- Tension force - the forces exerted by the two birds on the worm.
- Gravitational force - the force of gravity pulling the worm down.
- Frictional force - the force of friction between the worm and the ground.
If the worm moves towards the first bird, it means that the tension force exerted by the first bird is greater than the tension force exerted by the second bird. The resultant force acting on the worm is in the direction of the first bird, causing it to move towards the bird.