Apply Newton's laws of motion and the principles of force, momentum, stability and projectile motion to analyse sporting performance

What this dot point is asking

WACE expects you to use biomechanical principles as analysis tools. You should be able to state a principle, explain the underlying mechanics, then apply it to a named skill to justify good technique. Most marks come from the application, not the definition.

Newton's laws of motion

Newton's first law (inertia) states that a body stays at rest or in uniform motion unless an unbalanced external force acts on it. A stationary ball will not move until a foot applies force; a sprinter keeps decelerating after the line because of inertia.

Newton's second law states that the acceleration of a body is proportional to the net force applied and inversely proportional to its mass, written as force equals mass times acceleration. A greater applied force produces greater acceleration, which is why a harder kick sends the ball faster.

Newton's third law states that for every action force there is an equal and opposite reaction force. A sprinter pushes back and down on the blocks, and the ground pushes forward and up with an equal ground reaction force that drives the body out of the blocks.

Force summation

Force summation is the principle that maximum force is produced when as many body parts as possible are used, in the correct sequence and with correct timing. For a maximal throw the large, slow body parts move first (legs and trunk), transferring momentum to smaller, faster parts last (shoulder, elbow, wrist), so each segment adds to the velocity built by the one before. The sequence is proximal to distal, and the parts must be used in order rather than all at once.

Momentum and impulse

Momentum is mass times velocity. Because an athlete's mass is fixed during a skill, momentum is changed by changing velocity. Impulse is force multiplied by the time over which it acts, and impulse equals the change in momentum. This is why follow-through matters: extending the time a force is applied increases impulse and therefore the velocity given to a ball. The same relationship explains safe landings; bending the knees increases the time taken to stop, which reduces the force on the body for the same change in momentum.

Stability and balance

Stability is the resistance to a loss of balance. It increases with a lower centre of gravity, a larger base of support, keeping the line of gravity (the vertical projection of the centre of gravity) inside the base of support, and a greater body mass. A wrestler crouches low with feet wide to be hard to topple. The same principles explain instability: a sprinter in the set position raises the centre of gravity and moves the line of gravity to the front edge of the base so they can lose balance quickly forward at the gun.

Projectile motion

Once airborne, a projectile such as a ball or jumper follows a parabolic path governed by the angle, speed and height of release, with gravity acting downward and (in simple models) horizontal velocity constant. The horizontal and vertical components are independent: horizontal velocity covers distance while vertical velocity governs flight time. For release at ground level the optimal angle is about 45 degrees, but for releases above landing height (a shot put released from shoulder height) the optimal angle is lower, around 38 to 42 degrees, because the extra release height buys flight time.

How this maps to the exam

Expect a stimulus image and a command to apply a named principle to that skill. Always do three things: name the principle, explain the mechanics, then apply it specifically to the pictured athlete. Vague generic answers lose the application marks.