# Physics - Dynamics

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• June 3rd 2013, 01:57 PM
sakonpure6
Physics - Dynamics
Hi I am having trouble with the following question. Any help would be highly appreciated!
Quote:

A box accidentally falls from the back of a truck and hits the ground with a speed of 15.0m/s. It slides along the ground for a distance of 45m before coming to rest.
a) find the length of time the box slides before stopping
My attempt was using the equation d=v1*t + 0.5*a*t^2 where a = 9.8m/s^2 because the box fell out of the car. So i had to solve a quadratic and I got t about 111s. Is that right?

Thank you for your time!
• June 3rd 2013, 03:15 PM
BobP
Re: Physics - Dynamics
You need to find the (assumed) constant acceleration (retardation) of the box as it slides along the ground, and there is no reason to assume that this is -9.8m/s.
Start by using the formula
$v^{2}=u^{2}+2as$ to find $a,$ and then $v=u+at$ to find $t.$
• June 3rd 2013, 03:44 PM
Shakarri
Re: Physics - Dynamics
It is friction not gravity which is decelerating the box. In (quite accurate) theory the friction will decelerate it at a constant rate.