Rocket example

I took physics in high school AND college, but I’m befuddled by the math used in the Rocket example. I looked it up and thrust is a Force, which means that:


whose units are:

…which according to the interwebs is called Impulse. How do you get from an Impulse to an acceleration…because eventually you want to write:

Acceleration has the units m/s^2, so you would have to multiply thrustToApply by something with the units 1/kg-s:

Yet the text multiplies thrustToApply by a variable named direction. How does a direction have units 1/kg-s? So the thrust used in the text must not be a Force. Then, what is the thrust in the example?

Okay, I think I got it by working backwards:

acceleration = acceleration + thrustForce | | | V V V m/s^2 m/s^2 ?

thrustForce must also have the units of acceleration: m/s^2. What a great variable name! You couldn’t be any more misleading than calling a variable “Force” in a physics example when the variable actually represents an acceleration.


The direction variable sounds like it should be unit-less, and it is a Vector whose value is 1 in the y direction, so:

thrustForce = direction * thrustToApply | | | V V V m/s^2 = 1 * thrustToApply

…which means thrustToApply has to have the units m/s^2 (also an acceleration).


thrustToApply = thrust * thrustTime | | V V m/s^2 = thrust * s


thrust = m/s^2 * 1/s = m/s^3

And m/s^3 is the same as m/s/s/s, which is equivalent to a/s. So, I think thrust must be the measure of the change in acceleration over time, and

gives you the new acceleration of the rocket after 1 second.