Theme A — Space, Time and Motion
A.2.2 Newton’s First Law — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 13 of 63 1 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.
Lesson summary
A.2.2 Newton’s First Law — IB PHYSICS SL notes: State Newton's first law, explain inertia and apply the idea of equilibrium to bodies at rest and at constant velocity.Newton's first law — a body remains at rest or moves with constant velocity unless acted on by a resultant force.…
- Effect of a resultant force
- Correcting a belief
- Passengers in a braking car
- Tablecloth pulled quickly
- Seat belts
- Not a force
Section 1 of 9Statement of the law
1Newton's first law
- Newton's first law a body remains at rest or moves with constant velocity unless acted on by a resultant force.
- A body at rest stays at rest unless a resultant force acts.
- A body moving at constant velocity keeps that velocity unless a resultant force acts.
2Constant velocity
- Constant velocity it means constant speed and constant direction, so forces may act but must balance.
3Effect of a resultant force
- Effect of a resultant force it speeds the body up, slows it down or changes its direction.
4Correcting a belief
- Correcting a belief a force is needed to change the motion, not to keep it going.
- A puck on smooth ice slides almost unchanged because friction is small.
Section 2 of 9Inertia and mass
1Inertia
- Inertia the tendency of an object to resist changes in its velocity.
2Mass
- Mass the measure of inertia; the larger the mass, the larger the force needed for a given acceleration.
3Passengers in a braking car
- Passengers in a braking car they lurch forward because their bodies tend to keep their original velocity.
4Tablecloth pulled quickly
- Tablecloth pulled quickly the crockery stays almost in place.
5Seat belts
- Seat belts they supply the force needed to slow passengers with the car.
6Not a force
- Not a force inertia is not a force, so there is no forward "inertial force".
Section 3 of 9Equilibrium
Equilibrium
Translational equilibrium
- Translational equilibrium the resultant force is zero, so the acceleration is zero.
- Static equilibrium is at rest and dynamic equilibrium is constant velocity.
Balanced components
- Balanced components forces to the left equal forces to the right, and upward forces equal downward forces.
Three coplanar forces
- Three coplanar forces their vectors form a closed triangle.
Examples
- Examples a book on a table, a skydiver at terminal speed, a car cruising where driving force equals resistive force, and a hanging lamp.
Section 4 of 9Using the law in problems
Test
- Test if the velocity is constant, the forces balance.
Finding unknown forces
- Finding unknown forces at constant velocity the driving force equals the total resistive force.
- A boat at steady speed with 800 N thrust has 800 N total drag.
If thrust exceeds drag
- If thrust exceeds drag the boat accelerates until drag has grown enough to balance it again, at a new steady speed.
Link to the second law
- Link to the second law the first law is the special case F = ma with a = 0.
Common confusion
- Common confusion a body moving at constant velocity has balanced forces, but drag forces are often invisible.
Section 5 of 9Reference frames
Inertial frame
- Inertial frame one that is not accelerating; the first law holds in it.
- The ground and a train moving at constant velocity are approximately inertial.
Accelerating vehicle
- Accelerating vehicle objects seem to accelerate without a force, which is explained in the ground frame by their tendency to keep their velocity.
Hanging air freshener
- Hanging air freshener it swings back when a car accelerates because it tends to keep its old velocity.
Section 6 of 9Evidence for the law
1Air track
- Air track a glider on a cushion of air moves at almost constant velocity, and light gates show the same speed at both ends.
2Coin on a card over a glass
- Coin on a card over a glass the coin drops straight into the glass when the card is flicked away, because no horizontal force acts on it.
3Unsecured load
- Unsecured load a load on a lorry slides forward when the lorry brakes.
4Explanations
- Explanations name the missing or balanced force rather than saying the object "wants" to keep moving.
Section 7 of 9More from the specification
1
Translational equilibrium and weight balance
- Translational equilibrium and weight balance an object resting on level ground experiences equal magnitude upward contact force balancing downward gravitational weight.
2
Circular motion under centripetal forces
- Circular motion under centripetal forces a body travelling in a horizontal circle requires an inward centripetal force acting perpendicular to instantaneous velocity.
3
Terminal velocity in fluid resistance
- Terminal velocity in fluid resistance falling objects achieve constant velocity when upward fluid resistance force balances downward weight,
- resulting in zero net acceleration.
4
Newton's first law of motion
- An object remains at rest or continues in uniform motion in a straight line unless acted upon by a non-zero resultant external force.
5
Translational equilibrium condition
- Translational equilibrium requires the vector sum of all acting forces to equal zero (Sigma F = 0), producing zero linear acceleration.
6
Inertia and inertial reference frames
- Inertia is the intrinsic property of mass resisting changes in motion;
- Newton's laws are valid in non-accelerating inertial reference frames.
7
Fluid resistance
- Drag force from a fluid opposing motion.
8
Normal
- Line at 90° to a surface.
9
Tension
- Force transmitted along a string or rope.
10
Buoyancy
- Another name for upthrust.
11
Centripetal force
- Net force toward the centre, mv2/r.
12
Kinetic energy
- K = ½mv2, energy of motion.
13
Angular speed
- Magnitude of the angular velocity.
14
Centripetal acceleration
- Ac = v2/r toward the centre.
15
Normal reaction
- The contact force perpendicular to a surface.
16
Momentum
- P = mv, a vector.
17
Weight
- Gravitational force on a mass.
18
Collision
- Short interaction with large internal forces.
19
Linear
- A graph that is a straight line through the origin, so resistance is constant.
20
Impulse
- J = FΔt = Δp.What the symbols mean: F = force (N) · Δt = time the force acts (s) · Δp = change in momentum (kg m s−1); FΔt is the impulse (N s)
21
Radius
- Distance from the centre to the circular path.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Newton's first law | A body stays at rest or moves with constant velocity unless a resultant force acts |
| Inertia | Tendency of a body to resist changes in its velocity |
| Translational equilibrium | State in which the resultant force on a body is zero |
| Inertial frame | Frame of reference that is not accelerating |
| Mass | Measure of a body's inertia, in kg |
| Resultant force | Vector sum of all forces on a body |
Section 9 of 9Exam technique
1
Saying a force is needed to keep an object moving at constant velocity — no resultant force is needed.
2
Thinking zero velocity means no forces act — forces may act but balance.
3
Treating inertia as a force — it is a property of mass.
4
Taking weight and normal force as a third-law pair because they are equal — they act on one body and differ in type.
5
Believing constant speed along a curve is equilibrium — the direction of velocity changes.
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