding two forces pointing in the same direction, the magnitude of the resulting force is equal to the sum of the magnitudes of the two forces. Result: Big net force
	When adding two forces pointing in opposite direction, the magnitude of the resulting force is equal to the absolute value of the difference of the magnitudes of the two forces. Result: Smaller net force
	When adding two forces having the same magnitude but pointing in opposite direction, the resulting net force is zero. Result: The forces are balanced, we have zero net force.