Jessica's weight is 540 newtons. According to the Third Law, if the Earth pulls on Jessica with a force of 540 N, then Jessica pulls on the Earth with a force of 540 newtons that acts in the opposite direction. However, if Jessica stumbles while rollerblading, she will fall down to the ground. Explain why this happens.

She's been acted on by an outside force. Newton's First Law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. As Newton's third law states that for every external force that acts on an object there is a force of equal magnitude but opposite direction which acts back on the object which exerted that external force. As Jessica has been acted upon by a force that caused her to stumble, the forces will become unbalanced and therefore cause her to fall.