How to change speed ,change v or m?

[zxl777]

In a gravity environment and quality of objects,
I would like to change the objects in the movement direction of the movement, speed,
and I directly attribute the speed of objects to amend it?
Or modify the force?

[zxl777]

From the document, I have seen this text

Use forces to modify the rigid bodies if possible. This is likely to be the most stable.
Modifying a body’s velocity shouldn’t necessarily be avoided, but applying large changes can cause strange results in the simulation. Experiment freely, but be warned.
Don’t modify a body’s position every step unless you really know what you are doing. Otherwise you’re likely to get the position/velocity badly out of sync.