And that now must be the frictional force, because NP is still the frictional force.

这肯定是摩擦力，因为NP还是摩擦力。

youdao

And that now must be the frictional force, because NP is still the frictional force.

这肯定是摩擦力，因为NP还是摩擦力。

youdao