Sports Biomechanics: Levers, Rotation, Newton’s Laws
Third-Class Lever Example: Knee Extension
Q1: An example of a third-class lever is knee extension (like kicking a ball), where the force is between the fulcrum and load. The knee joint is the fulcrum (the pivot point where the lower leg rotates), the quadriceps tendon, inserting on the tibial tuberosity, provides the effort in the middle, and the leg and foot (plus any ball) form the load at the far end.
Landing Technique: Shock Absorption in Gymnastics
Q2: Gymnasts bend their knees and flex their hips when they land so their legs act like shock absorbers, increasing the time over which they come to a stop. By spreading the change in momentum over a longer time, the same impulse is applied with a smaller average force, which reduces the impact on their joints and lowers the risk of injury. This controlled slowing down also helps them maintain balance and stick the landing more effectively.
Moment of Inertia and Angular Velocity
Q3: Moment of inertia is how much a body resists changing its rotational motion, and it depends on how far the mass is distributed from the axis of rotation. Angular velocity is the rate at which the body is rotating. Therefore, if the moment of inertia changes, angular velocity must change in the opposite way.
In gymnastics on the floor, a gymnast changes body position during a somersault: when the gymnast tucks (knees to chest, arms in), moment of inertia decreases and angular velocity increases so they spin faster; when the gymnast opens into a straight body position, the moment of inertia increases and angular velocity decreases so the rotation slows for landing.
Tennis Serve: Newton’s Three Laws in Action
Q4: A tennis serve is a clear example of how one motor skill uses all three of Newton’s laws of motion at the same time. Before the serve, the ball and the player’s body are at rest and stay that way until forces are applied, which shows Newton’s first law (inertia). As the player swings, the legs, trunk, and arm apply a force to the racquet and then to the ball; because the ball’s mass is fixed, a larger force produces a larger acceleration and a faster serve, which is Newton’s second law F = ma. During this motion, Newton’s third law appears in each action–reaction pair: the player pushes down and back on the ground and the ground pushes up and forward on the player, helping them drive into the shot, and at impact the racquet pushes on the ball while the ball pushes back equally on the racquet, which the player must control to keep the serve powerful and accurate.
Improving a Beam Dismount Using Biomechanics
Q5: To improve a beam dismount, a gymnast can use four biomechanical principles. Stability (Principle 1) is increased by lowering the center of mass and using a solid, slightly wider stance before take-off and on landing, improving balance. Sequencing of joint rotation (Principle 3) is applied by extending in order from hips to knees to ankles and feet, with coordinated arm swing, to generate maximum rotational speed. The impulse–momentum relationship (Principle 4) is used by bending deeply, then pushing explosively against the beam to create a large impulse, giving more height and time in the air. Finally, the direction of movement principle (Principle 5) is applied by pushing down and slightly backward on the beam so the reaction force sends the gymnast up and forward, then pushing down/back on landing to slow and stabilize the body.
- Stability (P1): Lower center of mass, wider stance.
- Sequencing (P3): Hips → knees → ankles → feet with arm coordination.
- Impulse–momentum (P4): Deep bend then explosive push for greater impulse.
- Direction of movement (P5): Push down/back to direct reaction forces up/forward.
Bernoulli’s Principle and Lift
Q6: Bernoulli’s principle describes how moving air creates pressure differences that can generate lift. Diffusion is the movement of particles from an area of high concentration to an area of low concentration; by analogy, in fluids pressure differences cause air to move from high pressure toward low pressure. When air moves faster over the top of a wing, the pressure there becomes lower than the pressure under the wing. The air under the wing is like the “high concentration” or high-pressure side, and it pushes toward the “low concentration” or low-pressure side above the wing, creating an upward lift force.