1.2.2Tools and Inquiry — IB Physics skills
Newton’s First Law — IB Physics SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 23 of 33 Unit quiz
Curriculum Equation Key قوانين نيوتن للحركة - Newton Laws of Motion FormulasFast Exam Solution
F = m × a | W = m × g | F₁₂ = -F₂₁
Word Equation & Conditions: القوة المحصلة = الكتلة × التسارع | Force = Mass × Acceleration
Location & Specifications: الميكانيكا الكلاسيكية والتحصيلي وفيزياء 3 ثانوي و IGCSE Physics.
Part 1 of 4The First Law
Newton's first law describes what happens to motion when there is no net force.
Statement
- Statement An object remains at rest or moves with constant velocity unless acted on by a resultant (net) force.
Inertia
- Inertia Inertia is the tendency of a body to resist changes in its motion, and mass is the measure of inertia.
Equilibrium
- Equilibrium When the resultant force is zero the body is in translational equilibrium, with zero acceleration.
Vector sum
- Vector sum Forces must be added as vectors, so equal and opposite forces cancel but forces at angles combine by components.
Part 2 of 4Applying the Law
Use these steps to analyse any situation with balanced forces.
1
Draw a free-body diagram showing every force acting on the object, each as an arrow starting from the object.
2
Resolve forces that act at an angle into components along two perpendicular directions.
3
In each direction check whether the forces balance, which means the components sum to zero.
4
If the forces balance, state that the object is either at rest or moving at constant velocity, not that it has stopped.
5
If they do not balance, there is a resultant force and the object accelerates according to Newton's second law.
Part 3 of 4Everyday Examples
Everyday Examples
Constant velocity car
- Constant velocity car The driving force equals the total resistive force, so the resultant is zero and the speed stays constant.
Passengers in braking cars
- Passengers in braking cars Passengers continue forward when the car stops, because no resultant force acts on them until the seat belt does.
Terminal velocity
- Terminal velocity A falling object reaches constant velocity when upward drag equals the downward weight.
Space probe
- Space probe Far from other bodies a probe with its engine off keeps moving at constant velocity indefinitely.
| Situation | Resultant force | Motion |
|---|---|---|
| Book resting on table | Zero | Stays at rest |
| Skydiver at terminal speed | Zero | Constant velocity |
| Car accelerating from lights | Not zero | Velocity changes |
Part 4 of 4Exam Checklist & Tips
1.2.2 – Key Points for the Exam
- Inertia The resistance of a body to a change in its state of motion.
- Resultant force The vector sum of all the forces acting on an object.
- Translational equilibrium The state in which the resultant force is zero, so the acceleration is zero.
- Free-body diagram A diagram showing all the forces acting on a single object.
Common Mistakes
- Thinking a constant force is needed to keep an object moving at constant velocity.
- Saying that an object with zero resultant force must be at rest, when it may move at constant velocity.
- Including forces that the object exerts on other bodies in its free-body diagram.
- Treating inertia as a force instead of a property of mass.
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