(A Reflection of School-Time)
The button tip of the long metal épée swiftly approaches and the other child freezes, immobile. The flexible weapon bends into an arc immediately after the tip lands on the padded jacket. Touché! The attacker recovers easily from her lunge and the épée relaxes back into its straight form.
I teach fencing to fourth and fifth graders in my Title I school’s After School Program (ASP). Physics is a large part of the sport’s movements. Fencing is a smorgasbord of movements that require the balancing (or unbalancing) of the forces created most directly from mass and acceleration.
In the example above, Student A’s lunge creates the acceleration her extension needs to move the tip towards the target, while surprised novice Student B’s reaction is to do nothing. He is currently the object at rest and stays at rest throughout this bout exchange because his greater mass and the energy-absorbing safety gear prevent a significant transfer of momentum into his body (Newton’s First Law). Fencer A reflects this law as well because she herself had not moved forward until the energized force of her rear leg muscles pushed her — and her épée’s tip — medially forward along the fencing strip. (See Guide to lunging in fencing for a demonstration of how the back leg propels the fencer forward into a lunge.)
This brings Newton’s Second Law into the spotlight because Fencer A’s controlled movement had created an acceleration appropriate for both her mass and the distance she and her tip needed to cover in order to make a touch. If she had taken a step back, she would have needed to accelerate more in order to reach her opponent’s target area. If she had been closer, however, she would have run the risk of landing a touch with undue force, and/or the tip would have continued to slide, potentially off of the target completely, which brings the “an object in motion tends to stay in motion” portion of the First Law back into play, the angle of the jacket of the opposing fencer (as well as whether or not he moves) determining the angle in which the attacker’s tip slides past.
The immobility of Fencer B in the main example does not seem to reflect Newton’s Third Law, but the blade itself clearly illustrates the Law: “For every action there is an equal and opposite reaction”. When the épée tip landed, while the force was not enough to move the boy, his mass pushed back on the metal tip, and the weapon bent. An observer could argue that this bend resulted rather from Newton’s First Law — which of course would also be correct — but Fencer A (and everyone who has landed a good touch) would note that recovering out of this particular lunge felt easier, as if the force pushing against the tip helped push her out of her lunge — which is also true, as supported by Newton’s Third Law of motion!
The two young fencers talk after the touch, fifth-grade Fencer A reminding fourth-grade Fencer B about the parry defenses that we had been practicing during drills. I keep an eye and ear out for adjustments that might need to be made, and after she correctly reteaches him how to defend against her attack, I ask the fencers to think about how the energy flows throughout the movement. Here, for full fencing success, Newton’s First and Third Laws must be fully used, and not ignored.
The Olympics Channel (see link below) shows these two laws in action, how a parry deflects an attack — by actually physically moving the blade out of the way — and therefore opening a path to make a riposte, or return attack. During my ASP class, Fencer A lunges slowly at Fencer B, who moves his épée blade to make contact with her blade — click! His movement’s energy transfers to her blade, moving her tip off to the side and therefore causes it to no longer be pointing at his vulnerable target. At this point, he can use the energy flow to keep his épée in motion to continue pushing her épée away (Newton’s First Law), or he can opt to use the energy described by Newton’s Third Law as “an equal and opposite reaction” to allow his épée to bounce off of her blade and direct his tip towards her target in a hopefully smooth, successful riposte. I am constantly amazed that this click of the parry can result in both blades moving off target or just the parried one, depending on how the parrying fencer allows the inertia to be directed — even without adding acceleration or force to further direct the action.
Check out this video’s sections at 1:15-1:20 (parry 4) and at 1:41-1:47 (parry 6) to see the slow-motion movements of two fencing blocks that protect a fencer’s high-line target.
Video Links
Bashir, K. (2012, August 16). Guide to lunging in fencing. YouTube. https://www.youtube.com/watch?v=yY0LwH47n6Y
Olympics Channel. (2017, September 12). How to parry/defend in fencing: Olympians’ tips. YouTube. https://www.youtube.com/watch?v=vbEETULHshE