Newton's second law and direction of force

[neutron star]

Homework Statement

A particle is traveling in a straight line at a constant speed of 22.1 m/s. Suddenly, a constant force of 12.6 N acts on it, bringing it to a stop in a distance of 55.3 m.

Homework Equations

The Attempt at a Solution

(a) What is the direction of the force?
perpendicular to the direction of the particle's motion
*opposite the direction of the particle's motion
the same as the direction of the particle's motion
none of the above

(b) Determine the time it takes for the particle to come to a stop.
___ s

(c) What is its mass?
___ kg

F=ma
V_f=0
V_f ^2 = V_i ^2 + 2a(X-X_i)
0^2=(22.1m/s)^2 + 2a(55.3m)
0=(22.1m/s)+at

 

[rock.freak667]

F=ma
V_f=0
V_f ^2 = V_i ^2 + 2a(X-X_i)
0^2=(22.1m/s)^2 + 2a(55.3m)
0=(22.1m/s)+at

Yes so the deceleration is ? And the time taken is?

 

[neutron star]

0=22.1m/s^2+2a(55.3)
-110.6a=488.41m/s
a=-4.416m/s

0=22.1m/s^2+(-4.416m/s)t
22.1m/s^2/4.416m/s=5.004s
t=5.004s

Is this right?

How do I get mass from this? F=ma right, so m=F/a or 12.6=m(-4.416)
or m=12.6/-4.416. But it doesn't seem to work...

 

[rl.bhat]

c) Use 1/2*m*v^2 = F*d