Relativity help (1 Viewer)

jash1228

New Member
Question: As the rocket passes planet X, it fires its engines, producing constant net force on the rocket. State Why, from the point of view on planet X, this constant force results in a decreasing acceleration of the rocket.

The_Owl

New Member
Think about what happens to the rocket's mass from the perspective of planet X. (and remember F=ma)

pikachu975

I love trials
Moderator
More speed = more mass (mass dilation, state the formula to prove that mass increases)

Edit: F = ma doesn't work as mass is not constant - thanks Integrand

InteGrand said:
For the purposes of HSC Physics, just do what you did (like assume F = ma). However, the true formula for force is

F = dp/dt, where p = mv is momentum.

By the product rule, this yields F = m*dv/dt + v*dm/dt (provided we have a closed system).

If the mass is constant (dm/dt = 0), then we can see the formula reduces to the usual F = ma.

However, if the mass is changing over time (dm/dt is not identically 0), then we need to include that term. This is important for example for analysing motion of rockets that are propelled by burning fuel and ejecting spent gases. In fact, the equation then is slightly modified (since the system, called a variable mass system, is not closed). You can read more about this here: https://en.wikipedia.org/w/index.php?title=Newton's_laws_of_motion&mobileaction=toggle_view_desktop#Variable-mass_systems .

But this is beyond the syllabus (despite being the actual result), and for the purposes of the HSC, you are expected to use the (totally incorrect in this situation) formula F = ma.

Last edited:
• InteGrand