What is the Effect of Reduced Air Resistance on an Object's Acceleration?

[Dberry]

Homework Statement

If F is the force of air resistance on an object with mass m moving at a constant velocity, which of the following best describes the acceleration of the object when the force of air resistance is reduced by a factor of 4?

Homework Equations

F=ma

The Attempt at a Solution

The options listed are:
a) F/m
b) 1/2 F/m
c) 1/4 F/m
d) 3/4 F/m

My original thinking was that if there is constant velocity then there is no acceleration. But since that wasn't a choice I went with option c). However, the back of my book lists option d) as the answer, saying that the force propelling the projectile also has a force, so when the air resistance is reduced by a factor of 4 the net force must be F - 1/4 F = 3/4 F. I guess what I don't understand is why I have to subtract from the force projecting the projectile.

Thanks for your help!

 

[Doc Al]

To find the acceleration, you need the net force on the object. What two forces act on the object? When the object has constant velocity, what must the net force be? But when the air resistance force changes, so does the net force.

 

[Dberry]

To find the acceleration, you need the net force on the object. What two forces act on the object?

The forces acting on the object are air resistance and the force propelling it.

When the object has constant velocity, what must the net force be?

The net force must be zero because constant velocity means there is no acceleration, so according to F=ma F is zero.

The net force acting on the object right now are the two forces mentioned above and nothing else right? Or are the mass and velocity a part of the net force?

 

[Doc Al]

The forces acting on the object are air resistance and the force propelling it.

Right. And presumably the force propelling it does not change.

The net force must be zero because constant velocity means there is no acceleration, so according to F=ma F is zero.

Exactly. So that tells you how big the propelling force is. (In terms of F, at least.)

The net force acting on the object right now are the two forces mentioned above and nothing else right?

Right. The vector sum of those two forces.

Or are the mass and velocity a part of the net force?

No.

 

[Hioj]

To illustrate the problem I've created the following figures:

Before:
[URL]http://img63.imageshack.us/i/qbefore.png/[/URL]
In case image doesn't show (link): http://img63.imageshack.us/i/qbefore.png/

After:
[URL]http://img252.imageshack.us/i/qafter.png/[/URL]
In case image doesn't show (link): http://img252.imageshack.us/i/qafter.png/

As you see, the net force F_{net} increases as air resistance decreases.

Something wrong with the latex subscript..