How can motion be described and explained?

In this area of study students observe motion and explore the effects of balanced and unbalanced forces on motion. They analyse motion using concepts of energy, including energy transfers and transformations, and apply mathematical models during experimental investigations of motion. Students model how the mass of finite objects
can be considered to be at a point called the centre of mass. They describe and analyse graphically, numerically and algebraically the motion of an object, using specific physics terminology and conventions.

On completion of this unit the student should be able to investigate, analyse and mathematically model the motion of particles and bodies.

 Key knowledge

Concepts used to model motion

  • identify parameters of motion as vectors or scalars
  • analyse graphically, numerically and algebraically, straight-line motion under constant acceleration: v=u+atv^2=u^2+2ass=\frac{1}{2}(a+v)ts=ut+\frac{1}{2}at^2s=vt-\frac{1}{2}at^2
  • graphically analyse non-uniform motion in a straight line
  • apply concepts of momentum to linear motion: p=mv

Forces and motion

  • explain changes in momentum as being caused by a net force: F_{net}=\frac{\Delta p}{\Delta t}
  • model the force due to gravity, F_g, as the force of gravity acting at the centre of mass of a body, F_g=mg, where g is the gravitational field strength ( 9.8 Nkg^{-1}near the surface of Earth)
  • model forces as vectors acting at the point of application (with magnitude and direction), labelling these forces using the convention ‘force on A by B’ or F_{on\;A\;by\;B}=-F_{on\;B\;by\;A}
  • apply Newton’s three laws of motion to a body on which forces act: a=\frac{F_{net}}{m}F_{on\;A\;by\;B}=-F_{on\;B\;by\;A}
  • apply the vector model of forces, including vector addition and components of forces, to readily observable forces including the force due to gravity, friction and reaction forces
  • calculate torque: \tau=r_\perp F
  • investigate and analyse theoretically and practically translational forces and torques in simple structures that are in rotational equilibrium.

Energy and motion

  • apply the concept of work done by a constant force using:
    – work done = constant force × distance moved in direction of force: W=Fs
    – work done = area under force-distance graph
  • investigate and analyse theoretically and practically Hooke’s Law for an ideal spring: F=-k\Delta x
  • analyse and model mechanical energy transfers and transformations using energy conservation:
    – changes in gravitational potential energy near Earth’s surface: E_g=mg\Delta h
    – potential energy in ideal springs: E_s=\frac{1}{{2}}k\Delta x^2
    – kinetic energy: E_k=\frac{1}{{2}}mv^2
  • analyse rate of energy transfer using power: P=\frac{E}{t}
  • calculate the efficiency of an energy transfer system: \eta=\frac{usefull \;energy\;out}{total\; energy\; in}
  • analyse impulse (momentum transfer) in an isolated system (for collisions between objects moving in a straight line): I=\Delta p
  • investigate and analyse theoretically and practically momentum conservation in one dimension.

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