Question about terminal speed

[aimingforthat45]

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Hey guys, I was doing a problem on terminal velocity and I had trouble with finding out why the right answer is what it is. if anyone could shed light on this, that would be great. Below is the question:

"If an object is falling through a fluid at a speed less than its terminal speed, which one of the following best describes the relationship between the magnitudes of the drag force, buoyant force, and gravitational force on the object?"

A. Fd<Fb+Fg
B. Fg< Fb+Fd
C. Fd>Fb+Fg
D. Fg>Fb+Fd

I thought the answer was B, but the correct answer is D. Can anyone explain why? Thanks

 

[JMMTB]

When an object hits its terminal speed then it is no longer accelerating. Fg=Fb+Fd
The question states that we have not hit terminal speed which means that is must still be accelerating so Fg>Fb+Fd
If B were correct then the object would be going up the fluid and not down because there would be a net upward force on the object.

 

[DenTony11235]

When an object hits its terminal speed then it is no longer accelerating. Fg=Fb+Fd
The question states that we have not hit terminal speed which means that is must still be accelerating so Fg>Fb+Fd
If B were correct then the object would be going up the fluid and not down because there would be a net upward force on the object.

Good catch. The trick here is to realize that by "not at terminal speed" means "not yet at terminal speed" I'm thinking.

 

[DrknoSDN]

If B were correct then the object would be going up the fluid and not down because there would be a net upward force on the object.

Gotta be careful saying that because it's not always true.

The object does not need to be going up, merely needs to accelerating in the upward direction.
Fg< Fb+Fd could alternatively mean the object is beyond terminal velocity and decelerating (Jumping into a pool, or a bullet slowing after entering water).
Seems like you got caught by the "direction of motion must always be the same direction as acceleration" pitfall. 😛

 

[JMMTB]

Ya that makes sense because if it were traveling upward the forces acting in the downward direction would be Fg and Fd (opposing direction) and then upward Fb and it would then be Fb > Fg + Fd