AQA · A-Level · Physical Education
Course version: 7582 · Version from Sept 2016
First law (inertia), second law (acceleration), third law…
Full specification wording
Biomechanical principles: First law (inertia), second law (acceleration), third law (action/reaction). Force.
Newton's three laws describe how force changes motion, and each law maps onto a sporting action.
Checked against the AQA 7582 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Newton's three laws describe how force changes motion, and each law maps onto a sporting action. The first law (inertia) states that a body stays at rest or keeps moving at constant velocity unless a resultant external force acts, so a sprinter keeps travelling until friction and air resistance slow her. The second law (acceleration) states that resultant force equals mass times acceleration (F = m × a), so a larger force or smaller mass gives greater acceleration; a lighter rower accelerates her boat more quickly for the same applied force. The third law (action/reaction) states that for every action there is an equal and opposite reaction, so a swimmer pushes water backwards and the water pushes her forwards. Force is the push or pull, measured in newtons (N), that causes or resists this change in motion.
What earns the marks
- First law (inertia): a body remains at rest or moves at constant velocity unless acted on by a resultant external force.
- Second law (acceleration): resultant force equals mass multiplied by acceleration, F = m × a, so acceleration is proportional to resultant force and inversely proportional to mass.
- Third law (action/reaction): for every action force there is an equal and opposite reaction force acting on a different body.
- Force is a push or pull measured in newtons (N) that can change a body's state of motion or shape.
- Applying the laws to sport: a lighter performer accelerates more for the same force (second law), and a swimmer's backward push on water produces a forward reaction (third law).
Common mistakes and how to put them right
Thinking the first law means a moving object always needs a continuous force to keep moving.
Put it right: Constant velocity needs no resultant force, only a change in motion does.
Believing the third law action and reaction forces cancel on the same body.
Put it right: They act on two different bodies, so they do not cancel each other's effect on either one.
Confusing mass with force or using the wrong unit.
Put it right: Mass is measured in kilograms (kg) and force in newtons (N), and F = m × a links them.
Assuming a heavier performer always accelerates faster because they can push harder.
Put it right: For the same resultant force a larger mass gives smaller acceleration.
Exam tips
- Read the stem carefully to identify which law is being tested before looking at the options, since inertia, acceleration and action/reaction are easily confused.
- Check the units in each option: force is in newtons (N), mass in kilograms (kg) and acceleration in metres per second squared (m/s²).
- Eliminate options that describe action and reaction forces as acting on the same body, because that contradicts the third law.
- When a calculation is embedded, write F = m × a and substitute values before choosing, rather than estimating.
Try the lesson
The start of the Biomechanical principles: First law (inertia), second law (acceleration), third law (action/reaction). Force. lesson, made from checked teaching notes for this specification point.
Have a go first
Which quantity is defined as a push or a pull that can change the motion or shape of a body?
Listen
Newton's three laws describe how force changes motion, and each law maps onto a sporting action. The first law (inertia) states that a body stays at rest or keeps moving at constant velocity unless a resultant external force acts, so a sprinter keeps travelling until friction and air resistance slow her. The second law (acceleration) states that resultant force equals mass times acceleration (F = m × a), so a larger force or smaller mass gives greater acceleration; a lighter rower accelerates her boat more quickly for the same applied force. The third law (action/reaction) states that for every action there is an equal and opposite reaction, so a swimmer pushes water backwards and the water pushes her forwards. Force is the push or pull, measured in newtons (N), that causes or resists this change in motion. A strong answer earns marks for these points. First law (inertia): a body remains at rest or moves at constant velocity unless acted on by a resultant external force. Second law (acceleration): resultant force equals mass multiplied by acceleration, F = m × a, so acceleration is proportional to resultant force and inversely proportional to mass. Third law (action/reaction): for every action force there is an equal and opposite reaction force acting on a different body. Force is a push or pull measured in newtons (N) that can change a body's state of motion or shape. A common slip: Thinking the first law means a moving object always needs a continuous force to keep moving. Instead, constant velocity needs no resultant force, only a change in motion does. Exam tip: Read the stem carefully to identify which law is being tested before looking at the options, since inertia, acceleration and action/reaction are easily confused.
Key words
Say what each word means before you turn the card.
Newton's first law and inertia in sporting motion
Try answering aloud or in your head before turning the card.
Newton's second law and the F = m × a relationship
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Newton's third law and action/reaction force pairs
Try answering aloud or in your head before turning the card.
Force as a push or pull measured in newtons
Try answering aloud or in your head before turning the card.
See every definition
- Newton's first law and inertia in sporting motion
- A body stays at rest or keeps moving at constant velocity unless a resultant external force acts.
- Newton's second law and the F = m × a relationship
- Resultant force equals mass times acceleration, so a larger force or smaller mass gives greater acceleration.
- Newton's third law and action/reaction force pairs
- For every action there is an equal and opposite reaction.
- Force as a push or pull measured in newtons
- A push or pull, measured in newtons (N), that causes or resists a change in motion.
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