Newton’s Third Law and Ground Reaction Forces
Newton’s Third Law explains how forces occur in pairs. In IB Sports, Exercise and Health Science (SEHS), this law is crucial for understanding how athletes move by interacting with the ground or other objects. It forms the basis for analysing running, jumping, and pushing movements through ground reaction forces.
Newton’s Third Law states that for every action force, there is an equal and opposite reaction force.
Understanding Action–Reaction Force Pairs
When an athlete applies a force to an object or surface, that object or surface applies an equal and opposite force back on the athlete.
Key points include:
- Forces always occur in pairs
- The forces are equal in size
- The forces act in opposite directions
In sport, this principle explains how movement is generated despite forces being equal.
What Are Ground Reaction Forces?
Ground reaction force (GRF) is the force exerted by the ground on the body when the body applies force to the ground.
For example:
- A runner pushes down and backward against the ground
- The ground pushes up and forward on the runner
This reaction force propels the athlete forward. Without sufficient ground reaction force, acceleration and speed would be limited.
Ground Reaction Forces in Running
During running:
- Greater force applied to the ground results in greater reaction force
- Direction of force application affects movement direction
- Efficient technique maximises useful reaction force
Sprinters apply large backward forces to generate forward acceleration. Distance runners apply smaller forces over longer periods to maintain speed efficiently.
In IB SEHS, linking force direction to movement outcome is essential.
Ground Reaction Forces in Jumping
Jumping is another clear example of Newton’s Third Law.
During a jump:
- The athlete applies force downward against the ground
- The ground provides an upward reaction force
- The greater the applied force, the higher the jump
This explains why powerful concentric contractions are critical for explosive jumping performance.
Technique and Injury Considerations
Ground reaction forces are also important for injury analysis. Poor landing technique can increase the magnitude of reaction forces, placing stress on joints and tissues.
Understanding GRFs helps explain:
- Injury risk during landings
- Importance of shock absorption
- Role of technique in force distribution
Exam Relevance for IB SEHS
Newton’s Third Law questions often require applied explanations. High-mark answers clearly describe action–reaction force pairs and link ground reaction forces to movement, acceleration, or injury risk.
Avoid stating the law without applying it to sport.
RevisionDojo Call to Action
If Newton’s Third Law and ground reaction forces feel abstract, RevisionDojo helps IB SEHS students master force application with clear explanations and exam-style practice. It is the most effective way to turn physics principles into exam confidence.
Frequently Asked Questions (FAQs)
What is Newton’s Third Law in simple terms?
For every force applied, there is an equal and opposite force acting back.
What are ground reaction forces?
They are forces exerted by the ground on the body when the body applies force to the ground.
How is Newton’s Third Law tested in IB SEHS exams?
Students are often asked to apply the law to movements such as running or jumping and explain how force produces motion.
