Newton's three laws explain how athletes and sporting objects begin moving, accelerate, decelerate, change direction and interact with surfaces. Movement depends on the relationship between inertia, resultant force, mass and interacting force pairs.
The Reasoning and Mechanism
| Law | Principle | Application to sport |
|---|---|---|
| Newton's first law | An object remains at rest or continues moving at constant velocity unless acted upon by a resultant external force. This property is called inertia. | A stationary football remains still until kicked. A moving ball slows because friction and air resistance create a resultant force opposite its motion. |
| Newton's second law | Acceleration is proportional to resultant force and inversely proportional to mass: . | A greater resultant force produces greater acceleration. For the same force, a heavier athlete or object accelerates less. |
| Newton's third law | When one body exerts a force on another, the second body exerts an equal-magnitude, opposite-direction force on the first. | A sprinter pushes backward and downward on the track; the track exerts a forward and upward ground reaction force on the sprinter. |
For example, if a sprinter experiences a horizontal resultant force of , the acceleration is:
The second law refers to the resultant force, not one individual force. If propulsive force and resistive force are equal, the resultant force is zero, so there is no acceleration even though the athlete may continue at constant velocity.
A common misconception is that third-law forces cancel each other. They do not, because they act on different bodies: the athlete pushes on the ground, while the ground pushes on the athlete.
Exam Technique
For IB SEHS subtopic B.2.1, name the relevant law, state it accurately, and apply it to a specific sporting example. For calculations, show the formula, substitute values with units and give the final SI unit. When explaining the third law, identify both interacting bodies and the direction of each force.