# A level physics

A small sphere is moving horizontally through a viscous liquid.
(a) Stokes' law can be used to calculate the drag force on an object.
State the conditions that must apply for Stokes' law to be valid.
(b)There is a constant force of 2.3 × 10−5N acting horizontally on the sphere.
diameter of sphere = 4.5 × 10−3m
viscosity of liquid = 7.1 × 10−2Pa s
(i) At one instant, the speed of the sphere is 5.2 × 10−3ms−1.
Calculate the resultant horizontal force on the sphere.

Part a of this question was easy,but I'm confused for part (b).In mark scheme,they used Stokes' law formula,F=F(pi)(n)rv.But Stokes' law needs constant velocity,but how do we know that the speed we are given is a constant velocity?
(edited 5 months ago)
Original post by Anlasan
A small sphere is moving horizontally through a viscous liquid.
(a) Stokes' law can be used to calculate the drag force on an object.
State the conditions that must apply for Stokes' law to be valid.
(b)There is a constant force of 2.3 × 10−5N acting horizontally on the sphere.
diameter of sphere = 4.5 × 10−3m
viscosity of liquid = 7.1 × 10−2Pa s
(i) At one instant, the speed of the sphere is 5.2 × 10−3ms−1.
Calculate the resultant horizontal force on the sphere.

Part a of this question was easy,but I'm confused for part (b).In mark scheme,they used Stokes' law formula,F=F(pi)(n)v.But Stokes' law needs constant velocity,but how do we know that the speed we are given is a constant velocity?

Why do you think stokes law needs constant velocity? The fact it then asks for the resultant force (and so an acceleration) on the sphere strongly suggests that its not correct In fact the third part asks for the max / terminal velocity of the sphere, so the condition when the resultant force is zero and velocity is indeed constant.
(edited 5 months ago)
Original post by mqb2766
Why do you think stokes law needs constant velocity? The fact it then asks for the resultant force (and so an acceleration) on the sphere strongly suggests that its not correct In fact the third part asks for the max / terminal velocity of the sphere, so the condition when the resultant force is zero and velocity is indeed constant.

Oh!! So,Stokes law doesn't need terminal velocity? The v in the formula is just the velocity??
(edited 5 months ago)
Original post by Anlasan
Oh!! So,Stokes law doesn't need terminal velocity? The v in the formula is just the velocity??

Thats correct. Its constant when its terminal velocity (part c) of this question). Part a) asks for the assumptions and one of these is low speed/velocity, not necessarily constant. So v in the formula is a (low) velocity.
(edited 5 months ago)
Original post by mqb2766
Thats correct. Its constant when its terminal velocity (part c) of this question). Part a) asks for the assumptions and one of these is low speed/velocity, not necessarily constant. So v in the formula is a (low) velocity.

ohh okay! Thank you so much!!