4.2 How forces act Forces are pushes or pulls that try to change the shape or position of an object. Forces always act in pairs that work in opposite directions. If you are holding an object in the air, you are exerting an upward force on it, but the object is also exerting a downward force. Force diagram show the direction and size of forces on an object. Each force is shown as a force arrow. When the forces are equal, the force arrows are the same length. The downward force on an object is caused by gravity pulling it down.