When the neck supplies the force on the head necessary to rapidly change its motion, then the tissues of the neck will in return feel the same forces back from the head. If that force is large enough to cause injury, we refer the result as a whiplash injury.
Top: Forces on the head from the neck (black) and on the neck from the head (red) during rapid forward-back motion of the head. Bottom: Sites of whiplash injury. Image Credit: 3rd Law Whiplash is a derivative of Whiplash Injury by BruceBlaus, via Wikimedia Commons
The force exerted on the neck by the head is due to Newton's Third Law, which states that for every force applied by an object on a second object, a force equal in size, but opposite in direction, will be applied to the first object by the second object. The equal and opposite forces are known as third law pair forces (or third law pairs). The force on the head from the neck is a Third Law pair with the force on the neck from the head, as illustrated in the previous image.
Other Third Law pair forces include:
- The Earth pulls down on you due to gravity and you pull back up on the Earth due to gravity.
- A falling body pushing air out of its way and the drag force pushing back on the body.
- You pull on a rope and the rope pulls back against your hand via tension force (friction acts between rope and hand in both cases)
- You push on the wall, and the wall pushes back with a normal force.
- A rocket engine pushes hot gasses out the back, and the gasses push back on the rocket in the forward direction.
- You push your hand along the wall surface, and the wall pushes back on your hand due to kinetic friction.
- You push your foot against the ground as you walk, and the floor pushes back against your food due to friction (static if your foot doesn’t slip, kinetic if it does).
You may have noticed that in each of the cases above there were two objects listed. This is because Newton’s Third Law pairs must act on different objects. Therefore, Third Law pair forces cannot be drawn on the same free body diagram and can never cancel each other out. (Imagine if they did act on the same object, then they would always balance each other out and no object could ever have a net force, so no object could ever accelerate!)
Draw the free body diagrams necessary to show each force in the Third Law pairs listed above. How many free body diagrams will you need to draw for each Third Law pair? Keep in mind the rule about free body diagrams and Third Law pairs…