Physics help!

[Vin]

Anyone

Your Physics teacher.

[elemis]

Your Physics teacher.

She's on holiday

[@d!_†oX!©]

Can someone answer this please?

What is the meaning of viscous drag and upthrust?

For example: A submarine descends vertically at constant velocity. The three forces acting on the submarine
are viscous drag, upthrust and weight

[nid404]

Viscous drag- Resistance derived from friction between the surface of a body and the fluid through which it is moving. The fluid particles adjacent to the body slow down, causing turbulent flow. The magnitude of the drag depends on a number of factors; it increases with the relative velocity of fluid flow, the surface area over which the flow occurs; the roughness of the body surface; and the viscosity of the fluid.

Upthrust-Upthrust is the force that pushes an object up and makes it seem to lose weight in a fluid.

[@d!_†oX!©]

so then both are acting as resistive forces, right?

[elemis]

so then both are acting as resistive forces, right?

From my limited knowledge of A Level Physics :

Upthrust is acting against the weight of an object in water hence I would suspect it is resistive.

I might be wrong. 

[nid404]

so then both are acting as resistive forces, right?

Both are resistive forces but their action is in different directions.

[@d!_†oX!©]

Alright, I think I should post the complete question itself:

A submarine descends vertically at constant velocity. The three forces acting on the submarine
are viscous drag, upthrust and weight.

Which relationship between their magnitudes is correct?

A weight < drag
B weight = drag
C weight < upthrust
D weight > upthrust

[Vin]

I suppose it's D. But the velocity is constant. In the case of D, I'm guessing the velocity would increase.

[elemis]

D ?

Because Drag (D) + Upthrust (U) = Weight (W)

Hence B is wrong.

[@d!_†oX!©]

If it was just weight and upthrust or weight and drag, thn according to simple IGCSE Physics, I would answer B,  as at constant(terminal) velocity the downward forces must equal the upward forces. But the case is different here, with three forces acting. I neither know which year this question is from, nor do I have any mark scheme for this one.
Are you sure about this Ari?
       Drag (D) + Upthrust (U) = Weight (W)

[nid404]

Alright, I think I should post the complete question itself:

A submarine descends vertically at constant velocity. The three forces acting on the submarine
are viscous drag, upthrust and weight.

Which relationship between their magnitudes is correct?

A weight < drag
B weight = drag
C weight < upthrust
D weight > upthrust

It's descending vertically, so viscous drag doesn't come into picture. A and B are out.
Now if the submarine is sinking, then Weight is greater than upthrust.
If upthrust is equal to the weight, the submarine will float on the surface
If upthrust is greater, submarine will thrown up the surface

[@d!_†oX!©]

Viscous drag- Resistance derived from friction between the surface of a body and the fluid through which it is moving. The fluid particles adjacent to the body slow down, causing turbulent flow. The magnitude of the drag depends on a number of factors; it increases with the relative velocity of fluid flow, the surface area over which the flow occurs; the roughness of the body surface; and the viscosity of the fluid.

Why do we exclude drag because of vertical motion? Is there something like that in the definition?

[elemis]

It cant be B because there are 3 components involved.

B suggests that only two are present and hence equivalent..... the third force all must have some value thus B cant be correct.

[nid404]

Why do we exclude drag because of vertical motion? Is there something like that in the definition?

Wait. Have a look at this.



Like I said, drag and upthrust both act as resistive forces but their direction of action are different. Upthrust is the push on the object(acts vertically) and viscous drag opposes motion through the fluid (horizontal)