Thread: multple choice physics

you apply a 75-n force to pull a child's wagon across the floor at a constant speed of 0.5 m/s. If you increase your pull to 80 N the wagon will

A. continue to move at 0.5 m/s
B. speed up briefly and then move at a faster constant speed.
C. move with a continuously increasing speed.
D. stop

I think its C because it increase from zero to 5N

If this correct or I'm off

Rather than directly answer, I'll just make some observations. I'll just say this much: you seem to understand what's going on.

Interesting problem, in that it's secretly very dependent on considering friction. Constant speed means the net force is 0. If friction weren't included, then the only force with a horizontal component would be the 75 N force of pulling, which would then be a net horizontal force, and so the horizontal component of velocity wouldn't be constant.

So at 75 N, the net force is 0, and so the frictional force must be 75 N in opposition to the motion. So the question becomes, when you increase to 80 N, does the frictional force change?

If yes, and if it changes to exactly balance the new force, then the resulting velocity would become constant again (So answer = A or B).

If no, then there's now a net force on the object, and so it'll experience an ongoing acceleration (So answer = C).

So answering this requires knowing what determines frictional force, and if it will change with this new pulling force.

Actually the "correct" answer to this depends on assumptions that are not explicitly stated. In classical phyics classes you would conclude that the additional 5N is above and beyond the kinetic friction, which is assumed to be constant, and so the wagon would accelerate, reaching higher and higher speeds. Hence the answer would appear to be 'C,' and I would be that's the answer the teacher is looking for. But in reality friction actually increases with velocity, due largely to wind resistance - if this wasn't the case you could drive your car at light speed. So in practice if you increase the force by 5N the wagon increases speed until it reaches a new equilibrium with air resistance, and then it stops accelerating. Hence in the "real world" the answer is 'B.'