Newton’s Three Laws of linear motion applied to sporting movements.

The idea

This section requires you to apply Newton’s three laws of linear motion to sporting movements. Newton’s first law states that a body remains at rest or moves with constant velocity unless acted upon by a resultant force; for example, a stationary football remains still until a player kicks it. Newton’s second law states that the resultant force acting on a body is proportional to the rate of change of momentum, often expressed as F = m × a; for example, a larger force from a rugby player produces greater acceleration of an opponent. Newton’s third law states that for every action there is an equal and opposite reaction; for example, a sprinter pushes backwards on the blocks and the blocks push the sprinter forwards.

What earns the marks

• State Newton’s first law: a body remains at rest or moves with constant velocity unless acted upon by a resultant force.
• Apply Newton’s first law to sporting examples, such as a stationary ball remaining still until kicked.
• State Newton’s second law: the resultant force is proportional to the rate of change of momentum, often expressed as F = m × a.
• Apply Newton’s second law to calculate force, mass or acceleration in a sporting context.
• State Newton’s third law: for every action there is an equal and opposite reaction.
• Apply Newton’s third law to sporting examples, such as a sprinter pushing back on starting blocks to move forwards.

Common mistakes and how to put them right

• Thinking that Newton’s first law means a moving object always needs a continuous force to keep moving.

  Put it right: In fact, constant velocity requires no resultant force.

• Confusing Newton’s second and third laws.

  Put it right: The second law relates force, mass and acceleration, while the third law describes action–reaction pairs.

• Believing that action and reaction forces cancel out on the same object.

  Put it right: They act on different objects.

• Forgetting that Newton’s second law involves the resultant force, not just any single force.

Exam tips

• In MCQs, match the key phrase in the question to the law: 'constant velocity' points to the first law, 'F = m × a' to the second, and 'equal and opposite' to the third.
• When calculating with F = m × a, ensure mass is in kilograms and acceleration in m s⁻² to give force in newtons.
• For action–reaction questions, identify the two different objects involved to avoid the misconception that forces cancel.