Newton’s 2nd Law:

Question

A physics student pushed a 50 kg load across the floor, accelerating it at a rate of 1.5 m/s2. How much

force did she apply?

Answer 1

Hi Immaculate I.,

All you can give as an answer here is a minimum value. Why? The minimum value case applies if the student pushes directly from the side (parallel to the floor), AND the load has NO dynamic friction (most improbable!). Then F = ma .

But, if the load has dynamic friction, then you need to apply that as a frictional force resisting the motion of the load; the student must supply that amount additionally, when pushing parallel to the floor.

Worse, if she pushes downward and there is dynamic friction, a portion of her force goes into increasing the dynamic friction force, AND only a portion of her force (the decomposed parallel portion) is available to counteract the friction AND accelerate the load. This is a recipe for disaster!

SO -- what could she do instead, to use minimal force? A. Hitch a rope and pull the load instead! That way, a portion of her force (assuming she lifts some) actually decreases the friction; she can use proper body mechanics; and hitting obstructions doesn't ram the load into her!

An objection might be raised: why should she pull it, if, say, the load has no friction with the floor? The answer to that is, if the load has no friction neither does she, most likely, which is quite embarrassing if you're trying to accelerate a load.

So make sure, after you do your calculation, that you state "this much OR MORE" and why!

-- Cheers, -- Mr. d.