Motion
Displacement, velocity, acceleration

Displacement, velocity, and acceleration are all vectors.
Displacement measures how far something is from its starting point, whereas distance measures the length of the journey.
Speed is the magnitude of velocity, but velocity also has a direction. Speed is distance per time, velocity is displacement per time.
Acceleration is change in velocity over time. Since velocity has a direction, changing direction of travel is acceleration.

YouTube video about speed and velocity here.

Forces and Newton's laws

Forces act to push or pull an object. Examples: friction, electrostatic force, tension, gravitation, normal force, centripetal force.
Inertia is resistance to change in motion.
Newton's 1st law (the law of inertia): Velocity remains constant unless an unbalanced force acts.
Newton's 2nd law: Objects with more mass accelerate less when experiencing the same force.
Newton's 3rd law: A force on one object means an equal and opposite force on another object.

YouTube video about Newton's laws, inertia and momentum here.

Momentum, energy, work

Momentum is inertia in motion (mass × velocity) and is measured in kgms-1. Total momentum is always conserved.
Total energy is always conserved but often changes form. Examples: heat, light, sound, potential energy, kinetic energy.
If total kinetic energy is conserved during a collision, it is an elastic collision. If any changes form, it is inelastic.
Work done converts energy from one form to another.
All forms of energy are measured in J (joules).

YouTube video about energy and work here.

Motion with gravity

Weight is the force of gravity, measured in N (newtons). W = mg (m is mass in kg, g is acceleration due to gravity, usually 9.8 ms-2).
An object in free fall experiences only the forces of weight and air resistance. As its speed increases, air resistance increases. When the forces are equal, the object is at terminal velocity.
A projectile in the absence of air resistance follows a parabola. Its horizontal motion is constant and its vertical motion is accelerated by gravity.

Projectile calculations involve breaking the motion into a horizontal component and a vertical component.

  • at maximum height, the vertical component of velocity is zero
  • on level ground, at time of flight the vertical component of displacement is zero
  • launched horizontally, the projectile's initial vertical component of velocity is zero

Simulations of projectile motion: 1 2 3.

Projectile motion question practice here.