You are Here: Home >< Physics

# Terminal Velocity watch

1. Droplets in a deodorant spray have a mass of about 4*10^-12 kg and a radius of about 0.1mm
Estimate the terminal velocity of the droplets in air with a viscosity of 2 * 10^-5 Nsm^-2

How do I do this?
2. There's a simple explaination here: http://alevelnotes.com/?id=113.
3. use stokes law http://en.wikipedia.org/wiki/Stokes'_law
5. btw I used the formula on this website and the answer did not agree with that in my text book
I got 3 * 10^-4

6. First of all, from the information given it follows that droplets are less dense than air. Is it surely correct?
7. I don't know for sure it is just a question from a past paper
the answer is (according to mark scheme)
8. Taking the viscous force, upthrust and weight into account gives a terminal velocity of around 2.7x10^-4 m/s upwards.

If you ignore upthrust, you get around 1 mm/s downwards. To be honest, I've got no clue why this should be the way to do it. Especially that upthrust is greater than weight in this case.
9. ok, thanks so much for that!!!!

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 14, 2011
Today on TSR

### University open days

• Sheffield Hallam University
Wed, 17 Oct '18
• Northumbria University
Wed, 17 Oct '18
• Staffordshire University