# How NASA Finds the Mass of the Dust Grabbed From an Asteroid

It is vital to understand that the 2 sticks have the identical mass, however completely different moments of inertia. The second of inertia relies upon not simply on the full mass, however the place this mass is positioned with respect to the axis of rotation. Think about the you have got a bunch of particular person plenty (reasonably than a steady distribution like a stick). If every mass has a distance r from the axis of rotation then the second of inertia might be calculated because the sum of the merchandise of mass multiplied by distance squared. That sounds horrible, so I’ll write it as the next equation as an alternative. Sure, we use the image I for the second of inertia.

I do know that appears dangerous—nevertheless it’s not too horrible. The Σ simply means sum all of the elements (labeled with a altering index i), plus it seems cool. So with this definition, you may see why the 2 sticks have completely different moments of inertia (when rotated concerning the middle). For the follow the bottles on the finish, the gap values (r) are better and if you sq. them they get even larger. Meaning the outer bottle stick has a bigger second of inertia which makes it more durable to vary its rotational movement from one course to the opposite course.

OK, only a fast remark. The above expression for the second of inertia assumes there’s a fastened axis of rotation. It is doable that an object can rotate in very difficult methods and that expression simply will not work—nevertheless it’s nonetheless effective on this case. However now that we all know one thing concerning the second of inertia, how can we use it to measure the mass of some asteroid grime?

There are two extra issues to consider: torque and angular acceleration. Let’s begin with torque. In brief, it is the rotational equal of a pressure. Similar to a pressure causes adjustments in linear movement, a torque causes a change in rotational movement. In case you push on an object (off middle), you’ll exert a torque. That torque relies on magnitude of the pressure and the torque arm. The torque arm is the perpendicular distance from the purpose of rotation to the placement the place the pressure is utilized.

Suppose you have got a spacecraft (perhaps like OSIRIS-REx) and you’ve got a rocket thruster that exerts a pressure. Possibly it seems like this.

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