Ireally need with this, i dont know what to do first, i need due !
physics

f= 215 n
d= 13m
kef=0
vi= 7.2 m/s
m= ?

formulas to solve:
w=1/2mvf^2 - 1/2mvi^2 , w= kef - kei

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)

Mass = 107.83 kilograms

your answer is, orbit around the nucleus