Newton’s laws of motion — AQA A-Level Physics

Newton's laws of motion are three fundamental principles that describe the relationship between the motion of an object and the forces acting on it. They form the basis of classical mechanics and are essential for understanding topics such as dynamics, momentum, and energy. In AQA A-Level Physics, you will apply these laws to solve problems involving constant and non-constant acceleration, equilibrium, and systems of connected bodies.

These laws are crucial because they allow you to predict how objects move under the influence of forces, from everyday situations like a car accelerating to more complex scenarios like a rocket launching. They also underpin many other areas of physics, including circular motion, oscillations, and gravitational fields. A solid grasp of Newton's laws is therefore essential for success in both exams and practical work.

• →Newton's first law: An object remains at rest or moves with constant velocity unless acted upon by a resultant force. This introduces the concept of inertia and equilibrium.
• →Newton's second law: The resultant force acting on an object is directly proportional to the rate of change of momentum. For constant mass, F = ma, where F is the resultant force, m is mass, and a is acceleration.
• →Newton's third law: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A. These forces are of the same type and act on different objects.
• →Free-body diagrams: Essential for identifying all forces acting on a single object and resolving them to find the resultant force.
• →Equilibrium: When the resultant force on an object is zero, it is in equilibrium and its acceleration is zero (it may be at rest or moving with constant velocity).

• State the first law as constant velocity or rest when the resultant force is zero, and link it to inertia.
• Use the second law as resultant force equals rate of change of momentum, reducing to F = ma when mass is constant.
• State the third law as two forces equal in magnitude, opposite in direction, of the same type, acting on two different bodies.
• Apply the laws to a named situation by drawing a free-body diagram and resolving forces before calculating acceleration or identifying equilibrium.
• Distinguish equilibrium (zero resultant force, constant velocity) from a body with a non-zero resultant force, which accelerates.
• Identify F as the resultant force, found by adding all forces as vectors, not any single applied force.
• Substitute consistent SI units: newtons, kilograms and metres per second squared.
• Rearrange F = ma correctly to calculate acceleration or mass when required.
• Resolve forces on a slope or at an angle before applying F = ma along the chosen direction.
• Interpret a negative acceleration as deceleration or acceleration opposite to the chosen positive direction.

• 💡Name the law you are using and quote its condition before substituting values.
• 💡Draw and label a free-body diagram; examiners reward clear force identification.
• 💡Check units and directions: acceleration is in the direction of the resultant force.
• 💡Write the equation, rearrange symbolically, then substitute numbers to reduce arithmetic slips.
• 💡State the direction of the resultant force and acceleration in your answer.
• 💡For slope problems, show the resolved weight component before using F = ma.
• 💡Always draw a clear free-body diagram for each object in a problem. Label all forces and indicate directions. This helps you avoid missing forces and makes your reasoning clear to the examiner.
• 💡When using F = ma, ensure you use the resultant force. If forces are at angles, resolve them into perpendicular components before summing.
• 💡In explanations, be precise about which object forces act on. For Newton's third law, state the two objects involved and the type of force (e.g., gravitational, contact).

• Treating action and reaction as balanced forces on one body; correct by stating they act on different bodies and never cancel for a single object.
• Assuming a moving object always has a resultant force; correct by noting constant velocity means zero resultant force.
• Using F = ma with F as any single force rather than the resultant force; correct by summing vector forces first.
• Forgetting to subtract friction or drag from the driving force; correct by calculating the resultant force first.
• Mixing grams with kilograms or kilometres per hour with metres per second; correct by converting to SI units before substitution.
• Treating F = ma as valid when mass changes, such as a rocket burning fuel; correct by using the momentum form for variable mass.
• Misconception: Newton's third law pairs cancel out because they are equal and opposite. Correction: They act on different objects, so they do not cancel; each force affects a different body.
• Misconception: An object moving at constant speed has no forces acting on it. Correction: It may have balanced forces (resultant force zero), but individual forces can still be present.
• Misconception: Mass and weight are the same. Correction: Mass is a scalar measure of inertia (kg), while weight is a force (N) given by W = mg, where g is the gravitational field strength.

1. 1Day 1-2: Review the three laws and their statements. Learn definitions and units. Practice identifying Newton's third law pairs in simple scenarios.
2. 2Day 3-4: Master free-body diagrams. Draw diagrams for objects on inclines, in lifts, and connected by strings. Resolve forces into components.
3. 3Day 5-6: Apply Newton's second law to solve problems with constant acceleration. Include problems with friction and tension. Check your answers for reasonableness.
4. 4Day 7-8: Tackle more complex problems: systems of connected masses, objects in equilibrium, and motion with variable acceleration (using calculus if required).
5. 5Day 9-10: Practice exam-style questions under timed conditions. Review mark schemes to understand what examiners expect. Focus on explanation questions and calculations.

• 📋Calculation of resultant force or acceleration using F = ma: Often involves resolving forces or using kinematics to find acceleration first. Advice: Draw a free-body diagram, resolve forces, and show all steps clearly.
• 📋Explanation of Newton's third law in a given context: E.g., explain the forces involved when a person walks or a rocket accelerates. Advice: State the two objects, the type of force, and that the forces are equal in magnitude and opposite in direction.
• 📋Analysis of motion with equilibrium: E.g., a car moving at constant speed, or an object on an inclined plane. Advice: State that resultant force is zero, resolve forces parallel and perpendicular to the plane, and solve for unknowns.
• 📋Practical-based question: E.g., describe an experiment to investigate Newton's second law. Advice: Include measurements of mass, force, and acceleration; mention controlling variables and using light gates or data loggers.