How to find the time

1. Oct 22, 2011

Snaveekim

1. The problem statement, all variables and given/known data
Traumatic brain injury such as concussion results when the head undergoes a very large acceleration. Generally, and acceleration less then 800 m/s^2 lasting for any length of time will not cause injury, whereas an acceleration greater then 1000 m/s^2 lasting for a least 1 millisecond will cause injury. suppose a small child rolls off a bed that is 0.40 m above the floor. if the floor is hardwood, the child's head is brought to rest in approximately 2.0 mm. if the floor is carpeted, this stopping distance is increased to about 1.0 cm. Calculate the magnitude and duration of the deceleration in both cases, to determine the risk of injury. Assume the child remains horizontal during the fall to the floor. Note that a more complicated fall cound result in a head velocity greater or less than the speed you calculate

2. Relevant equations
Vf=at+Vo

3. The attempt at a solution
a=-9.8 m/s^2
t=?
Vo=0
Xo=0
Xf=.4m

The only problem I have with this question is, finding time, and without time I couldn't really start. so any tips on how to start this? I don't want the answer really this is my first year of physics and I would love to understand this.
So if someone could teach me how to solve for time that would be great!

2. Oct 22, 2011

PhanthomJay

You first need to find the speed of the child's head at the instant it hits the floor. You can do this without knowing the time of the fall, by using another kinematic equation. then move on to the 2nd step to calculate the acceleration during , and duration of , the impact.

Welcome to PF!

3. Oct 22, 2011

Snaveekim

I don't know many Equations so would you mind showing me the way?

and thank you and congrats on 4,444 posts :)

4. Oct 22, 2011