# Homework Help: Force/Kinematics problem

1. Sep 29, 2011

### miglo

1. The problem statement, all variables and given/known data
Paleontologists estimate that if a Tyrannosaurus rex were to trip and fall, it would have experienced a net force of approximately 261,500 N acting on its torso when it hit the ground. Assume the torso has a mass of 3800 kg.

Find the magnitude of the torso's upward acceleration as it comes to rest. (For comparison, humans lose consciousness with an acceleration of about 7g.)

Assuming the torso is in free fall for a distance 1.40 m as it falls to the ground, how much time is required for the torso to come to rest once it contacts the ground?

2. Relevant equations
F=ma
V=V_0+at
V^2=(V_0)^2+2a(x-x_0)

3.

i figured out the first part, but im having trouble with finding the time required for the torso to come to rest once it contacts the ground. from the first part of the question i got a=59m/s^2, am i supposed to use this value for acceleration in the second question or should i use a=-9.81m/s^2, also i tried getting initial velocity but i kept getting the square root of a negative number so i know im wayy off

2. Sep 29, 2011

### PhanthomJay

Your acceleration value for the first part appears incorrect. You have factored in the dinosaur's torso's weight, but the net force is already given.
For part 2, you need to use the acceleration while in contact with the ground to solve for the impact time, not the acceleration value while falling to the ground. You can first use the kinematic equation you chose to solve fopr V, taking down as the positive direction to avoid the sq rt of a negative, but in this case, the acceleration is the free fall value.