# Initial velocity and time of motion through water

1. Feb 25, 2016

### gruba

1. The problem statement, all variables and given/known data
What initial velocity has to have a ball at the height $h=1m$ in order to sink in the water to depth of $s=4m$? How long is the motion of a ball through water? A ball is made of material that has density $\rho=770 kg/m^3$. Surface tension and friction are negligible.

2. Relevant equations
Classical mechanics - free fall
3. The attempt at a solution
Ball will fall on water surface after $t_1=\sqrt{\frac{2h}{g}}$.
Time of falling to the bottom of water is $t_2=\sqrt{\frac{2s}{g}}$.
Motion through the water is $t_2-t_1=0,451 s$.
Is this correct?
How to find the initial velocity? Why is the density of a ball $\rho$ given?

2. Feb 25, 2016

### kotone

Yes, you are correct in that you use classical mechanics. However, you must also think about other forces that could be present, not just gravity.
What forces are exerted on an object partially or totally immersed in a fluid? How would density be relevant?

3. Feb 25, 2016