2 physics multiple choice questions pls help! Lots of points & brainiest

What is the force that opposes motion and works against the downward pull? a) friction b) gravity c) weight d) acceleration

Ahorizontal force of 400.0 n is required to pull a 1760 n truck across the floor at a constant speed. find the coefficient of sliding friction.

When can a theory be modified if a new type of technology allows for new observations that raise new questions? a-immediately, while the questions about the theory are being asked b-after new hypotheses related to the theory are tested in experiments c-only after the new observations disprove all parts of the theory d-while scientists begin to think about how the theory could improve