# Freefall problem of a diver jumping horizontally upward

1. Dec 1, 2015

### RaeZ

1. The problem statement, all variables and given/known data
A diver jumps vertically straight up off of a platform 3 meters above the surface of a pool below. The diver hits the water and comes to rest in 0.80 seconds after traveling through 3.6 vertical meters of water. (See the diagram below). Assume the acceleration of the diver through the water is constant. The goal of this problem is to work backwards to eventually find the velocity of the diver at the instant the diver leaves the platform.

Find both the velocity with which the diver hits the water and the acceleration that the diver experienced when slowing down through the water.

2. Relevant equations
d=v_0t+.5at^2
v=v_0+at

3. The attempt at a solution
I have no idea how to determine the final velocity of the diver(when they hit the water) without either a known initial velocity or the distance they traveled from their max height to the water.
I drew the diagram for this and looked at previous problems but none gave as little information. I also tried to use the third equation with v_0=0 and d=3 for the fall from it's maximum height to the water, but this doesnt make sense since the displacement was more than three since they jumped upwards from 3

Last edited: Dec 1, 2015
2. Dec 1, 2015

### RaeZ

I drew the diagram for this and looked at previous problems but none gave as little information. I also tried to use the third equation with v_0=0 and d=3 for the fall from it's maximum height to the water, but this doesnt make sense since the displacement was more than three since they jumped upwards from 3

3. Dec 1, 2015

### TSny

Welcome to PF!

See if you can find the the speed the diver hits the water by considering just the motion while in the water.

4. Dec 1, 2015

### RaeZ

I have the same problem with that though, I could use the third equation and solve for v_0, with a final velocity of zero, but i do not know their acceleration in the water...

5. Dec 1, 2015

### TSny

You can try to combine the equations that you wrote in such a way that you eliminate the acceleration and express V0 in terms of the time and distance.

Or, try to use the concept of average velocity.

6. Dec 1, 2015

### RaeZ

How can I eliminate acceleration, the acceleration water is different than the acceleration in air

7. Dec 1, 2015

### azizlwl

Two unknown, so you need 2 equations.

8. Dec 1, 2015

### TSny

"Eliminating the acceleration" means to combine the equations to produce a new equation in which the acceleration does not appear in the new equation but the initial velocity, time, and distance do appear. So, you won't need to know the acceleration to solve for the initial velocity.

Or, another approach is to use the concept of average velocity. If an object has constant acceleration while the velocity changes from v0 to vf, how can you express the average velocity?