Motion, Laws of Motion & Forces

An object changes its position with time.

A fixed reference point (origin) to compare against.

No — they are relative, depending on the observer's reference point.

At rest with respect to the train, but in motion with respect to the platform. Both views are correct.

The total length of the actual path travelled. It is a scalar.

The shortest straight line from start to finish, with direction. It is a vector.

Large distance but zero displacement.

Yes — zero or even negative; but distance is always positive.

If how much is enough it is scalar; if you also need which way it is vector.

Speed & distance are scalars; velocity and displacement are vectors. Mass ↔ Weight is another exam pair.

Speed = Distance ÷ Time; a scalar, SI unit m/s.

Velocity = Displacement ÷ Time; a vector, SI unit m/s.

Equal displacements in equal times along a line.

2v₁v₂ ÷ (v₁+v₂) — the harmonic mean, not the simple average.

Average speed uses total distance; average velocity uses total displacement.

The rate of change of velocity with time; it is a vector.

a = (v − u) ÷ t, SI unit m/s².

Increasing velocity → positive; decreasing velocity → negative (retardation/deceleration).

Yes — a change in speed, direction, or both causes acceleration.

Only when acceleration is constant (uniform).

v = u + at — gives final velocity after time t.

s = ut + ½at² — gives distance covered in time t.

v² = u² + 2as — used when time is not given.

u = initial velocity, v = final velocity, a = acceleration, t = time, s = distance. Derived from the velocity-time graph.

= speed.

Slanting = uniform speed; flat = the object is at rest.

Gives an upward-bending curve.

= acceleration.

= distance travelled.

Uniform = straight slanting line; zero = horizontal line.

A push or a pull.

Start, stop, speed up, slow down or change the direction of motion — and also change the shape of an object.

Contact: friction, muscular, tension, normal. Non-contact: gravitational, magnetic, electrostatic.

The newton (N); 1 N = 1 kg·m/s². (Newton)

The tendency of an object to resist any change in its state of rest or motion.

A body stays at rest or in uniform motion unless an external force acts.

Force = mass × acceleration (F = ma); it also equals the rate of change of momentum.

Every action has an equal and opposite reaction.

Due to inertia of rest.

Rocket propulsion and recoil of a gun.

p = mass × velocity; a vector, SI unit kg·m/s.

Total momentum of an isolated system stays constant.

Impulse = Force × time = change in momentum.

To increase the time of catching, reducing the force felt.

Shock absorbers and bending knees while landing.

The force that opposes relative motion between surfaces in contact.

Static acts before motion starts; kinetic (sliding) acts during motion.

Friction exactly equals the applied force.

Rolling friction is much smaller than sliding friction.

Convert sliding friction into rolling friction to reduce it.

Useful (walking, brakes, grip) but also harmful (wear, heat, energy loss).

Because its direction changes constantly.

Acts towards the centre and keeps the body on its circular path.

The apparent outward force felt in a rotating frame — a pseudo-force.

This principle — water is flung outward through the drum holes.

A stone whirled on a string, a car turning, planets orbiting the Sun.

Defined/measured speed using inclined planes; showed all objects fall at the same rate regardless of mass.

The three laws of motion and universal gravitation.

Relativity; E = mc².