# motion due to gravity

• Sep 5th 2010, 11:26 PM
furor celtica
motion due to gravity
When an object falls through a liquid, three forces act on it: its weight, the buoyancy and the resistance of the liquid. Two spheres, of mass 0.5 kg and 1.5 kg respectively, have the same radius, so that they have the same buoyancy of 3.2 newtons, and the same resistance formula, 5v newtons when falling at speed v ms^-1. Both spheres enter the liquid falling vertically at 1 ms^-1. Calculate the terminal speeds of the two spheres, and the acceleration or deceleration when they enter the liquid.
(Here I found: for mass 0.5 kg, terminal speed 0.36 ms^1 and acceleration -6.4 ms^2; for mass 1.5 kg, terminal speed 2.36, acceleration 4.53 (to 3 s.f.))
If the depth of the liquid is 10m, show that the heavier sphere reaches the bottom after a time between 4 and 10 seconds. Find bounds for the time that the lighter sphere takes to reach the bottom.

Ok to starting with the heavier sphere I thought it would be logical to proceed like this:
To find higher bound: distance = 10, u=1, v<2.36, a= (15 – 3.2 – 5v)/1.5 = (11.8 -5v)/1.5, equation s=ut + 0.5at^2, find v in terms of t and find interval for which v<2.36 is true
No problem here, I found t<10
I used a similar method to find the lower bound using s=vt-0.5at^2 and v=1, u>1, but this time the expression had no real roots, so I’m wondering if I’m not using the wrong method altogether, and if my first result was just lucky.
Can someone show me how to work through this?
• Sep 6th 2010, 02:02 AM
sa-ri-ga-ma
I didn't get the idea of lower bound and higher bound. Can you elaborate?
• Sep 6th 2010, 03:02 AM
CaptainBlack
Quote:

Originally Posted by sa-ri-ga-ma
I didn't get the idea of lower bound and higher bound. Can you elaborate?

If the sphere enters the liquid at less than terminal velocity its' intial speed is the minimum speed it will have and terminal velocity is a upper bound on speed.

If the initial speed is greater than the terminal velocity then that is the maximum speed it will achive, and teminal velocity is a lower bound on speed.

CB
• Sep 6th 2010, 03:55 AM
furor celtica
yeah i know that.