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3.1.5Dynamics, forces and pressure

Newton’s Laws of Motion

5 min read 8 parts Lesson 8 of 24 1 quiz

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Lesson summary

3.1.5 Newton's Laws of Motion — AS PHYSICS 9702 notes: State and apply Newton's three laws, identify third-law pairs and recognise equilibrium.A body remains at rest or moves with constant velocity unless acted on by a resultant force.Balanced forces — the resultant force is zero…

  • Unbalanced forces
  • A force is needed to change velocity
  • It describes inertia
  • A car at constant velocity
  • In momentum form
  • What the symbols mean

Section 1 of 8Newton's first law

1
A body remains at rest or moves with constant velocity unless acted on by a resultant force.
2Balanced forces
  • Balanced forces the resultant force is zero and the body is in equilibrium, with no change in motion.
3Unbalanced forces
  • Unbalanced forces a resultant force changes the motion;
    • the object may speed up, slow down or change direction.
4A force is needed to change velocity
  • A force is needed to change velocity not to keep it moving.
5It describes inertia
  • It describes inertia passengers lurch forward when a bus brakes suddenly.
6A car at constant velocity
  • A car at constant velocity the frictional forces equal the driving force.

Section 2 of 8Newton's second law

A resultant force on a body causes a change in its motion in the direction of the force.
In momentum form
  • In momentum form the rate of change of momentum is proportional to the resultant force.
F = ma
  • F = ma for constant mass.
    What the symbols mean: F = resultant force (N) · m = mass (kg) · a = acceleration (m s−2)
    • (see 3.1.2 and 3.1.4)
The first law is the special case F = 0.

Section 3 of 8Newton's third law

Newton's third law
Whenever two bodies interact
  • Whenever two bodies interact the forces they exert on each other are equal and opposite.
A third-law pair must be
  • A third-law pair must be the same type of force, the same magnitude, opposite in direction and acting on different objects.
Pairs never cancel
  • Pairs never cancel since they act on different objects.
Earth pulls a book down with gravity; the book pulls the Earth up with an equal gravitational force.
  • The table's push on the book is not the partner of weight because it is a contact force acting on the same object.
Avoid "every action has an equal and opposite reaction"
  • Avoid "every action has an equal and opposite reaction" it does not say which object each force acts on.

Section 4 of 8Applying the laws

1A book at rest on a table
  • A book at rest on a table weight and normal reaction are equal and opposite, so the resultant is zero (first law), but they are not a third-law pair.
2A rocket
  • A rocket pushes exhaust gas backward and the gas pushes the rocket forward with an equal force, without needing air.
3Horse and cart
  • Horse and cart the forces between them are equal and opposite;
    • the cart accelerates because the horse's force on it exceeds the friction on the cart.
4Always identify which object is being analysed
  • Always identify which object is being analysed before listing the forces on it.

Section 5 of 8Equilibrium

Equilibrium
An object is in equilibrium
  • An object is in equilibrium when the resultant force is zero, so the force components along any direction sum to zero.
Constant velocity counts as equilibrium.
A hanging sign on two cables
  • A hanging sign on two cables the vertical components of tension add to the weight and the horizontal components are equal and opposite.
Three forces in equilibrium
  • Three forces in equilibrium form a closed triangle drawn tip to tail.
Forces in equilibrium are not necessarily third-law pairs
  • Forces in equilibrium are not necessarily third-law pairs they simply add to zero on the same object.

Section 6 of 8Limits of the laws

Limits of the laws
They describe everyday speeds and sizes well.
Special relativity
  • Special relativity is needed near the speed of light.
Quantum mechanics
  • Quantum mechanics is needed for atomic-scale objects.
They assume an inertial frame
  • They assume an inertial frame one that is not itself accelerating.
The third law makes impulses equal and opposite in collisions
  • The third law makes impulses equal and opposite in collisions the basis of conservation of momentum.

Section 7 of 8Key terms

TermMeaning
Newton's first lawA body stays at rest or at constant velocity unless a resultant force acts.
Newton's second lawThe resultant force equals the rate of change of momentum.
Newton's third lawForces between two interacting bodies are equal and opposite.
InertiaThe reluctance of a body to change its motion.
EquilibriumA state in which the resultant force is zero.
Force pairTwo third-law forces of the same type on different objects.
Inertial frameA frame of reference that is not accelerating.
Resultant forceThe vector sum of all forces on a body.

Section 8 of 8Exam technique

1
Calling weight and the normal reaction an action-reaction pair — they act on the same object and are different types.
2
Thinking an object needs a force to keep moving at constant speed.
3
Believing action-reaction forces cancel — they act on different bodies.
4
Saying the larger object exerts a larger force in a collision — the forces are always equal.
Physics 0625 & 9702 SUVAT Mastery Graph Interpretation

Constant Acceleration Equations (SUVAT) & Slopes

v = u + at  |  s = ut + ½at2  |  v2 = u2 + 2as  |  s = ½(u + v)t

  • Displacement-Time Graph: Gradient = Velocity (curve = accelerating).
  • Velocity-Time Graph: Gradient = Acceleration; Area under curve = Distance travelled / Displacement.
  • Free Fall in vacuum: a = g ≈ 9.81 m/s2 downwards (air resistance negligible).