Newton’s Laws & Space Travel (SQA National 5 Physics): Revision Note

Newton’s laws & space travel

• Newton's laws can be applied to space travel, including the launch and landing of a rocket

• Newton's third law states:

  • If object A exerts a force on object B, then object B exerts an equal and opposite force on object A

• Newton's second law states:

  • The acceleration of an object is proportional to the unbalanced force acting on it and inversely proportional to the object's mass

• Newton's first law states:

  • An object will remain at rest or continue to move with constant velocity unless acted on by an unbalanced force

Rocket launch

Newton's third law during launch

• When rocket fuel burns, exhaust gases are expelled downwards at high speed

• The rocket exerts a force on the gases, so according to Newton's third law, the gases must exert an equal but opposite force on the rocket

• As the gases push back on the rocket, this provides the thrust which propels it upwards

Newton's second law during launch

• During lift-off, the forces acting on the rocket are

  • Thrust acting upward

  • Weight acting downward

• The rocket’s thrust must exceed its weight to achieve lift-off

• This produces an unbalanced force equal to:

• According to Newton's second law, the unbalanced force causes the rocket to accelerate upwards

• Where:

  • = acceleration of the rocket, measured in metres per second squared (m s^-2)

  • = unbalanced force on the rocket, measured in newtons (N)

  • = mass of the rocket, measured in kilograms (kg)

• As the altitude increases, the acceleration of the rocket increases

• This is because the unbalanced force on the rocket increases as the weight of the rocket decreases

• There are several reasons for this:

  • The mass of the rocket decreases as it burns fuel

  • When the fuel is used up, the fuel tank is released, so the mass decreases even further

  • Gravitational field strength decreases with distance from the surface

Applying Newton's laws to a rocket launch

Space travel

Newton's first law during space travel

• Once free of Earth’s atmosphere, a spacecraft no longer needs continuous thrust because there is no friction to oppose its motion

• It continues moving at constant velocity until acted upon by another unbalanced force

Newton's third law during space travel

• Newton's third law also explains how ion propulsion works

• The ejection of ions in one direction causes the spacecraft to move in the opposite direction

• This provides a small, unbalanced force which allows spacecraft to

  • achieve a high velocity when applied over an extended period of time

  • perform precise manoeuvres in zero-friction environments, such as docking with the ISS

Rocket landing

• The same principles applied to launching a rocket can also be applied to landing a rocket

Newton's third law during landing

• During landing, the spacecraft expels exhaust gases, producing a downward force on the gases

• The gases push back on the spacecraft, providing an equal upward force

• This produces a thrust which slows the descent of the spacecraft

Newton's second law during landing

• During launch, the rocket is stationary, so it has a velocity of zero

  • So, when the unbalanced force is applied, it causes the rocket to accelerate

• Whereas, during landing, the spacecraft has a downward velocity that it is trying to reduce to zero

  • Therefore, the same unbalanced force is required to cause it to decelerate

  • i.e. the rocket’s thrust must exceed its weight to slow its descent

• Some spacecraft deploy parachutes to reduce the unbalanced force and land at a lower terminal velocity