Year Twelve Topic One

1. Distinguish between inertial and non-inertial frames of reference.
Frames of reference are coordinate systems or objects with respect to which we take measurements. (1)
A non-inertial frame of reference is one that is accelerating with fictitious or inertial forces. In these frames observers have to predicate the existence of these ‘forces’ to maintain the validity of Newton’s laws.
An inertial frame of reference is one that is moving with constant velocity or is it at rest, holding Newton’s first law of Inertia. No frame is more correct then another, however, some are simpler. An example could be the stars.(2)
  2. Analyse the changing acceleration of a rocket during launch in terms of the:
Law of conservation of momentum and
In its simplest form a rocket is a chamber enclosing gas under pressure. A small opening at one end allows this gas to escape, and in doing so provides a thrust that propels in the opposite direction. The engine creates thrust by burning propellants or fuel and expelling the resulting gases. (3)
The Law of Conservation of momentum states that the change in momentum of a rocket and its exhaust gases is zero (as the gases move in one direction, the rocket moves in the other and therefore cancel each other out)*. We can also determine that the momentum of the rocket plus the momentum of the exhaust gases equals zero**, as momentum equals mass times velocity***. The momentum of the exhaust gases remains quite constant and as the fuel is burned and the mass of the rocket decreases, the velocity of the rocket increase to compensate the loss and the rocket accelerates.(4)
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Forces experienced by astronauts
The mass of the system decreases as the fuel is burnt and the exhaust gases expelled.   Through Newton’s second law we know that the acceleration is proportional to the thrust and inversely proportional to the mass, therefore as the mass decreases the acceleration increases. Hence the forces on the astronauts increase. The...