Can I tell you what makes this problem so hard ?
It's having all the data WITHOUT HAVING THE STORY !
We first have to figure out what all those things are. I mean, we don't even know what F is, what d is, what Kef or Vi is, or how W figures in to the whole thing. You really have no mercy !
If my hunch is correct, the story goes like this:
-- There's an object sailing along, minding its own business, not bothering anybody, and its speed is 7.2 meters per second.
-- Somebody jumps out in front of the object and begins to push back on it with 215 Newtons of force, trying to slow it down and stop it.
-- The object is only able to go another 13 meters, pushing the guy backwards but slowing down, and then it stops.
-- The question is: What is the mass of the object ?
Now I'll go ahead and solve the problem that I just invented:
-- Kinetic energy = (1/2) (mass) (speed²)
Before anybody touched it, the object's kinetic energy was
KE = (1/2) (mass) (7.2 m/s)²
KE = (25.92) x (mass)
-- Since that's the energy the object had, THAT's how much work the guy has to do in order to make the object stop.
Work = (force) x (distance)
Work = (215 N) x (13 meters)
Work = 2,795 N-m
-- And there you go. The work the guy did to stop the object is the amount of energy the object had before he came along.
(25.92) x (mass of the object) = 2,795 N-m
Divide each side by 25.92:
Mass of the object = (2,795 N-m) / (25.92)
<em>Mass = 107.83 kilograms</em>
